2007 SOUTHEASTERN NATURALIST 6(3):505–522
The Herbivores of Solanum carolinense (Horsenettle) in
Northern Virginia: Natural History and Damage Assessment
Michael J. Wise*
Abstract - I studied interactions between the herbaceous weed Solanum carolinense
(horsenettle) and its herbivore community in northern Virginia from 1996–2002.
Thirty-two species regularly fed on the plant, including 31 insects from 6 taxonomic
orders and Microtus pennsylvanicus (meadow vole). An intensive field experiment
on 960 horsenettle individuals in 2001 revealed high levels of damage to all parts of
the plants. Two chrysomelid beetles—Epitrix fuscula (eggplant flea beetle) and
Leptinotarsa juncta (false potato beetle)—damaged leaves on nearly every plant.
Roughly half of the flowers were destroyed by herbivores, with Anthonomus nigrinus
(potato bud weevil) destroying 30%. Nearly three-fourths of the fruits were damaged
by three species: the tephritid fly Zonosemata electa (pepper maggot), false potato
beetle, and meadow vole. The weevil Trichobaris trinotata (potato stalk borer) bored
in stems of 73% of the plants, and the most damaging root feeder was the moth
Synanthedon rileyana (Riley’s clearwing). A literature review on the horsenettleherbivore
community is integrated with new observations as a guide for applied and
basic research on this economically significant species.
Solanum carolinense L. (Solanaceae) (horsenettle) is a perennial herbaceous
plant native to the southeastern United States. It has expanded its range
northward to Ontario and westward to California, and it has become an invasive
weed in temperate and tropical areas in Europe and Asia (Ilnicki et al. 1962,
Imura 2003, NAPPO 2003). Horsenettle reproduces vegetatively through a
rapidly expanding root system, and sexually through flowers and fruits (Ilnicki
et al. 1962). Racemes mature from base to tip and may bear 1 to over 20 flowers
(mean 8). Horsenettle blooms in Virginia from late May until early October.
Mature fruits are round, yellow berries with an average diameter of 1.5 cm.
Several species of birds and mammals have been reported to feed on horsenettle
fruits (Bassett and Munro 1986, Cipollini and Levey 1997b, Martin et al. 1951).
Nevertheless, it is common for many fruits to remain on horsenettle plants
throughout the winter, indicating that seed dispersal may be suboptimal.
Horsenettle has traits that are likely to function as herbivore deterrents,
including spines, stellate trichomes, and a variety of secondary chemicals,
especially toxic alkaloids (Bassett and Munro 1986, Ilnicki et al. 1962,
Nichols et al. 1992, Thacker et al. 1990). Despite these potential defenses,
horsenettle is subject to feeding by numerous herbivores, and no part of the
plant is immune to attack.
*Department of Biology, Duke University, Durham, NC 27708. Current address -
Department of Biology, Bucknell University, Lewisburg, PA 17837;
506 Southeastern Naturalist Vol. 6, No. 3
Horsenettle is one of the most significant weeds in the eastern United
States. It is an economically important pest of grain and fruit crops
(Banks et al. 1977, Bassett and Munro 1986, Frank 1990, Hackett et al.
1987, Prostko et al. 1994) and is toxic to livestock (Bassett and Munro
1986, Gorrell et al. 1981). Horsenettle may have an indirect adverse
impact on solanaceous crop plants (e.g., eggplants, potatoes, tomatoes,
and peppers) by serving as a wild reservoir for insect pests (Aguilar and
Servín 2000, Burke 1976, Faville and Parrott 1899, Foott 1968, Ilnicki et
al. 1962, Judd et al. 1991, Mena-Covarrubias et al. 1996). Any efforts to
use biological control against horsenettle (and efforts to control pest insects
that horsenettle shares with crop plants) must be based on a sound
knowledge of the herbivore community and the type and amount of damage
In addition to its agricultural significance, horsenettle and its herbivores
are increasingly being used as study species by ecologists and evolutionary
biologists. This system has been used to address topics such as the impact of
herbivores on plant fitness (Solomon 1983; Wise and Cummins 2002, 2006;
Wise and Sacchi 1996), plant defensive chemistry (Cipollini and Levey
1997a, b; Cipollini et al. 2002a; Thacker et al. 1990), interspecific competition
among herbivores (Cipollini et al. 2002b, Wise and Weinberg 2002),
and maternal care in subsocial insects (Hardin and Tallamy 1992, Loeb
2003). Horsenettle has been the focus of studies on the evolution of sexallocation
strategies and self-incompatibility systems (Elle 1998, 1999; Elle
and Meagher 2000; Richman et al. 1995; Solomon 1985, 1986; Steven et al.
