Diptera: Anthomyiidae)

Journal of Insect Behavior, Vol. 11, No. 3, 1998 Choices and Consequences of Oviposition by Hylemya (Delia) Sp. (Diptera: Anthomyiidae) Michael Zimmerman1,3 and Alison K. Brody2-4 Accepted November 3, 1997; revised November 18, 1997 Hylema sp.5 females oviposit on the undersides of sepals of developing buds of both Ipomopsis aggregata and Polemonium foliosissimum. Eggs deposited on the latter are significantly more likely to be fully protected by the sepal than are eggs deposited on the former. Unexposed eggs have a significantly greater likelihood of successfully developing to the larval stage than do exposed eggs. The difference in frequency of egg exposure on the two plant species can be attributed to differences in sepal morphology: I. aggregata sepals are significantly narrower than those of P. foliosissimum. The hypothesis that females preferentially oviposit on larger flowers was unconfirmed by a manipulative choice experiment. Plants differing in the size of their flowers were potted together and presented to Hylemya in arrays in the field. Flowers of the larger- flowered pair were no more likely to be oviposited on than flowers of the smaller- flowered pair. However, there were significant negative correlations between the corolla length and the percentage of flowers laid on per day at each of two sites. There was also a significant positive correlation between the corolla width and the percentage of flowers laid on at one site. Thus females appear to be using some measure of flower morphology, or a correlated trait, in making oviposition decisions. The degree to which Hylemya is making suboptimal choices between host plant species is discussed and requires further examina- tion. KEY WORDS: oviposition behavior; host selection; offspring performance; predispersal seed pre- dation; Anthomyiidae; Hylemya; Delia. 1Office of the Dean, College of Letters and Science, University of Wisconsin Oshkosh, Oshkosh, Wisconsin 54901. 2Biology Department, University of Vermont, Burlington, Vermont 05405. 3The Rocky Mountain Biological Laboratory, P.O. Box 519, Crested Butte, Colorado 81224. 4To whom correspondence should be addressed at Biology Department, University of Vermont, Burlington, Vermont 05405. e-mail:[email protected]. 5Hylemya has recently been classified as belonging to the genus Delia, however, to avoid confusion with previously published work, we use the former classification. 371 0892-7553/98/0500-0371$15.00/0 © 1998 Plenum Publishing Corporation 372 Zimmerman and Brody INTRODUCTION A lack of correspondence between oviposition choices and offspring performance has frequently been observed for herbivorous insects (e.g., Karban and Courtney, 1987; Courtney and Kibota, 1990; Valladares and Lawton, 1991; Burstein and Wool, 1993; Larsson et al., 1995). However, the underlying cause of such a surprising pattern is often unknown. To understand fully why that correspondence is lacking, it is necessary to know how females go about choosing oviposition sites. One may then determine if host plant traits eliciting oviposition behavior are the same, correlated with, or indicative of traits important for offspring success. Toward this end, in the present paper we examine the degree to which flower morphology is important in determining both which flowers Hylemya5 sp. (Diptera: Anthomyiidae) oviposit on and the success of their eggs on those flowers. Hylemya sp. females regularly oviposit on buds of two confamilial Rocky Mountain herbs, Ipomopsis aggregata and Polemonium foliosissimum [Pole- moniaceae (Zimmerman, 1979a, b, 1980b, c, 1982; Zimmerman et al., 1984)]. Eggs are attached to the undersides of sepals of both species. After an egg hatches, the larva bites an entrance hole into the plant's ovary and then preys upon developing seeds. Two striking differences exist between the patterns of use of the two plant species. First, females ovipositing on P. foliosissimum secrete an oviposition-deterring pheromone (Zimmerman, 1979a, b) while those utilizing I. aggregata as a host species do not, although they do respond appro- priately to experimentally applied pheromone (Zimmerman, 1980c, 1982). Sec- ond, egg mortality on the two host species differs significantly (Zimmerman, 1980c). While mortality was only approximately 9.4% on P. foliosissimum, it was 53.4% on I. aggregata. Additionally, there is a lack of correspondence on both hosts between oviposition choice and larval performance (Brody and Waser, 1995), despite flowers of I. aggregata chosen as oviposition sites having a higher rate of fruit set than flowers not chosen (Brody, 1992b; but see Campbell, 1991). Such striking differences in egg mortality on the two hosts lead to two obvious questions. First, why is I. aggregata used as a host at all? Second, what is the cause of egg mortality? Although the issue of host plant choice has been addressed in some detail (Zimmerman et al., 1984), a satisfactory reso- lution to the first question has not yet been achieved. Given that equal larval food is present in developing fruits of both