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Sex-specific Attraction of Moth Species to Ultraviolet Light Traps
Heath W. Garris and John A. Snyder

Southeastern Naturalist, Volume 9, Issue 3 (2010): 427–434

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2010 SOUTHEASTERN NATURALIST 9(3):427–434 Sex-specific Attraction of Moth Species to Ultraviolet Light Traps Heath W. Garris1,* and John A. Snyder2 Abstract - Phototactic behavior toward ultraviolet light varies among nocturnal flying insects. By recording sex ratios of 28 southeastern US moth species, we tested the commonly held belief that UV light-trap collections of moths are significantly skewed toward males. Twelve species demonstrated a statistically significant male preponderance, but a wide range of sex ratios was found. Two of the 28 species demonstrated both significant male and female bias during different observation periods, illustrating the need to collect over the entire flight period. Since the sex ratio of collected organisms varies by species and by time, this must be taken into consideration when using light-trap collection to make population estimates and to gather information for conservation or control of any particular species. Introduction Adults of many moth species demonstrate phototaxis toward ultraviolet (UV) light in a range of wavelengths. Attraction to artificial light sources has become progressively more important for insect populations as urban lighting has increased (Frank 1988). This taxis has long been exploited by entomologists through the use of traps that include emitters of UV light. Light traps are used for a variety of purposes, such as gaining information on diversity (Thomas and Thomas 1994), geographic ranges, and migration patterns (Gregg et al. 1994), estimating the density of populations (Thomas 1989), and controlling populations of agricultural pests in fields (with variable success; Cantelo et al. 1972, Frank 1988). It is important to know whether the male-female ratio of moths captured in light traps is representative of the actual ratio in the population. Limited observations in other studies (e.g., Levine 1989, Steinbauer 2003, Worth and Muller 1979, Yathom 1981) and anecdotal records have indicated that males dominate light-trap catches. A number of explanations are possible. For instance, the sexes might perceive and respond to UV light in different ways, one sex might have a more limited flight range, the sex ratio might be skewed from 1:1 at eclosion of adults, or some combination of these and other factors might be occurring. In this study, we document the adult malefemale ratio for 28 nocturnal moth species attracted to UV light in Greenville County, SC and changes in that ratio during the observation period. Field-site Description Over 90% of specimens were collected from a single location (34°55'20"N, 82°22'38"W, elevation = 342 m), 300 m south of the boundary 1Department of Integrated Bioscience, University of Akron, Akron, OH 44325. 2Department of Biology, Furman University, Greenville, SC 29613. *Corresponding author - hwg3@uakron.edu. 428 Southeastern Naturalist Vol. 9, No. 3 of Paris Mountain State Park, Greenville County, SC. Situated in eastern temperate forest, the habitat is comprised of anthropogenically maintained open grass immediate to the trap (within 20 m, all of which was shorter than the trap collection bucket), surrounded by mixed forest typical for this region—hardwoods, e.g., Quercus spp. (oaks), Liriodendron tulipifera L. (American Tulip Tree), and Liquidambar styraciflua L. (Sweetgum), and softwoods, e.g., Pinus strobus L. (Eastern White Pine) and Pinus virginiana L. (Virginia Pine). This area is progressively suburban-urban to the south and west toward the city of Greenville and is abruptly less so to the immediate north, constituting the State Park. An area encompassing 0.40 km2 with the primary collection site at the center yielded 74% forest cover, 24% cover devoted to anthropogenic activities (right of ways, roads, dwellings, and disturbed or early successional scrub-brush habitats), and 2% visible lake surface determined using ArcGIS® (ESRI of Redlands, CA) from Landsat 7 ortho-imagery, May 2005 (USGS 2005). The second sample location (34°54'35.74"N, 82°24'41.77"W, elevation = 311m; 3.3 km southwest of the primary sampling site) was situated on open grass overlooking a small pond, with broad-scale forest cover, anthropogenic cover, and lake surface components similar in composition to the primary site. Methods Moths were collected with