1999; Stone 2004, Uyenoyama 1997; Vallejo-Marín and Rausher, in press).
As with applied research, the conclusions of ecological and evolutionary
studies will be stronger if interpreted within the context of the whole community
of horsenettle herbivores.
Despite the applied and basic interest in horsenettle and its herbivores, a
comprehensive overview of the plant’s native herbivore community is not
available. To fill this need, I report here on observations from 7 years of
research in northern Virginia. This paper has 3 specific objectives: (1)
provide a comprehensive list of horsenettle herbivores within the plant’s
native range; (2) report on a study of the magnitude of damage imposed by
herbivores; and (3) integrate new natural history information with older
literature on the herbivores.
From 1996–2002, I performed a series of observational and experimental
studies of horsenettle and its community of herbivores in old
fields in and around Blandy Experimental Farm (Shenandoah Valley at
39ºN, 78ºW, Clarke County, VA). Blandy Farm receives an average of 94
cm of rain per year.
2007 M.J. Wise 507
Field experiment setup
In the spring of 1997, I collected root material from 30 horsenettle plants
(i.e., genets) growing at least 10 m apart along transects in each of 4
populations within a 13-km radius of Blandy Farm. Plants were propagated
in pots in commercial growing media (Wesco Growing Media III, Wetsel
Seed Company, Harrisonburg, VA) each year through 2002. Propagation
procedures may be found in Wise (2003). In early May of 2001, I cut roots
from plants of 40 different genets (10 from each of the 4 source populations)
into 2-cm3 segments. I planted at least 38 segments from each of the 40
genets individually in growing media in 3.8-liter (1-gallon) pots.
Between 28 June and 2 July of 2001, I chose 24 ramets (individual stems)
from each of these 40 genets and transplanted them into an old agricultural
field at Blandy Farm that already had an extensive population of horsenettle.
These 960 ramets were planted in a randomized block design with 3 spatial
blocks, each consisting of 10 rows separated by 2 m. Each row contained 32
ramets spaced 1.5 m apart, and the ramets were flagged and labelled with
plastic tags in the soil. The growing media from the pot of each ramet was
covered with a layer of field soil and dead grass from the field so that the
transplanted plants would blend with native horsenettles. The density of
horsenettle in the field before transplantation was approximately 12 ramets
per m2; thus, the ratio of native to transplanted ramets within experimental
blocks was about 30:1.
Field experiment data collection
Data collection on floral herbivory began immediately after transplanting
and continued until flowering ceased in late September. Each
plant was checked every 3–5 days, and the fate of each flower was recorded
as either aborted in the bud stage unrelated to herbivory, killed by
one of several herbivore species, or successfully opened. Fruit abortion,
maturation, and herbivory were also noted throughout the growing season.
Each raceme was collected once its fruits started ripening (before
they could be dispersed); fruits were kept in plastic bowls in a growth
chamber to rear out insects. After the emergence period, I dissected each
fruit to search for additional evidence of feeding. A series of hard freezes
from 8–10 October killed all stems and stopped development of fruits. I
was unable to identify any insects that may have been in these unripe
fruits at the time of this frost.
Data on leaf herbivory were taken in 2 cycles, corresponding to the peaks
in damage by folivores. In August, I measured damage by Epitrix fuscula
(eggplant flea beetles), Leptinotarsa juncta (false potato beetles), and
Tildenia inconspicuella (eggplant leafminers) (see Appendix 1 for insect
nomenclature). In early September, I measured damage by Gratiana
pallidula (eggplant tortoise beetles), Gargaphia solani (eggplant lace bugs),
and Prodiplosis longifila (citrus gall midges). The presence of other species
of herbivores was noted each time the plants were checked. Here, I report
508 Southeastern Naturalist Vol. 6, No. 3
only the proportion of the 960 plants that displayed damage by folivores.
Details of the methods used to quantify the damage more specifically for
each species may be found in Wise (2003).
I harvested stems down to the roots once plants had senesced, beginning
on 30 September and ending after the killing October frosts. I dissected each
stem to collect insects and note signs of stem boring.
Results and Discussion
Overview of herbivore damage
From 1996–2002, I documented 31 species of insect herbivores that
regularly feed on horsenettle, including members of 6 taxonomic orders
(Table 1, Appendix 1). The only non-insect herbivore was Microtus
pennsylvanicus (Ord) (Cricetidae: Arvicolinae) (meadow vole). A variety of
feeding modes were utilized by these insects, including chewing, piercingand-
sucking, leaf mining, galling, and stem boring. The herbivores ranged
from horsenettle specialists, such as the moth Frumenta nundinella and the
Table 1. Geographic ranges of 11 major horsenettle herbivores in the US. Specific records are
noted with state abbreviations.
