WILLIAMS, J.B.*; LEE, R.E.; University of Nevada Las Vegas; Miami University: Plant senescence cues entry into diapause in the gall fly, Eurosta solidaginis: resulting metabolic depression is critical for water conservation.

Mechanisms and possible cues for seasonal increases in desiccation resistance in larvae of the goldenrod gall fly, Eurosta solidaginis, were examined before and after natural and premature plant senescence, or after being removed from their gall and placed in either100, 95, or 75% RH treatments. Rates of water loss were 8.6-fold lower, averaging 0.7 � 0.2 �g�mm^-2�h^-1, in larvae from senescent gall tissue and all RH treatments compared to control larvae from pre-senescent plants. Contrary to previous reports, the majority of the increased desiccation resistance (~85%) was due to reduced respiratory transpiration with the remainder being the result of a lowered cuticular permeability. Reduced cuticular rates of water loss were cued by the presence of a vapor pressure gradient between the larval hemolymph and environmental water vapor and were likely due to increases in cuticular lipids and/or production of the cryoprotectant glycerol. Metabolic rate was reduced by over four-fold, averaging 0.07 � 0.01 μl CO₂�g^-1�h^-1, in larvae from senescent gall tissue and all RH treatments compared to larvae from pre-senescent plants. The magnitude of the reduction in rates of metabolism indicated that these larvae had entered diapause. In addition, larvae entered diapause in response to removal from or degeneration of the gall tissue they feed on rather than seasonal changes in temperature or photoperiod. The low metabolic rates of the diapausing larvae likely allowed them to dramatically reduce their respiratory transpiration and total rate of water loss compared to non-diapausing controls. Thus, diapause, with its associated lowered metabolic rate, may be essential for conserving water in overwintering temperate insects, which may be dormant for six or more months of the year.