Reproductive Responses to Photoperiod by a South Florida Population of the GrasshopperRomalea microptera(Orthoptera: Romaleidae)

Abstract

Reproductive tactics of some insects are affected by photoperiod because daylength is a reliable cue indicating seasonal change in environmental suitability. We tested the hypothesis that late-season (short-day) photoperiod affects the length of the oviposition cycle of the eastern lubber grasshopper, which is from a subtropical area with a winter dry season. We predicted that oviposition would occur sooner in short-day (11.5:12.5 [L:D] h) than in long-day (13.75:10.25 [L:D] h) individuals, and that costs associated with earlier oviposition would be expressed as smaller eggs or fewer eggs per female under the short-day photoperiod. Two diets (low and high food quantity) were tested at each photoperiod. Although MANOVA yielded no significant main effect of photoperiod on the timing of oviposition, number of eggs, and mean egg dry mass, there was a significant multivariate interaction between photoperiod and food for these variables. In the low food treatment, the short-day animals produced fewer, smaller eggs than did the long-day animals, but this multivariate difference between short-day and long-day animals was absent in the high-food treatment. Photoperiod and its interaction with food did not affect timing of oviposition in either multivariate or univariate analyses. The absence of the predicted change in timing of reproduction in an autumn photoperiod suggests that seasonal constraints on reproduction do not exert a strong influence on reproductive timing of this population. Our alternative hypothesis is that the combination of short days and low food availability serve as a cue to lubber grasshoppers of an oncoming dry season with potentially limiting food, and that they respond to this cue by reducing the mass of reproductive output but not by accelerating reproductive timing. Thus, we propose that these subtropical grasshoppers adjust reproductive investment in response to seasonal changes in resource availability.