Herbivore species Geographic range in the US References
Gargaphia solani AL AZ AR CT DC GA IA IL IN Henry and Froeschner (1988)
KS MA MD MS MO NC NJ OH
OK PA SC TN TX VA
Leptinotarsa juncta Southern US; AR Fl IN LA NC Downie and Arnett (1996);
OH PA VA Jaques (1951)
Epitrix fuscula Eastern half of US; AL Fl IN NY Downie and Arnett (1996);
OH OK Jaques (1951)
Gratiana pallidula AZ CA Fl GA MD NY PA VA Downie and Arnett (1996)
Anthonomus nigrinus AR CT DC GA IL IN KS KY LA Downie and Arnett (1996)
MI MO MS NC NJ NY OK SC TN
TX VA WV
Trichobaris trinotata CO CT DC Fl IA IL IN KS MD Downie and Arnett (1996)
MI MO MS NC NJ NY PA SC TX
Prodiplosis longifila Fl Peña et al. (1989)
Zonosemata electa AL CT DC Fl GA IA IL IN KS Foote et al. (1993)
MD MO MS NJ NY SC TN TX
Tildenia inconspicuella AK AL DE Fl GA IA IL IN KS Gross (1986)
KY LA MD MO NC NJ OK PA
SC TX VA WV
Frumenta nundinella Eastern US; AK GA IL IN MO Bailey and Kok (1982)
PA TX VA
Microtus pennsylvanicus East of Rocky Mountains, south Maser and Storm (1970
to NM and GA
2007 M.J. Wise 509
false potato beetle, to extreme generalists, such as the meadow spittlebug
and the salt marsh caterpillar.
In the field experiment, every type of plant organ was subject to attack by
at least 1 and usually several different species. Leaves were attacked by the
greatest diversity of herbivores, with 6 different species of folivores, each
damaging at least 6% of the plants (Fig. 1A). Evidence of stem boring was
found in 73% of the plants. Slightly fewer than half of the flowers and only
about a quarter of all fruits (excluding those that aborted undamaged or were
killed by frost) escaped or survived herbivory (Fig. 1B, C). About 10% of
the fruits were destroyed by frost before maturing. Any insects that might
Figure 1. Damage levels of major herbivores
in an experimental population of
960 horsenettle ramets. A. Leaf herbivory:
% of ramets displaying leaf
damage. B. Floral herbivory. The “other
herbivores” include eggplant flea
beetles, eggplant tortoise beetles, tobacco
hornworms, and an unidentified
microlepidopteran. The fates of 2% of
flowers were unknown. C. Fruit herbivory.
The “other herbivores” include
potato bud weevils and tobacco hornworms.
The “aborted” fruits only include
those that did not receive any
damage. The “frost damaged” fruits may
have been infested by pepper maggots,
but maggots were destroyed by hard
freezes along with unripe fruits.
510 Southeastern Naturalist Vol. 6, No. 3
have been in these fruits were also destroyed, along with evidence of their
feeding. Almost 15% of the fruits aborted before the frosts without showing
any evidence of herbivore damage.
Eggplant flea beetle. This small, black beetle was the most ubiquitous
herbivore species in this study. Adults feed by chewing trichomes off a
small area of a leaf, consuming leaf tissue that was under the trichomes,
and then moving to a new area of the leaf or a different leaf. Their
feeding results in a distinctive shotgun pattern of small holes in a leaf’s
surface. This leaf damage may be severe, and no horsenettle plant (and
virtually no leaf) in the field experiment escaped flea beetles entirely.
Adults may be found from the time of horsenettle emergence in the spring
until the first killing frost in the fall. Larvae feed on horsenettle roots and
cause relatively mild damage compared to adults. While I occasionally
encountered other species of Epitrix on horsenettle, eggplant flea beetles
were by far the most abundant.
False potato beetle. This congener of the infamous L. decemlineata
(Colorado potato beetle) has also been referred to as the horsenettle beetle or
the false Colorado potato beetle (Jacques 1988). The false potato beetle,
which is a specialist on horsenettle, is found throughout the southeastern US
(Hsiao 1986, Jacques 1988). Adult beetles emerge in late spring, coinciding
with the emergence of horsenettle shoots. Bright orange eggs are laid in
small clusters on the underside of horsenettle leaves or on nearby plants.
