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 predation; 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

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 resolution 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 manipulated 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. The experiment was terminated when other naturally growing plants began blooming. First, we analyzed the results by comparing the total number of eggs laid on each member of the pair and the percentage of flowers that received eggs for each member of each pair using a paired t test. Comparisons were made separately by site. Second, we removed all pots (i.e., pairs) that were never oviposited upon and again compared the total number of eggs and the percentage of flowers laid on for each pair using a paired t test. Third, we examined if there was a significant relationship between any of the five measurements of floral morphology and the total or percentage of flowers laid on for that plant by simple linear regression.

RESULTS Eggs were significantly more likely to be exposed when oviposited on I. aggregata than on P. foliosissimum (Table I). On P. foliosissimum, only 22% of the eggs deposited were exposed, while on I. aggregata the sepals failed to cover half (49.3%) of the eggs. Oviposition in Hylemya (Delia) So. 375

Table I. Frequency of Exposed and Unexposed Hylemya Sp. Eggs Found on Flowers of Ipomopsis aggregata and Polemonium foliosissimum*

Plant species Eggs exposed Eggs unexposed

Ipomopsis aggregata 83 89 Polemonium foliosissimum 64 227

*X2 = 33.21, 1 df, P < 0.0001.

Three measures of sepal size differed between I. aggregata and P. foliosissimum (Table II). Sepals of the latter species were significantly shorter than were sepals of the former. Additionally, P. foliosissimum sepals were significantly wider, both at the base and at the tip, than were those of I. aggregata. Significantly fewer exposed eggs than unexposed eggs successfully made the transition to the larval stage (Table III). On I. aggregata, egg mortality for exposed eggs was 48.4%, while for unexposed eggs it was only 28.8%. We found no difference in the propensity of females to oviposit on the "large" versus the "small" member of the intertwined pair of plants. The total number of eggs laid on flowers of the two treatments was not significantly different (Figs. 1A and 2A), nor was the percentage of the flowers receiving eggs among pairs of the two treatments significantly different (Figs. 1B and 2B). Corolla length was negatively correlated with egg load at both sites (Fig.

Table II. Three Measurements (mm) of Sepals from Ipomopsis aggregata and Polemonium foliosissimum

Measurement Ipomopsis aggregata Polemonium foliosissimum

Length X 6.12 4.17 SD 1.14 0.63 N 100 100 P < 0.005 Width-base X 1.56 1.95 SD 0.32 0.30 N 100 100 P < 0.005 Width-tip X 0.46 1.27 SD 0.18 0.21 N 100 100 P < 0.005 376 Zimmerman and Brody

Table III. Fate of Exposed and Unexposed Eggs of Hylemya Sp. Oviposited upon Ipomopsis aggregata*

Fate

Location Survived Died

Exposed 32 30 Unexposed 57 23

*X2 = 4.95; 1 df, P < 0.025, one-tailed test.

3), and corolla width was positively correlated with egg load at one site (Ava- lanche Acres) but not the other (Dining Hall; Fig. 3). We found a significant correlation between sepal length and sepal width (r = 0.26, P = 0.02; N = 82), neither of which was a strong predictor of egg load. However, corolla length and corolla width were not significantly correlated (r = 0.05, P = 0.67; N = 82).

DISCUSSION The shape of I. aggregata sepals differs significantly from that of the sepals of P. foliosissimum; the former are longer and thinner than are the latter. Because of the thinness of I. aggregata sepals, Hylemya eggs oviposited on this species have a much greater tendency (by a factor of 2.2) to stick out beyond the margin

Fig. 1. The average number of eggs laid on small- versus large-flowered members of intertwined pairs of Ipomopsis aggregata at the two sites. All pairs are represented in A (N = 29 for both sites); only pairs for which at least one flower of one of the plants was oviposited upon are represented in B (N = 23 and 18 for Avalanche Acres and Dining Hall, respectively). Oviposition in Hylemya (Delia) Sp. 377

Fig. 2. The percentage of flowers oviposited on for small- versus large- flowered members of all intertwined pairs of Ipomopsis aggregata at the two sites (A) and only those pairs receiving at least one egg (B). Sample sizes as in the legend to Fig. 1.

