Host Discrimination In Callosobruchus Maculatus

ABSTRACT

The bean beetle, Callosobruchus maculatus, is a common agricultural pest that lays its eggs on bean crops and develops larvae within the bean. Female bean beetles use host discrimination (detecting the number of eggs or larvae already present on a bean) to determine whether or not to lay their eggs on the bean. The purpose of this experiment was to experimentally determine if female bean beetles have a bean preference for egg laying based on additional factors such as surface area or bean color. Four male and four female bean beetles were placed in a petri dish containing four pinto beans and four black beans and were left for a week. The number of eggs on each bean was counted and recorded to measure bean preference. Analysis of results showed no significant difference between the number of eggs laid on pinto beans versus the number of eggs laid on black beans. The relatively even spread of eggs on both bean types shows that while egg-laying was not based on specific bean preference, it was likely based on host discrimination.

Introduction

Bean beetles, Callosobruchus maculatus, are a common agricultural pest in the African and Asian continents. They lay eggs on beans, presenting a challenge to farmers. Their larvae develop within the beans. Upon emerging from the bean, bean beetles mate and lay more eggs before dying (Beck 2014). Bean beetles prefer to lay eggs on legume beans (like pinto and black beans) because of their seed coat penetrability for larvae to burrow into the seed and obtain nutrients from the embryo (Souza et. al 2011). Female bean beetles have the ability to detect how many eggs and larvae are already present on a bean and use this information to determine whether or not to lay eggs on that bean, a process known as host discrimination (Messina 2002). Laying eggs on a bean that has no eggs on it provides the greatest chance of survival and fitness for that female’s offspring (Horng 1997). This means that female beetles are selecting beans that have fewer or no larvae to lay eggs on. However, additional factors, such as bean size and color could contribute to the female’s choice. Larger surface area could allow for more eggs to thrive on the bean, while a bean color close to that of the eggs could provide camouflage. Studying the bean preference and egg distribution of bean beetles is relevant to the agricultural industry because it can help farmers to make informed decisions regarding crops they choose to grow. The purpose of this experiment was to determine if female bean beetles have a bean preference for egg laying. I hypothesized that the bean beetles would lay more eggs on the pinto beans because of their larger size (which allows for more eggs) and similarity in color to bean beetle eggs.

Methods

Four male and four female bean beetles were placed in a petri dish with a divider that separated the dish into halves. The beetles were able to cross this barrier. Four pinto beans were placed on one side of the divider, and four black beans were placed on the other side. The petri dish was taped shut and left in a room-temperature environment for one week. Eight trials with this set-up were conducted. After one week, the number of eggs laid on each bean were counted and recorded. This information was used to determine if beetles have a bean preference for egg laying.

Results

To determine if there was any significant difference between the number of eggs laid on pinto beans and the number of eggs laid on black beans, a one-tailed t-test was performed in Excel. I hypothesized that the bean beetles would lay more eggs on the pinto beans because of their lighter color and larger surface area. The p-value, the probability of finding significant results against the null hypothesis, was calculated to be 0. 279. To show significant difference between the pinto and black beans (against the null hypothesis), the p-value would need to be less than 0. 05. Because the number from our results was much higher, this concludes that there was no significant difference between the number of eggs laid on pinto beans and number of eggs laid on black beans, thus failing to reject the null hypothesis. Although the pinto beans had a slightly higher average of 79. 375 eggs than the black beans at 65. 625 eggs, it is most likely that this distribution of eggs was simply due to chance.

Discussion

I hypothesized that bean beetles would lay more eggs on the pinto beans because of their lighter color for camouflage and larger surface area. My hypothesis was not supported by our data, as the p-value showed no significant difference between the number of eggs on black beans and number of eggs on pinto beans. The results from our data are supported by previous research. Bean beetles typically lay eggs on the first bean available but have shown that they are capable of host discrimination, in which a female can determine if there are already too many eggs or larvae present in the bean by detecting chemical cues (Horng 1997). The relatively even spread of eggs on both bean types in most trials may represent that while egg-laying was not based on specific bean preference, it was likely based on host discrimination in which the female detected other eggs present on the bean and chose to lay eggs on a different, less crowded bean to decrease larval competition (Horng 1997). Bean beetles also exhibit rapid adaptation to new host plants, which allows them to be more successful in reproducing; this could also explain why there was no significant bean preference (Messina 2002).

The implications of this research are particularly important for the agriculture industry. Bean beetles are known agricultural pests that destroy bean crops (Beck 2014). Our data, supported by other research, shows that bean beetles are a serious threat to agriculture, as they are able to adapt and lay eggs on a variety of bean types. This research could also be used to work towards a solution to containing bean beetle populations in agricultural settings. Various pesticides and fertilizers have shown to be effective in containing some bean beetle populations, however these solutions are not sustainable or attainable for some farmers and more research is required to find a feasible solution for this issue (Fan et al 1993). While bean beetle reproduction and oviposition habits have been thoroughly studied, the data from research in this area is critical to finding a solution to containing outbreaks of these pests.