Though they appear to be bivoltine in northern Virginia (Wise and Sacchi
1996), larvae may be found continuously until nearly the end of
horsenettle’s growing season. Both adults and larvae feed mainly on leaves,
chewing from the edges toward the midveins. Larvae often feed in groups,
which may help them to overcome the physical toughness of the leaves
(Hsiao 1986). These beetles may also cause considerable damage to
horsenettle flowers and developing fruits.
Despite reports of feeding by the similar looking Colorado potato beetle on
horsenettle in other geographic regions (Mena-Covarrubias et al. 1996), I
rarely encountered Colorado potato beetle on horsenettle through the duration
of this study. It has been reported that the extension of Colorado potato beetle's
geographic range has caused a drastic decline in populations of false potato
beetle, either because the former depleted its food source or interbreeding
genetically swamped false potato beetles to near extinction (Jacques 1988).
Experimental evidence argues against these reports (Neck 1983). For instance,
Hare and Kennedy (1986) found that survival of Colorado potato beetle larvae
from Virginia potato fields was extremely low on horsenettle, suggesting that
the food sources of the 2 species overlap very little. In addition, Boiteau (1998)
found that behavioral and gametic barriers prevent the 2 species from interbreeding.
These findings, combined with the relative abundance of the beetles
in this study, argue that the false potato beetle is not suffering from invasions by
its infamous congener, at least in northern Virginia.
2007 M.J. Wise 511
Eggplant tortoise beetle. This species was the third most common chrysomelid
beetle on horsenettle, with 42% of the ramets in the field experiment
exhibiting tortoise beetle damage. Both larvae and adults feed almost exclusively
on leaves, concentrating on the interior of the leaf surface, away from
the leaf edges and between major veins. Their damage is easily distinguished
as uniform oval-shaped holes, up to about 1 cm in length, depending on the
size of the beetles. They were occasionally observed feeding on flower petals.
Margined blister beetle. Adults of this beetle appear in horsenettle populations
in mid-to-late summer. They tend to congregate in small groups, so
their feeding damage is often localized. While they may damage some
flowers, their feeding is concentrated on the lower, older leaves of
horsenettle. These blister beetles feed from the edges of leaves like potato
beetles, but they tend to avoid major veins, with the result that their leaf
damage often appears in characteristic triangular segments. Because adults
concentrate their feeding on older leaves near the end of the season and
because larvae do not feed on horsenettle (they feed on grasshopper eggs),
the margined blister beetle does not appear likely to have a serious impact on
horsenettle fitness in this study area.
Eggplant lace bug. Adults and nymphs feed by sucking contents from
leaf parenchyma cells, causing yellowing and premature abscission of leaves
(Loeb 2003). Most damage is caused by nymphs, which feed in groups of up
to several hundred guarded by an adult female (Loeb et al. 2000). Eggplant
lace bugs may have as many as 8 generations per year in Virginia (Tallamy
and Denno 1982), and their densities may reach very high levels within a
horsenettle population; thus, damage may be locally quite severe. However,
these lace bugs also appear prone to population crashes. I have found that
heavily damaged horsenettle populations may remain essentially free of lace
bugs for several successive years.
Citrus gall midge. Horsenettle is a new host record for the citrus gall
midge, and before this study the species had not been reported north of
Florida (Gagné 2004, Peña et al. 1989). Although this gall midge generally
feeds on flowers (Gagné 1989), on horsenettle the larvae feed in rolled-up
leaves, usually in terminal clusters at the plant’s apex or the end of an
axillary shoot. The mature larvae drop from the plants and pupate in the soil.
The citrus gall midge may be abundant in horsenettle populations in moist or
shaded areas. When on plants in relatively open fields, the gall midges
almost always occurred on leaves that were near the ground and shaded by
other plants. Though the damage these gall midges do is small in terms of
leaf area consumed, their impact on horsenettle fitness may be disproportionately
large because their feeding often damages apical or axillary
meristems, thus stunting plant growth.
Eggplant leafminer. This gelechiid moth attacks horsenettle in multiple,
overlapping generations from early summer until the first frost of autumn.
Female moths deposit eggs singly on leaf surfaces, and larvae tunnel into the
leaves, eventually forming a blotch mine from the edge of the leaves (Gross
512 Southeastern Naturalist Vol. 6, No. 3
1986). I have found between-year variation in population sizes of the eggplant
leafminer to be substantial. At low leafminer population densities, it is
relatively uncommon for more than 1 mine to occur on a leaf. At higher
densities, multiple mines may occur on a single leaf, and several larvae may
share a communal mine. Damaged leaf areas turn brown, curl up from the
edges of the leaves, and may eventually fall off. This is the only species of
leafminer I observed on horsenettle during the study period.
