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The effect of bean type on the number of eggs oviposited by bean beetles, Callosobruchus maculatus. ABSTRACT A factor that affects bean beetles, Callosobruchus maculatus, oviposition preference is the nutritional value of the bean. The purpose of this experiment was to determine the bean beetle oviposition preference for soybeans and navy beans. I hypothesized that, if bean beetles were given soybeans and navy beans they would prefer soybeans over navy beans because soybeans have a high-fat content than navy beans. To test this, 39 petri dishes were prepared containing 4 of each type of bean with 3 males and 5 females. After 6 days the number of eggs oviposited on the beans was counted and recorded. The results found by the experiment did not support my hypothesis, soybeans were preferred by bean beetles for oviposition than navy beans. The number of eggs oviposited on the soybeans was not significantly different from the number of eggs oviposited on the navy beans. This suggests that bean beetles do not have a preference over these two types of beans.

INTRODUCTION Been beetles, Callosobruchus maculatus, are herbivorous insects that are found in Africa and Asia with a lifespan of 10-14 days, and the adults fully maturing after 24-36 hours after emergence (Blumer and Beck 2007). They are agricultural pests and are parasites to the beans they use as hosts. The females oviposit on the surface of the bean (Hudaib et al. 2010). Once the larvae hatch from the bean they consume the bean and do not need to eat or drink for the rest of their lives (Blumer and Beck 2007).

Since larvae don’t move between seeds, female oviposition decisions determine the environment in which the larvae develop (Hudaib et al. 2010). The type of bean that the larvae will be oviposited will influence its growth, survival, and future reproduction. While female bean beetles typically prefer to oviposit on the same type of bean they emerged from, they will readily lay eggs on other bean types if the natal bean is not available (Blumer and Beck 2007). One of the factors that contribute to the bean preference is the nutritious value of the bean. In a previous experiment, it was found that female bean beetles prefer beans that could provide them with more energy because the type of bean can cause a variance in their lifetime (Paukku and Kotiaho 2008). This experiment was conducted to test the effect of bean type on the number of eggs laid by the female bean beetles. I chose to compare soybeans and navy beans since their color, size and the region they are harvested is the same and because there's a lack of research on these two types of beans. I hypothesize that if the female bean beetles are given the choices between soybeans and navy beans, then there will be a significant oviposition preference on soybeans over navy beans because soybeans have a high-fat content of 47% than navy beans which only have 3% of fat, thus soybeans providing a higher percentage of energy than the navy beans (Messina 1999).

METHODS Four of each soybeans and navy beans were put on the two sides of the Y plate petri dishes. Y plates were used to avoid contamination between the two types of beans

and to make it easier to count the number of oviposited eggs because some eggs fall off the bean. Three males and five females were put into the third compartment of each Y plate to make them decide which bean they want to oviposit on. Bean beetles were given six days to oviposit with 12 hours of light and 12 hours of dark. The petri dishes were stored at 22°C. A total of 39 replications were done. A one-tailed t-test was used to analyze the data.

RESULTS There was no statistical difference between the number of eggs oviposited on the soybeans (µ = 28.03 ± 8.60) and the navy beans (µ = 30.28 ± 5.76; t(38) = -1.35, p=0.09). The eggs were oviposited randomly in the two types of beans in all of the replications. Some petri dishes had more eggs oviposited on soybeans while the others had more eggs oviposited on navy beans. The highest number of eggs oviposited was forty, which was in 2 petri dishes on the soybean side.

Figure 1. Average number of eggs oviposited (Mean ± SD) in soybeans and navy

beans. The difference in the number of eggs oviposited on the two types of beans has no significant difference (n=39; p=0.09).

DISCUSSION The hypothesis of this experiment which was bean beetles would prefer oviposition on soybeans over navy beans because soybeans have a high-fat content than navy beans was not supported by the results. The results portrayed that female bean beetles didn’t have an oviposition preference for soybeans and navy beans leading to having no difference in the number of eggs oviposited. Previous research done on the oviposition preference of bean beetles had found that bean beetles preferred the beans with high nutritional value (Paukku and Kotiaho 2008) but it is found in our study that higher fat content did not make the bean beetles prefer one bean over the other. Further experimentation is needed to reconcile what type of nutrition would make the bean beetles choose one bean over the other. The type of bean chosen by female bean beetles determines the survival development and future reproduction of the offspring. A previous study has shown that bean beetle preference of larger beans over smaller beans as a factor that influence female bean beetle oviposition preference (Cope and Fox 2002). However, our study used two beans of similar sizes so we did not expect there to be a female preference between two of these beans because they are the same size and in fact, we did not find that preference in our study. Another study has found that bean beetles prefer lightcolored seeds over dark-colored seeds for oviposition (Baidoo et al. 2015). Yet, our two types of beans were similar in color, they were both off white. Our females did not have a preference for these beans so these colors were not enough difference to make the

females prefer one bean over the other. The conditions that the bean beetles are kept in can affect oviposition. Certain environments and temperatures allow female bean beetles to oviposit more eggs. Bean beetles can act in different ways based on where they are and the type of bean. Bean beetles, who prefer nutritional beans for oviposition, didn't have a preference for soybeans in this experiment as I expected. For future studies, the two types of beans can be placed at a different temperature or alter the hours the bean beetles are exposed to light and dark. This will help to determine if the environment affects the bean beetle from ovipositing more on soybeans. This experiment can also be done in 12 days instead of 6 days to observe whether a change will occur.

REFERENCES

Messina M.J. 1999. Legumes and soybeans: overview of their nutritional profiles and health effects. Am. J. Clin. Nutr. 70(3):439-450. doi:10.1093/70.3.439.

Blumer L.S, Beck C.W. 2007. Oviposition Substrate Choice by Bean Beetles, Callosobruchus maculatus. ABLE. 29(1):51-66.

Paukku S, Kotiaho J.S. 2008. Female Oviposition Decisions and Their Impact on Progeny Life-History Traits. J Insect Behav. 21:505–520. doi:10.1007/10905-008-9146. Hudaib T, Hayes W, Brown S, Eady P.E. 2010. Effect of seed moisture content and d‐ limonene on oviposition decisions of the seed beetle Callosobruchus maculatus. Entomol. Exp. 137(2):120-125. Baidoo P.K, Kwansa N.A, Annin C.P. 2015. The Role of Seed Coat and Its Pigmentation on the Acceptance of Bambara Groundnut (Vigna subterranea L. Verdc.) Cultivars by the Cowpea Beetle, Callosobruchus maculatus (F.). J. Sci. Res. 3(4):125-131. doi: 10.4236/ae.2015.34015. Cope J.M, Fox C.W. 2002. Oviposition decisions in the seed beetle, Callosobruchus maculatus

(Coleoptera: Bruchidae): effects of seed size on superparasitism. J. Stored Prod. Res. 39(4):355-365....