ultraviolet lights at two locations within Greenville County, SC, between 13 June and 19 August 2005, and between 21 May and 20 July 2006. The light trap consisted of a UV light source (PestWest Quantum BL UV bulb with output peak at 365 nm) vertically attached at the center of 4 metal vanes positioned over a collecting bucket charged with ethyl acetate as a killing agent. In each case, the bulb was illuminated beginning at dusk for approximately 12 h on most dates throughout the observation period. Each specimen was identified, and its sex was determined either by gross exterior anatomy or by dissection. A representative of each species was prepared as a voucher specimen and deposited in the Furman University Zoological Collection. Sex differences were examined for species with aggregates of 10 or more individuals over the period of observation. The sex ratio of each species, summed across sites and its observed flight period, was evaluated using a Yates-corrected chi-squared test (Hassard 1991). A sequential Bonferroni correction for multiple comparisons (Rice 1989) was applied. We also tested the hypothesis that sex ratio changes progressively throughout the majority of the flight period, by correlating percent males with the date of capture (Spearman rank correlation). Any correlation from these evaluations might describe, to some extent, patterns that were hidden when data were summed from the entire observation period for the chi-squared analysis. The species chosen were the subset of those used for analyzing gross sex ratios which had sample sizes that were sufficiently large to provide some confidence in correlative statistical analysis and for which the sample sizes of individual sexes were termed sufficiently balanced to impart analytical relevance. 2010 H.W. Garris and J.A. Snyder 429 Results A total of 1101 individuals representing 56 species were examined. An aggregate of 843 males and 258 females was found. Of the 28 species for which at least 10 individuals had been collected, 12 demonstrated a signifi- cant sex bias toward males, 1 species demonstrated a significant sex-bias toward females, and the sex ratio of 15 species did not differ significantly from 1:1 following a sequential Bonferroni correction (Table 1). Figure 1 shows the distribution of male percentages among the 28 species. When we analyzed possible sex differences in observed flight periods by comparing the percentage of males captured over the observation period, there were no significant changes in male bias for Atteva punctella, Desmia funeralis, Halysidota sp., Polygrammate hebraeicum, and Spodoptera ornithogalli. One species, Thioptera nigrofimbria (Fig. 2) showed a significant trend toward females caught at light traps as flight days progressed (P = 0.0005, rs = -0.7049), although the total catch showed no significant bias toward either sex. Despite exhibiting a significant overall male bias in the chi-squared analysis (P = 1.31 x 10-4), Tetanolita mynesalis (Fig. 3) also Table 1. Yates corrected chi-squared analyses performed for 28 species collected at UV light. Numbers represented in the ID column reflect species number designations in Figure 1. * denotes statistical significance retained at the 2-tailed P = 0.05 level after sequential Bonferroni correction was applied (Rice 1989). ID Family Species ♂ ♀ χ2 value P-value 21 Acrolophidae Acrolophus sp. 18 1 13.47 2.42 x 10-4 * 8 Yponomeutidae Atteva punctella (Cramer) 30 21 1.26 0.2620 22 Tortricidae Pandemis limitata (Robinson) 42 2 34.57 4.11 x 10-9* 7 Limacodidae Prolimacodes badia (Hübner) 7 6 0.00 1.0000 12 Crambidae Desmia funeralis (Hübner) 12 5 2.12 0.1450 15 Pyralidae Dolichomia olinalis (Guenée) 10 3 2.77 0.0960 14 Geometridae Epimecis hortaria (Fabricius) 13 4 3.76 0.0520 1 Hypagyrtis unipunctata (Haworth) 0 18 18.00 2.21 x 10-5* 26 Euchlaena amoenaria (Guenée) 10 0 8.10 4.43 x 10-3 19 Lasiocampidae Malacosoma americanum (Fabricius) 26 4 14.70 1.26 x 10-4* 27 Saturniidae Dryocampa rubicunda (Fabricius) 13 0 13.00 3.11 x 10-4* 17 Anisota stigma (Fabricius) 23 5 13.32 2.63 x 10-4* 23 Notodontidae Datana perspicua Grote & Robinson 42 2 34.57 4.11 x 10-9* 20 Nadata gibbosa (J.E. Smith) 103 7 83.78 5.53 x 10-20* 16 Arctiidae Hypoprepia fucosa Hübner 10 3 2.77 0.0960 10 Pyrrharctia isabella (J.E. Smith) 7 4 0.35 0.5540 25 Spilosoma latipennis Stretch 65 2 57.37 3.61 x 10-14* 24 Apantesis vittata (Fabricius) 150 6 131.08 2.38 x 10-30* 28 Grammia parthenice (W. Kirby) 23 0 21.04 4.50 x 10-6* 5 Halysidota sp. 17 20 0.11 0.7400 11 Noctuidae Tetanolita mynesalis (Walker) 72 32 14.63 1.31 x 10-4* 18 Hypena scabra (Fabricius) 24 5 11.17 8.31 x 10-4* 9 Thioptera nigrofimbria (Guenée) 28 19 1.36 0.2440 6 Acronicta haesitata (Grote) 5 5 0.10 0.7520 3 Polygrammate hebraeicum Hübner 7 14 1.71 0.1910 4 Spodoptera ornithogalli (Guenée) 10 18 1.75 0.1860 2 Agrotis ipsilon (Hufnagel) 4 15 5.26 0.0218 13 Xestia dolosa Franclemont 14 5 3.37 0.0664 430 Southeastern Naturalist Vol. 9, No. 3 demonstrated a significant increase (P = 0.0174, rs = -0.5680) in females captured as the study period progressed. Spodoptera ornithogalli showed Figure 1. Distribution of evaluated species (labeled 1–28) in ascending order from 0% males to 100% males captured at a UV light source. Species 1, Hypagyrtis unipunctata, reflects 0% males as all of the individuals captured were female. Figure 2. The percentage of captured males over time in days beginning 13 June 2005 and ending 19 August 2005 for Thioptera nigrofimbria (Noctuidae). Analysis of Spearman’s rank correlation coefficient revealed a significant trend toward females caught at light traps as flight days progressed (P = 0.0005, rs = -0.7049). 2010 H.W. Garris and J.A. Snyder 431 Figure 3. The percentage of captured males over time in days beginning 13 June 2005 and ending 19 August 2005 for Tetanolita mynesalis (Noctuidae). Analysis of Spearman’s rank correlation coefficient revealed a significant trend toward females caught at light traps as flight days progressed (P = 0.0174, rs = -0.5680). a similar yet not statistically significant change (P = 0.0580, rs = -0.51763) toward females. None of the remaining species analyzed yielded significant correlations signifying a change in sample sex-bias over time. Discussion Half of the evaluated species exhibited more males than females at a UV light trap. This result matches the limited published data for other species, such as Surattha indentella (Kearfott) (Sorensen and Thompson 1984), Agapeta zoegana (L.) (Story et al. 2001), Hydraecia immanis (Guenée) (Levine 1989), four saturniid and one sphingid species (Worth and Muller 1979), and Earias insulana (Boisduval) (Yathom 1981), all of which showed significant bias toward males at UV light traps. However, our data for some species were quite different: species demonstrated sex-bias to varying degrees along a spectrum from female-predominant to male-predominant (Fig. 1). It is notable that bias in a particular direction or to a particular degree was not family specific (Table 1); however, studies should be performed to compare a broader range of species, especially species within a single genus, to determine if degree of UV attraction is conserved in related taxa. A published study with some parallel to ours is that of Persson (1976). He reported that 48.0% of all collected noctuids were female. However, he found that 13 noctuid species (of more than 300 surveyed) showed significant deviation from a 1:1 sex ratio, ranging from 68% males to 78% females. The percentage of female noctuids in our study was lower at 40.1%, 432 Southeastern Naturalist Vol. 9, No. 3 but comparable to Persson’s results, we found a wide range of percentages among individual species. Although no noctuid species in our study showed significant bias toward females, females were predominant in 2 of the 8 species. Some important differences do exist between our studies: Persson’s work was carried out in a humid subtropical environment, it used a mercury bulb (with different UV and visible emission maxima than our bulb), and its reported species did not overlap ours. However, our studies are in agreement in that both report a wide range of sex ratios among captured species. A number of factors might contribute to the observed sex-ratio variation among our surveyed species. For some species, UV-trap collection may be a consequence of the sex ratio of adult moths as they emerge from the pupa stage, rather than a sex-specific attraction to the light. At fertilization, the initial sex ratio of most moth species should be 1:1 since one sex is heterogametic (De Prins and Saitoh 1999). That initial ratio could be offset by variability in survival throughout embryonic and larval development as a result of genetic and environmental factors. Studies of populations where adult female moths are significantly more numerous than males have variously attributed this to meiotic drive (Seiler 1920), to parthenogenesis (Lokki et al. 1975), and to bacterial infections selectively killing male larvae (Hurst 1993, Hurst and Majerus 1993). It would be instructive to check the sex ratio of