Salt marsh caterpillar and tobacco hornworm. Larvae of two species
of moths, Estigmene acrea (salt marsh caterpillar) and Manduca sexta
(tobacco hornworm), were regularly, though not commonly, observed to
feed on horsenettle throughout the study period. Salt marsh caterpillars
generally feed on leaves, but they sometimes cause substantial damage to
flowers. Large hornworm caterpillars sometimes strip entire horsenettle
ramets of leaves, flowers, and developing fruits. In the transplant study, 5
of the 960 plants incurred measurable hornworm damage. Larvae and
pupae of several species of tortricid moths were occasionally found on
horsenettle, but none appeared to cause appreciable damage.
Frumenta nundinella. This gelechiid moth has an unusual natural history.
In the spring, female moths lay eggs near the apices of young
horsenettle ramets (Bailey and Kok 1982). Upon hatching, a larva glues
terminal leaves together to form a roughly spherical structure in which it
feeds on the apical meristem (Solomon 1981). These structures, which reach
a diameter of about 2 cm (Solomon 1983), are usually found at the plant
apices, but they may be found as well on axillary meristems of larger plants.
The insects pupate inside the structures, and adults emerge in early summer.
The females of this generation generally lay their eggs on or near horsenettle
flower buds (Solomon 1980). Once a larva enters a flower bud, a fruit begins
to form parthenocarpically around the larva, perhaps in response to damage
to the style, and the larva feeds on ovules of the unfertilized fruit (Solomon
1980). The infested fruits look relatively normal from the outside (though
often a bit lumpy), but they usually contain no seeds, or at most a very few
seeds. Prior to pupation, the larva chews an emergence hole in the side of its
fruit, leaving a thin, membranous window (Solomon 1981). Pupation occurs
within the fruits, and the second-generation adult moths emerge in the late
summer. In the field study, larvae inside fruits were commonly parasitized
by 2 species of wasps: Conura dema (Burks) (Chalcididae) and Bracon sp.
(Braconidae). Foott (1967) recorded parasitism by Bracon mellitor Say on F.
nundinella in fruits in Canada. Solomon and McNaughton (1979) reported
that larvae in both leaf chambers and fruits were commonly parasitized by
the wasp Scambus pterophori (Ashmead) (Ichneumonidae) in central New
York, but this species was not observed in this study.
A number of homopteran sap feeders regularly attack horsenettle, including
Aphididae (aphids), Cercopidae (spittlebugs), Cicadellidae
(leafhoppers), Flatidae and Acanaloniidae (planthoppers), Membracidae
2007 M.J. Wise 513
(treehoppers), and Aleyrodidae (whiteflies). Adult Philaenus spumarius
(meadow spittlebugs) were quite common on horsenettle throughout most of
the plant’s growing season, but feeding by nymphs was extremely rare. Over
the course of this study, I also observed numerous species of leafhoppers and
planthoppers on horsenettle, but it was not always apparent whether they
were actually feeding or just resting between stops on their host plants. The
treehopper Entylia bactriana was commonly found feeding on the midveins
of horsenettle leaves in the fall, particularly in populations near woodlots in
years with a late killing frost. In most horsenettle populations, whiteflies
also are not present until autumn. The later in autumn the first hard frost
occurs, the more abundant whiteflies become. In years with especially late
first frosts, virtually all horsenettle plants in some populations became
heavily infested by whiteflies. I have also observed whiteflies on horsenettle
in the middle of the summer, but only in isolated plant populations with very
little damage from other herbivores.
Potato bud weevil. A rather inconspicuous black beetle, Anthonomus
nigrinus (potato bud weevil),was the main floral herbivore of horsenettle.
Overwintering adults begin to emerge in the spring in general synchrony with
the sprouting of horsenettle ramets. Before flower buds are produced, the
weevils can often be found congregating on horsenettle leaves. Adults will
feed on leaves, but they seem to prefer flowers (Burke 1976). Most of the
floral damage occurs, however, when females oviposit. A female chews a hole
into a flower bud and oviposits onto an anther. She then plugs the hole with a
fecal pellet and proceeds to chew the flower’s pedicel. Such flower buds either
fall off immediately or die attached to the raceme and fall a few days later.
Larvae feed and pupate inside the unopened flower buds. Development is
rapid, sometimes lasting less than a month from egg to adult (Chittenden
1895). Usually 1, but up to 3 adults may emerge from a single bud (Chittenden
1895, Tuttle 1956). Adults may be found on horsenettle through the end of
the flowering season, and they will occasionally feed on developing fruits.
