The effects of aging on the ovipostitional behavior of the Cabbage Butterly (Pieris rapae)
McMurry, Morgann Christina
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As with many plant feeding insects, the larvae of the cabbage white butterfly, Pieris rapae, develop on a single host plant selected by the adult female. For this reason, host selection behavior is considered to play a key role in understanding the evolution of plant-insect interactions. Age is expected to have a significant effect on the host selection process, because it affects both the time available for host selection and the relative egg availability. However, relatively little experimental data exists regarding the effects of aging on host selection by ovipositing female insects. The purpose of this experiment was to examine the effects of aging on the ability of the cabbage butterfly to identify and select a host. P. rapae are specialists on plants in the Brassicaceae family, but exhibit significant preferences between plats within this family. The hosts that the cabbage butterfly were able to choose from included mustard greens, collard greens, and cabbage. A non-host plant was also used to determine if the butterflies made host selection mistakes. By viewing the butterflies in an observation arena at the various stages of aging, the effect of aging on host preference was determined. In addition, the time required to make decisions during the ovipostitional process was quantified and used to determine the effects of aging on ovipositional behavior.
The Effects of Aging on the Ovipositional Behavior of the Cabbage Butterfly (Pieris rapae) Morgann C. McMurry, Dr. Rodrigo Mercader Department of Biology Washburn University Introduction As with many plant feeding insects, the larvae of the cabbage white butterfly, Pieris rapae, develop on a single host plant selected by the adult female. For this reason, ovipositional behavior by adult females is considered to play a key role in determining host range evolution in plant feeding insects. Identifying the factors that influence host-selection behavior will therefore help us gain a mechanistic understanding how novel associations between plants and plant feeding insects evolve. Due to its effect on both the time available for host selection and the relative egg availability, age is expected to have a significant effect on the host selection process. However, relatively little experimental data exists regarding the effects of aging on host selection by ovipositing female insects. The purpose of this experiment was to examine the effects of aging on the ability of a specialist insect, the cabbage white butterfly, to identify and select a host. The cabbage white butterfly is a specialist on plants in the Brassicaceae family, but exhibit significant preferences between plants within this family. Here we subjected the cabbage white butterfly to choice assays consisting of two known host plants, Brassica juncea (mustard greens) and Brassica oleracea (collard greens), and one non-host plant, Phaseolus vulgaris (green beans), belonging to the ancestral plant family used by the Pieridae. In addition, no-choice oviposition assays were conducted including only the non-host plant to get a better appreciation of ovipositional mistakes. These assays were conducted on 8 females over the course of 2 weeks for each butterfly to examine the influence of aging on ovipositional behavior. Materials and Methods Results Ethogram Discussion The cabbage white butterflies were raised on an artificial diet to avoid the potential confounding effects of larval host experience on our ovipositional assays. Once the larvae pupated they were placed in a large flight cage and marked on the right front wing for identification. Eight female butterflies were observed and video recorded over a period of three weeks using an observation arena. Females were inserted into the observation arenas for a 15 minute period prior to the start of the host selection assays to allow them acclimate to the arena. Females were placed in the arenas in randomly selected groups of two. Each pair of butterflies were first subjected to a no-choice assay consisting of a single leaf of P. vulgaris. To minimize the influence of leaf shape and size leaves were placed on a glass container and parafilm placed along the rim of the glass container (see Figure). In this manner the height and surface area of each leaf used throught this study was identical. Following the no-choice assay, the butterfly pair was subjected to a three choice assay consisting of P. vulgaris, B. juncea, and B. oleracea. Following each assay, the number of eggs laid on each leaf was observed and recorded. The videos collected during these assays were then viewed using program, JWatcher (Blumestein et al. 2011). The behaviors assayed are represented in the ethograms below. Acknowledgements I would like to thank Dr. Rodrigo Mercader for allowing me to do research with him. I have learned so much from him throughout the course of the semester. I would also like to thank my family for supporting me in all of my endeavors. I couldn’t have done any of this without their love and guidance. Literature Cited Blumstein, D., Daniel, J., and Evans, C. (2011). JWatcher. Retrieved from http://www.jwatcher.ucla.edu/ Rosenheim, J. A. (1999) Characterizing the cost of oviposition in insects: a dynamic model. Evol. Ecol. 13 (2): 141-165 1999. Miller, J. R. and K.L. Strickler (1984) Finding and accepting host plants. Pp 127-158. In J.W. Bell and R. T. Carde [eds.] Chemical Ecology of Insects. Chapman and Hall, NY. Although preliminary, the results of this study suggest that as the cabbage white butterfly ages, two opposing factors appear to influence the host selection behavior of the cabbage white butterfly. The first factor is time limitation. As the butterflies aged their propensity to lay eggs on the non-host during the no-choice trials greatly increased. This effect was clearly noticeable during days 10 and 12 when the butterflies laid their maximum number of eggs. Egg laying on suboptimal hosts is expected to be observed in female insects when the cost of a missed opportunity exceeds the cost of laying eggs on a suboptimal host (Rosenheim 1999). Motivational models based on insect physiology also predict an increased propensity to lay eggs on suboptimal hosts when egg load becomes high (Miller and Strickler 1984). However, in this case we observed these butterflies laying large numbers of eggs on non-hosts, not simply suboptimal hosts. These results indicate that ovipositing females of this specialist insect that fail to encounter hosts in the wild are likely to lay eggs on non-host plants, which may lead to potential local divergence in host use if a host at least marginally suitable for larval development is selected. Butterflies in our choice assays showed a distinct preference for the, B. juncea, followed by the B. oleracea, and P. vulgaris were least favored. The second factor observed in these assays is a greater fidelity to the preferred host during the choice assays. This result indicates that the butterflies may be capable of learning and reducing their error rate. Furthermore, the mean approach time decreased during the choice assays, while the mean landing time increased. These results indicate that the butterflies were capable of making faster decisions as they aged. The butterflies’ tendencies to make errors were also influenced by their affinity for egg laying. A selection of the butterflies was determined to be the most active egg layers. These butterflies made the greatest number of mistakes in selecting a host plant. The butterflies that laid a reduced number of eggs made less host selection errors. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1 2 3 4 5 Mean Proportion of Eggs Laid Butterfly Proportion of Eggs Laid on Mustard Green - Choice Test 3 5 8 10 12 14 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1 2 3 4 5 Mean Proportion of Eggs Laid Butterfly Proportion of Total Eggs Laid in No Choice Test 3 5 8 10 12 14 0 10 20 30 40 50 60 70 80 90 1 2 3 4 5 Mean Number of Eggs Butterfly Egg Load 3 5 8 10 12 14 Figure 1: Egg load for butterflies laying at least 10 eggs during the study by day. Figure 2: Proportion of eggs laid on the bean leaf substrate during “no-choice” assay relative to the total number of eggs laid by the female in all assay conducted that day. Figure 3: Proportion of eggs laid on the preferred host B. juncea during choice assays relative to the total number of eggs laid by the female in all assay conducted that day. Figure 4: Mean approach and landing times for each butterfly subject throughout the trials. Bean Host Only Drum Egg Land Leave Approach Curl Curl Mustard Green, Collard Green, and Bean Hosts Approach Land Drum Egg Leave The ethograms depict the egg laying behaviors. The arrows indicate the behavioral pathways. Butterflies depict other behaviors as well, but they were excluded in order to focus on the predominant behaviors. The boxes are scaled to indicate which behavior was most common. -1000 0 1000 2000 3000 4000 5000 6000 approach land approach land approach land 3 3 5 5 12 12 Time (ms) Day Mean Approach and Landing Time 1 2 3 4 5 Mean