Since phytophagous insects, or “plant-eating” insects, have different species or different populations of the same species that are often specific to a particular host plant species, a change in the availability of a particular host plant or the introduction of a new host plant can lead to a shift in the host plant that is mainly used. Thus, this shift can then lead to strong natural selection for adaptation to the new host plant. For example, a preference for a novel host evolved in female checkerspot butterflies, or the Euphydryas editha, so they rejected their native host in only seven years (Singer et al. 1993). Similar to this concept, female bean beetles, or the agricultural pest insects called Callosobruchus maculatus, lay their eggs on the type of bean that they were born on (Messina 2004). Since the beetle larva burrows into the bean and cannot move after the eggs are deposited on the bean, the quality of food resources available in a bean will influence the developing individual’s growth, survival, and future reproduction (Mitchell 1975, Wasserman and Futuyma 1981). Thus, since the determination of larval growth, survival, and future reproduction depends on the ability to efficiently use the resources of the host bean, we expect the populations to adapt rapidly to the host bean species that are readily available.
Furthermore, environmental changes in the local or global environment, such as the availability of certain nutrients and the presence of detrimental factors like temperature, can lead to reduced fitness, unless the individuals of a species have a quick evolutionary response to the environmental changes. Environmental changes influence reproductive rates and can explain the effective population sizes inferred from genetic data that are often much smaller than the census population size (Melbinger et al. 2015).
In this experiment, environmental changes will be implemented on the bean beetles; the three different type of bean beetles will be placed on either petri dishes of only one kind of bean or petri dishes that are divided into thirds for the Adzuki, Mung, and Black Eyed Peas beans.
Methods: Bean beetles (cowpea seed beetles), or Callosobruchus maculatus, are common agricultural pests that originate in Africa and Asia. The bean beetles used in this experiment contained adults that were raised on Mung beans (Phaseolus aureus) for a large number of generations. There were also bean beetles that were originally from Mung beans but were then transferred to Adzuki beans (Phaseolus angularis) or Black Eyed Peas (Vigna unguiculata). These beans were all in relatively good condition, and none, according to the best of our knowledge, were defective. In total, 45 petri dishes were split into thirds, in which one third of the petri dish contained a fine layer of Mung beans, one third for Adzuki beans, and one third for Black Eyed Peas. Exactly 15 of these divided petri dishes contained one male and one female Mung bean beetles, 15 of the petri dishes contained one male and one female Adzuki bean beetles, and the remaining 15 petri dishes contained one male and one female Black Eyed Peas bean beetles. Out of the remaining 15 petri dishes, five petri dishes contained a complete and fine layer of only Mung beans, five petri dishes had a fine layer of only Adzuki beans, and five petri dishes contained a fine layer of only Black Eyed Peas. In the five petri dishes with Mung beans, the Mung beetle was added. These 15 petri dishes served as the (normal egg laying) control, in which the corresponding bean beetles were reared on the associated bean type for many generations that were only given the corresponding bean to coexist and reproduce in. Although it is not clear whether there were the same exact number of beans in each petri dish, there were approximately similar amounts as all petri dishes had one fine layer of the beans. Additionally, tweezers and brushes were used to transfer one male and one female bean beetle into each petri dish. The data presented in this report were from two laboratory experiments and analyzed through comparison of bars once all of the data were put into bar graphs (Graph A).
Results: In the treatment trials, the Mung bean beetle laid eggs more times when among the Mung beans rather than when among the Adzuki beans or the Black Eyed Peas. Similarly, Adzuki bean beetles laid eggs more times when among the Adzuki beans rather than when among the Mung beans or the Black Eyed Peas. However, Black Eyed Peas bean beetles laid eggs more times among the Adzuki beans instead of the Black Eyed Peas or the Mung beans. In the control trials, there were significant increases in the egg laying of the bean beetles in each group.
Discussion: The most significant limitation to this experiment was the buying of geriatric beetles that barely laid any eggs. Another limitation was that the diligence in sorting through the beetles and ensuring one male and one female beetle in each petri dish was not maintained for all of the petri dishes; thus, the number of eggs laid on the beans were certainly influenced. In the future, experiments can be repeated after buying geriatric beetles that did lay eggs. This change would certainly improve the accuracy in the bean beetles’ egg laying habits on the different type of beans. Another future experiment is that all of the petri dishes can be checked multiple times to make sure that there were one male and one female beetle; this way, we would know that reproduction is possible, and if there were no eggs, it was because the two beetles chose not to reproduce with each other, not that there were single gendered beetles.