In the field experiment, potato bud weevils destroyed nearly a third of
horsenettle’s entire flower crop and almost 1% of the fruits.
The potato bud weevil is common in cultivated potatoes as well, but
because it is not considered a pest species, it has not been studied nearly as
much as many other potato-feeding insects (Burke 1976, Chittenden 1895).
It seems unlikely that destruction of flower buds will negatively affect
potato plants’ growth, survival, or tuber production. In a separate controlled
study, horsenettle plants subjected to heavy herbivory by bud weevils had
significantly greater underground growth than those protected from floral
herbivory (Wise and Cummins 2006). If the loss of flowers has the same
effect on underground growth in potatoes, potato bud weevils may actually
benefit tuber production.
A close relative of the potato bud weevil, Anthonomus eugenii Cano
(pepper weevil), feeds on Capsicum (pepper) crops in the southern US from
514 Southeastern Naturalist Vol. 6, No. 3
Florida to California as well as in Hawaii (Berdegue et al. 1994, Capinera
2005). The pepper weevil has also been reported to damage flowers and
immature fruits of horsenettle in Florida (Aguilar and Servín 2000, Capinera
2005). These weevils do not diapause and thus they need a continuous
source of food throughout the year (Capinera 2005). Because their host
plants are not available in the winter in Virginia, it is not surprising that I did
not encounter pepper weevils in my study.
Thrips. Several species of thrips were collected from flowers of horsenettle
over the course of this study, including several common crop pests: Thrips
tabaci (onion thrips), Frankliniella fusca (tobacco thrips), and F. tritici and F.
occidentalis (eastern and western flower thrips). These thrips fed on anthers,
causing brown spots and premature wilting. Their damage does not appear to be
extreme, but it is possible that they affect pollen production, survival, or
removal by pollinators.
Meadow voles. Voles were the only non-insect herbivores of horsenettle
observed in this study. Voles are generalist herbivores, but horsenettle does not
appear to be a preferred food item (Burt and Grossenheider 1980, Pascarella
and Gaines 1991). In the field experiment, voles sometimes cut down entire
horsenettle ramets if they happened to be transplanted in the voles’ runways.
Voles also killed a large number of horsenettle flowers and immature fruits.
However, this damage often seemed incidental, as the voles tended not to eat
the flowers or fruits they cut down. They were more likely to just gnaw on the
cortex of the racemes’ peduncles and fruit pedicels and leave the fruits or
flowers dangling on the plant or lying beneath the plant. In the fall and winter,
however, meadow voles do consume horsenettle fruits and may cache and
disperse their seeds (M.A. Bowers, University of Virginia, Charlottesville,
VA, pers. comm.).
Pepper maggot. Adults of the tephritid fly Zonosemata electa (pepper
maggot) appear in horsenettle populations in July when the first fruits are
maturing. Females inject eggs into the fruits, and larvae feed on the pulp.
Larvae emerge from fruits in late summer or early fall to pupate in the soil.
Many eggs may be laid per fruit, and I have found over twenty immature larvae
inside a single fruit. This oviposition behavior differs from Foott’s (1968)
observations in Ontario, in which generally only 1 egg, but occasionally up to 3
eggs, were laid per fruit. Out of the many hundreds of pepper maggots I reared
from horsenettle fruits over the course of my studies, I almost never observed
more than one larva emerge from a fruit. Therefore, larval cannibalism is likely
quite common. Successful parasitism, in contrast, was extremely rare. The only
evidence of a parasitoid was a single wasp of the genus Diachasmimorpha
(Braconidae) that emerged from a pepper maggot puparium.
Pepper maggots feed on placentas of fruits and do not appear to affect the
seeds, although infestation despoils the pulp. Infested fruits often ooze
through oviposition holes when ripening, and they turn black and harden in
the fall, while uninfested fruits are yellow and smooth. If the rotten fruits are
2007 M.J. Wise 515
less appealing to fruit-feeding mammals and birds, then pepper maggots
may have a detrimental effect on horsenettle seed dispersal. The effect of
pepper maggot infestation on fruit and seed dispersal is worth further study.
Potato stalk borer. Adult Trichobaris trinotata (potato stalk borers)
emerge from previous-year stems in the spring, and females deposit an egg in
a slit chewed in the axil of a terminal leaf of a young horsenettle ramet. The
larva hatches and spends the entire summer feeding on stem pith, eventually
pupating at the junction of the stem and root in a nest constructed of fibrous
stem shavings (Somes 1916). Adults usually eclose in less than 2 weeks but
remain in the stems throughout the winter (Cuda and Burke 1986, Faville and
Parrott 1899). None of 960 stems in the field experiment contained more than
1 stalk borer, but I have previously found 2 adults inside larger horsenettle
ramets, with 1 at the base of the stem and 1 much higher up.
