HANKISON, S.J.*; LEE, R.E.: Water content of the gall regulates susceptibility to inoculative freezing in larvae of the goldenrod gall fly, Eurosta solidaginis
We examined seasonal changes in susceptibility to inoculative freezing in larvae of the freeze-tolerant goldenrod gall fly, Eurosta solidaginis (Diptera, Tephritidae). During the autumn the goldenrod plant senesces and the water content of the gall decreases markedly from >50% to near 10%. In late September, when galls were green and water content was ~ 55%, more than half of the larvae within their galls froze when held at -2.5�C for 24 h and nearly all larvae froze when held -4 or -6�C. However, at this time less than 15% of larvae survived freezing at -6�C. Gall water content decreased to 33% in October and the number of larvae that froze at -4 and �6�C also decreased though the proportion of larvae that survived freezing increased markedly. By November, gall water content dropped to its lowest value (10%) and few larvae froze during exposure to subzero temperatures. During the winter, rain and melting snow transiently increased gall water content to values as high as 64% which caused many larvae to freeze at temperatures as high as -4�C. However, during periods of no precipitation the gall tissues dried and, as before, larvae were not as susceptible to inoculative freezing. These results indicate that in nature larvae may freeze earlier in the autumn and/or at higher temperatures than would be predicted based on the supercooling point of isolated larvae.