a population of newly emerged adults in a species where our study has found a significant skewing toward one sex at UV traps. Even with a 1:1 proportion of the sexes upon emergence as adults, a factor in producing an offset toward males taken in light traps could be the relative flight activity of the two sexes. If males tend to spend a greater portion of the night-time hours flying to forage for food or searching for females, this behavior favors their perceiving and being attracted to a stationary UV source. The concept of a diminished nightly flight time of females is consistent with their having sex-specific energetic expense (egg production, increased body mass when bearing eggs, and ovipositing activity). A parallel phenomenon is seen in certain butterfly species where a 1:1 sex ratio occurs in laboratory-reared populations, but a significant skewing toward males is found in field-caught populations (Brussard and Ehrlich 1970). Those workers concluded that greater flight activity (and thus visibility to collectors) by males is the most likely cause of their being captured more frequently. If light trapping occurs over less than the full flight period for a species, an apparent skewed sex ratio may result from differential emergence times from the pupal stage. For example, females of Panolis flammea typically eclose before males (Leather and Barbour 1983), resulting in a population-level bias toward females during the first part of the species’ flight period. As an exploration of this phenomenon, we analyzed the temporal distribution of some species whose sex-ratio bias might be masked when combining data from the entire flight period. It will be recalled that a correlation analysis revealed significant change from predominantly capturing males to predominantly capturing females of Thioptera nigrofimbria (Fig. 2) and Tetanolita mynesalis (Fig. 3), which is consistent with a hypothesis that the sex ratio has changed over the two-month survey period. The resulting change in light-trap capture 2010 H.W. Garris and J.A. Snyder 433 counts could be further accentuated if unmated females increased their nightly flight durations and/or ranges toward the end of the male flight period. If a light-trap survey is limited to the last portion of a species’ flight period, another possible factor coming into play is that one sex has a longer average lifetime in the adult stage. In such a case, a skewed sex ratio would be observed even if the adult population began with equal numbers. Once again, the importance of surveying over the entire flight period is evident. For several moth species, Persson (1976) found that females are most abundant at a light trap during the first half of the night. This should have no bearing on our study, since we collected specimens only after the entire night had elapsed with the trap continuously in operation. It could be posited that gravid females tend to fly closer to the ground because of the greater mass imparted by their mature eggs, and therefore would be less well represented in trap catches if the UV light source is considerably above ground level. Placing multiple traps at significantly varying heights above the substrate during identical sampling periods would test for this. Finally, differences in visual perception and response may lead to one sex being disproportionately represented in UV light-trap catches throughout the flight period. This differential capture rate may occur by attracting a particular sex from a greater distance or by eliciting a stronger phototactic response at any distance. This potential factor could be determined in species where appropriate numbers of newly eclosed adults can be obtained and tested in a controlled setting. This study serves as a base set of observations for determining the extent of male versus female sex bias in their attraction to light in the UV range. Clearly, there are not always significantly more males represented in catches at a UV light trap. Additional studies should be conducted to evaluate the relative influence of female versus male flight periods, relative levels of activity, and surviving emergent adult sex ratios of a variety of species to determine the effect of each in producing a sex ratio at UV light traps. Understanding relative sex ratios for species caught at UV light traps may serve to improve populationestimation techniques of moths and other nocturnal insects as well as provide information for the conservation or control of individual species. 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