Almost three-fourths of the horsenettle stems in the field experiment
exhibited signs of damage by the potato stalk borer. However, only 57% of
the stems contained live borers upon dissection. Some of the larvae died
of unidentified causes, but many were parasitized by Neocatolaccus
tylodermae (Ashmead) (Hymenoptera: Pteromalidae) and a species of
Heterospilus (Hymenoptera: Braconidae).
Although the potato stalk borer is one of the most destructive pests of
cultivated potatoes (Cuda and Burke 1986, Faville and Parrott 1899),
horsenettle appears to tolerate its presence very well. In a controlled study,
infestation by potato stalk borers had no effect on any horsenettle growth or
reproductive measures (M.J. Wise, unpubl. data). In the field study reported
here, infested plants produced more seeds on average than those without
stalk borers (Wise 2003). This apparent fitness benefit may have been an
artifact of the weevils preferentially ovipositing in larger, more vigorous
Gall midge. Stem galls of cecidomyiid flies were regularly found in
some populations over the course of this study, but they were never
common, and single ramets rarely contained more than one or two galls.
These galls consist of small swellings on the main stem, lateral stems, or
on racemes between fruits. Only 4 of the 960 plants in the transplant
experiment were galled. All of the individuals I have reared from these
galls were Lasioptera solani, though a species of Neolasioptera is reported
to be more common in horsenettle (Gagné 1989). Rearing of these
gall midges proved difficult because the vast majority of galls were parasitized
by proctotrupoid wasps. The small size of the galls and low densities
of these gall midges suggest that they are likely to have relatively
little impact on horsenettle.
Riley’s clearwing. Besides larvae of the eggplant flea beetle, the only other
common root-feeding herbivores were larvae of the moth Synanthedon
516 Southeastern Naturalist Vol. 6, No. 3
rileyana (Riley’s clearwing). The adults, which mimic yellow-jackets, are
found on the wing in late summer in northern Virginia. It has been reported that
females oviposit on horsenettle stems and leaves, and larvae bore through the
stem and downward into the root, eventually pupating in the soil (Somes 1916,
Williams et al. 1999). This information is somewhat suspect, however, as the
author who reported that adults emerge in August and September also reported
that third instar larvae were found in stems in May (Somes 1916). Because
horsenettle stems do not survive the winter, it is not clear how the larvae could
be present in living stems earlier in the season than the emergence of adults.
In my studies, I have found no evidence of insects other than potato stalk
borers and their parasitoids in the stems of horsenettle. All of the Riley’s
clearwing larvae I have seen were feeding on taproots or thick lateral roots.
They appear to bore into the root from the outside and feed close to the
cortex, rather than tunneling into roots down through the interior of the stem.
Thus, it is likely that females oviposit at the base of horsenettle stems rather
than on leaves. I have found clearwing larvae at different stages of development
feeding on roots both in the fall and early spring, indicating that the
overwintering stage is the larva, and that larval development may not conclude
Over seven years of observations in northern Virginia, I recorded 32
species of herbivores that regularly fed upon the leaves, flowers, fruits,
stems, or roots of horsenettle. Of these, 11 species consistently caused
considerable damage in at least some horsenettle populations. The false
potato beetle was the most conspicuous herbivore, damaging large numbers
of leaves, flowers, and fruits. The other main folivores were the eggplant
flea beetle, eggplant leafminer, eggplant tortoise beetle, eggplant lace bug,
and citrus gall midge, in general order of abundance. The potato bud weevil,
the pepper maggot, and the potato stalk borer were consistently the most
damaging herbivores of flowers, fruits, and stems, respectively. The moth
Frumenta nundinella damaged leaves, meristems, and flowers. The only
non-insect herbivore, the meadow vole, destroyed large numbers of flowers
and fruits in some populations.
The University of Virginia’s Blandy Experimental Farm provided logistical and
financial support throughout the 7 years of this research via graduate research
fellowships, field station grants from the National Science Foundation (NSF-BIR-
9512202) to M.A. Bowers and E.F. Connor, and an REU site grant
(NSF-DBI-9912164) to M.A. Bowers and D.E. Carr. This work was also supported
by an NSF Dissertation Improvement Grant (NSF-DEB-00-73176) to M.D. Rausher
and M.J. Wise, a US EPA STAR Fellowship (U-915654-01-0) to M.J. Wise, and NSF
grant (DEB-0515483) to W.G. Abrahamson and M.J. Wise. Any opinions, findings,
and conclusions expressed in this material are those of the author and do not
2007 M.J. Wise 517
necessarily reflect the views of the US Environmental Protection Agency or the
National Science Foundation.
