Sample Data

The following data were collected by students in the Ecology Laboratory course at Morehouse College during spring semester 2007. A total of 17 independent trials were conducted in which five mung beans without eggs and five mung beans with a single egg laid by a previous female were placed together in a 35mm Petri dish. One female (a different female than the one who had previously laid single eggs on beans) was placed in each Petri dish and permitted to lay eggs for 24 hours. A total of 100 new eggs were laid in the 24 hour period and there were significantly more eggs laid (77) on beans without previously laid eggs (binomial test, p<0.001) than on beans that had one egg from a previous female (33). An alternative statistical analysis comparing the mean number of eggs laid on beans without previously laid eggs to those beans that had an egg already present also showed a highly significant preference for beans without eggs (Figure 1, paired t-test, t=3.7, df=16, p=0.002) suggesting that female bean beetles are very sensitive to the potential for competition among their offspring in a given bean.

Figure 1. Number of eggs laid by female bean beetles on beans with or without previously laid eggs. The mean ± SE of the number of new eggs laid by a second female are shown (n=17). The total number of new eggs laid on mung beans that had no previous eggs and mung beans that had one egg from a previous female are given at the top of each bar.

Mung beans (N=137) with one, two, or three eggs were isolated individually in the wells of 12-well tissue culture plates and allowed to develop for 5-weeks. Beans were evaluated for the number of adult beetles that emerged from each bean. Only 34 of the 137 beans with eggs successfully yielded adult beetles, and only one adult emerged from a bean regardless of the number of eggs that had been laid on that bean. Among those beans from which an adult emerged, eggs on two egg beans had a 50% success rate and eggs on three egg beans had a 33% success rate. Overall, the success rates (percentage of eggs that yielded adults) varied among beans with different numbers of eggs (Figure 2), but these differences were not significant (X²=0.44 , df=2 , p=0.80). However, given the success rate for single egg beans, the success rates for eggs on two and three egg beans, should be 36% and 54%, respectively, in the absence of any competitive effects. Therefore, intraspecific competition seems to decrease emergence success.

Figure 2. The effect of the number of eggs on a bean on the number of emerged and non-emerged adults.

In fall 2007, students in the Ecology course at Emory University examined the effect of previous exposure of beans on female oviposition preference. Females were given access to 3 beans exposed to another female and to 3 unexposed beans, and allowed to oviposit for 7 days. Based on 14 replicates, the frequency of eggs laid did not differ between the two types of beans (binomial test, p = 0.37; Figure 3). Furthermore, the mean number of eggs laid on the two bean types was not significantly different (paired t-test, t=0.49, df=13, p=0.63).

Figure 3. Number of eggs laid by female bean beetles on with or without previous exposure to another female. The mean ± SE of the number of new eggs laid by a second female are shown (n=14). The total number of new eggs laid on mung beans that had no previous exposure to a female and mung beans that had been exposed to a previous female are given at the top of each bar.