species (Zimmerman, 1980c) and that P. foliosissimum flowers have a greater probability of being pollinated and becoming fruits than do those of I. aggregata (Zimmerman 1980a; L. Wolf, personal communication), Zimmerman et al. (1984) hypothesized that Hylemya females were making suboptimal oviposition choices. That oviposition choice was an active decision-making process rather than the result of uneven distri- butions of the two host species was demonstrated by controlled experiments in Oviposition in Hylemya (Delia) Sp. 373 which females were presented with equal numbers of both species and allowed to choose between them (Zimmerman et al., 1984). The actual cause of the difference in egg mortality has not been investi- gated, although Zimmerman (1980c) postulated that eggs oviposited on I. aggregata may desiccate at a far greater rate than those deposited on P. foliosissimum. The hypothesis was that the floral structure of the two species is such that P. foliosissimum provides a better microenvironment for eggs than does I. aggregata. The present paper addresses this point explicitly by comparing sepal dimensions and oviposition locations on the two species, as well as by examining egg mortality on I. aggregata as a function of oviposition location. In addition, we experimentally examine the degree to which flower morphology is important to females ovipositing on I. aggregata. Given that females commonly use I. aggregata, despite higher egg mortality on it, we sought to examine the degree to which within-plant variability in flower morphology governs oviposition decisions. We hypothesized that females use morphological cues in choosing flowers of I. aggregata and that such a choice could prevent wasting eggs on buds having sepals too narrow to protect the eggs. This hypothesis was prompted by a positive correlation between flower size and egg presence for I. aggregata in 1 year (Brody, 1992b), however, the relationship had not been examined experimentally. Here we manip- ulated apparent within-plant flower size on I. aggregata to test experimentally the importance of floral morphology on choices of oviposition sites within plants, in addition to correlating egg loads with various morphological traits. MATERIALS AND METHODS Egg mortality studies and female choice experiments were conducted in the field along the Kebler Pass Road, Gunnison National Forest, Gunnison County, CO, and at the Rocky Mountain Biological Laboratory (RMBL) near Crested Butte, CO, respectively, during the summers of 1989 and 1995. In examining egg mortality, the flowers of both Ipomopsis aggregata and Polemonium foliosissimum were censused for eggs. Each time an egg was found, its position was noted as either exposed or unexposed. An exposed egg was one that protruded from the sepal under which it had been oviposited, while an unexposed egg was fully covered by the sepal. Three measurements were taken from the sepals of 100 I. aggregata and 100 P. foliosissimum. The length of each sepal was recorded, as were two measurements of sepal width: at the base of each sepal and 2 mm below the tip. Mann-Whitney U tests were used to test for statistically significant differences. Ipomopsis aggregata flowers found to be hosting a single exposed or unex- 374 Zimmerman and Brody posed egg were marked. When these flowers matured into fruits, they were dissected and the fate of the egg was determined. The egg was considered to have hatched successfully if either a larva or a larval frass was found within the developing fruit. Conversely, egg mortality was considered to have occurred where no evidence of larval presence was found. To examine the role of flower morphology in governing oviposition behavior, we conducted the following experiment. Early in the season, before plants were in bloom at the RMBL, we transplanted I. aggregata plants into pots from two lower elevation sites near Almont and Crested Butte, CO. For 40 plants from each site, we measured sepal length, sepal width, corolla length, corolla width, and carpel length on three randomly selected flowers of each plant as in Brody (1992a). We then ranked each set of 40 plants from the two sites using their first principal-component score (an overall measure of size) and chose pairs of plants of different floral sizes (by pairing the 1st with the 21st, the 2nd with the 22nd, and so on). We potted each pair of dissimilar plants in the same pot with their stems as close together as possible and their flowers entwined. We transported them back to RMBL, where we watered them with root stimulator and allowed them to overcome transplant shock before placing them in the field. After 3 days, all plants had fully recovered and we placed each pot randomly in rectangular arrays of 20 pots each at two sites near the RMBL—"Dining Hall" and "Avalanche Acres." We then censused all flowers and buds for Hylemya eggs over the next 2 weeks. When we found an egg, we marked the outside of a sepal of that flower or bud to prevent counting the same egg twice.

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