I thank J.A. Leachman and J. Byrd for providing invaluable assistance in the field
study of 2001. Thanks also to W.G. Abrahamson, C.P. Blair, N. Dorchin, P.J. March,
J.E. Rawlins, and several anonymous reviewers for helpful comments on the manuscript
and to the Bucknell University Biology Department for financial support while
preparing the manuscript. J.E. Rawlins kindly provided geographic distribution data
for the herbivores. Several taxonomists from the Systematic Entomology Laboratory
(Agricultural Research Service, US Department of Agriculture) provided identifications
of some of the herbivores and parasitoids: S.H. McKamey (Homoptera), S.
Nakahara (Thysanoptera), A.S. Konstantinov (Epitrix), R.J. Gagné (Cecidomyiidae),
D. Adamski (Frumenta), J.W. Brown (Tortricidae), and M.W. Gates (Hymenoptera).
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Appendix 1.Taxonomy, feeding location, and host range of insect herbivores of horsenettle observed in northern Virginia from 1996–2002.
The common names are from the Entomological Society of America (2007). For species without common names, the common name of the
family or order is shown in parentheses. The plant organs are: Lf = leaf, St = stem, Fl = flower, Fr = fruit, and Rt = root. The host ranges are:
monophagous = specialist on horsenettle; oligophagous = restricted largely to species in family Solanaceae; polyphagous = feeds on a large
number of species, including non-solanaceous plants; and ? = unknown.
Order, Family Species Common name fed upon Host range
Tingidae Gargaphia solani Heidemann Eggplant lace bug Lf Oligophagous
Membracidae Entylia bactriana Germar (Treehopper) Lf Polyphagous
Cercopidae Philaenus spumarius (Linnaeus) Meadow spittlebug Lf, St Polyphagous
Cicadellidae Draeculacephala antica (Walker) (Leafhopper) Lf Polyphagous
Flatidae Ormenis sp. (Planthopper) Lf ?
Acanaloniidae Acanalonia bivittata (Say) (Planthopper) Lf Polyphagous
Aleyrodidae Trialeurodes abutilonea (Haldeman) Bandedwinged whitefly Lf Polyphagous
T. vaporariorum (Westwood) Greenhouse whitefly Lf Polyphagous
Aphidae Unidentified (Aphid) Lf, St ?
Thripidae Frankliniella fusca (Hinds) Tobacco thrips Fl Polyphagous
F. occidentalis (Pergande) Western flower thrips Fl Polyphagous
F. tritici (Fitch) Flower thrips Fl Polyphagous
Thrips tabaci Lindeman Onion thrips Fl Polyphagous
522 Southeastern Naturalist Vol. 6, No. 3
Order, Family Species Common name fed upon Host range
Meloidae Epicauta pestifera Werner Margined blister beetle Lf, Fl Polyphagous
Chrysomelidae Leptinotarsa juncta (Germar) False potato beetle Lf, Fl, Fr Monophagous
L. decemlineata (Say) Colorado potato beetle Lf, Fl, Fr Oligophagous
Epitrix fuscula Crotch Eggplant flea beetle Rt, Lf Oligophagous
Gratiana pallidula (Boheman) Eggplant tortoise beetle Lf Oligophagous
Curculionidae Anthonomus nigrinus Boheman Potato bud weevil Fl Oligophagous
Trichobaris trinotata (Say) Potato stalk borer St Oligophagous
Cecidomyiidae Prodiplosis longifila Gagné Citrus gall midge Lf Polyphagous
Lasioptera solani Felt (Gall midge) St Monophagous
Tephritidae Zonosemata electa (Say) Pepper maggot Fr Oligophagous
Gelechiidae Tildenia inconspicuella (Murtfeldt) Eggplant leafminer Lf Oligophagous
Frumenta nundinella (Zeller) (Moth) St, Fl, Fr Monophagous
Sesiidae Synanthedon rileyana (H. Edwards) Riley’s clearwing moth Rt Oligophagous
Tortricidae Argyrotaenia velutinana (Walker) Redbanded leafroller Lf Polyphagous
Platynota flavedana (Clemens) (Moth) Lf Polyphagous
Sparganothis sulfureana (Clemens) Sparganothis fruitworm Lf Polyphagous
Sphingidae Manduca sexta (Linnaeus) Tobacco hornworm Lf, Fl, Fr Oligophagous
Arctiidae Estigmene acrea (Drury) Salt marsh caterpillar Lf, Fl Polyphagous