What is the null hypothesis, when there is no research hypothesis?
So your independent variable is the bug, and your dependent variable is apple size.

The question asks whether you are testing a directional hypothesis. 
Given that researchers are divided about the direction of the effect of the IV on the DV, it makes more sense to treat it as a non-directional hypothesis:

i.e., H0 is that apple size is equal with and without bugs.
The alternative hypothesis is that they are not equal. Or in some senses, there are two alternative hypotheses, one suggesting an increase and another suggesting a decrease.

Of course, you ultimately want to conclude whether there is a difference and if so what is the direction of the effect. Thus, confidence intervals from a frequentist perspective or a posterior density from a bayesian perspective might be more useful in understanding the direction and degree of any effect.

You may also want to read about suggestions for testing the hypothesis of no group differences.