Fig. 3. The relationship between the percentage of flowers (arcsin square root transformed) oviposited on and the corolla length and corolla with of Ipomopsis aggregata plants at the two sites. Avalanche Acres: F1,57 = 4.42, P < 0.05, and F1,57 = 0.29, P > 0.05, for corolla length and corolla width, respectively. Dining Hall: F1,57 = 4.33, P < 0.05, and F1,57 = 5.88, P < 0.05, for corolla length and corolla width, respectively. 378 Zimmerman and Brody of the sepal than do those eggs oviposited on P. foliosissimum. Exposed eggs have a much greater tendency (by a factor of 1.7) to die than do fully protected eggs. Taken together, these two variables suggest that egg mortality should be 3.7 times greater on I. aggregata than on P. foliosissimum. Zimmerman (1980c) found that the Hylemya egg mortality experienced on I. aggregata was actually approximately 5.7 times greater than that on P. foliosissimum. The difference in sepal morphology between the two species with presumed desiccation of exposed eggs thus explains a major portion (65%) of the observed difference in egg mortality found on I. aggregata and P. foliosissimum. If cues available at oviposition are correlated with larval success, one would expect females to employ them in oviposition decisions. In this case, the width of the sepals protecting eggs is a strong predictor of egg hatching success. But females apparently do not discriminate between wide and narrow sepals. How- ever, we found significant relationships between corolla length of I. aggregata and egg loads at both sites, and between corolla width and egg loads at one of the two sites. These results suggest that some aspects of flower morphology or, perhaps, correlated characters, are important to ovipositing Hylemya. Why should corolla morphology be important to Hylemya ovipositing on I. aggregata? At least three possibilities exist. First, the correlations are spurious and corolla length and/or width are, in fact, unimportant. However, corolla length was the strongest predictor of whether or not a flower was chosen as an oviposition site in both this study and a previous one (Brody, 1992a). However, here the correlation was negative, whereas previously it was positive (Brody, 1992a). It is notable that corolla length is sometimes strongly favored through pollination, but phenotypic selection on corolla length varies in magnitude and direction among years (Campbell, 1991). Thus the combined effects of pollinators and seed predators may help explain the maintenance of variability in this trait. Second, corolla morphology is indicative of pollination success and therefore females use it as a direct measure of the probability of a flower setting fruit. Evidence from other studies both support and contradict this hypothesis. Morphological traits had virtually no predictive value for fruit set in a study by Mitchell (1994) once overall plant size and flower number were accounted for, whereas Campbell (1991) found that corolla length was positively correlated with seed set in some years but not others. The third explanation is that corolla length and/or width may be correlated with some other trait indicative of sub- sequent flower success. This is likely since females often lay eggs before the flowers of I. aggregata have fully expanded and thus are probably not measuring corolla length or width per se. Brody (1992b) found a positive correlation between the overall flower size and the presence of Hylemya eggs. By choosing plants of dissimilar-sized flowers and potting them together, we sought to magnify "within-plant" differences in flower size. We hypothesized that flies would choose the largest member of Oviposition in Hylemya (Delia) Sp. 379 the pair on which to lay their eggs. That prediction was not upheld, though at both sites the trend was for larger flowers to be laid on more frequently than smaller. A number of differences between the two experiments might help to explain the varied results. First, we did not measure each individual flower— laid on or not—as did Brody (1992b). However, the current design is a stronger test of the hypothesis that females choose larger flowers since we manipulated flower size "within plants" per se. Second, the experiments were conducted at different times of the year—early versus late in the season. The behavior of animals foraging for oviposition sites is likely to be influenced by both external and internal factors that may change over the season or over the animal's lifetime (Papaj and Rausher, 1983; Bell, 1990). Thus it is conceivable that fly behavior measured early in the season in one year might be different from that measured late in the season in another year. However, one might expect females to be more rather then less choosy early in the season (Wiklund, 1981; Singer, 1986). Alternatively, the motivation to oviposit may be high early in the season due to high egg loads, perhaps resulting in a lower threshold for discrimination (Court- ney et al., 1989; Odendaal, 1989; Singer et al., 1992; Browne, 1993). The data presented here indicate that females are not using overall flower size as a cue in making oviposition decisions within plants (at least early in the season), but they may be responding to more subtle differences in flower morphology. In addition, it is important to note that most flowers on all plants were not laid on. Therefore, our results are unlikely to be due to an "ideal free distribution" (sensu Fretwell and Lucas, 1969) whereby high-quality hosts are deval- ued by their prior use as oviposition sites. Although the present work confirms Zimmerman's (1980c) hypothesis that egg mortality was due, in large part, to exposure of poorly placed eggs, it does not bring about a resolution to the evolutionary question concerning host plant choice by egg-laying females. The cost of ovipositing on I. aggregata appears to be considerably more than the cost of ovipositing on P. foliosissimum, and without compensatory benefits. Egg mortality is higher on I. aggregata as demonstrated here, while food resources are equivalent (Zimmerman 1980c). Added to this, there appears to be a lack of correspondence between oviposition choices and larval success on either host (here, and see Brody and Waser, 1995). Until additional work can be performed, the possibility that host plant choice is suboptimal (Zimmerman et al., 1984) cannot be dismissed.

ACKNOWLEDGMENTS We thank Elan Cohen, Christine Douglas, Rebecca Irwin, Isaac Leader, Todd Palmer, Elena Pinto-Torres, Kim Rand, Nick Waser, and Brooke Zanetell for very capable field assistance. Gary Entsminger and Svata Louda provided valuable discussions of the ideas. The research was supported by National Sci- 380 Zimmerman and Brody ence Foundation grants BSR-8805643 and DEB-9318602 and by a grant to Oberlin College from the Howard Hughes Medical Institute.

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