The Journal of Experimental Biology 203, 783–789 (2000) 783 Printed in Great Britain © The Company of Biologists Limited 2000 JEB2373 EXTREME RESISTANCE TO DESICCATION IN OVERWINTERING LARVAE OF THE GALL FLY EUROSTA SOLIDAGINIS (DIPTERA, TEPHRITIDAE) HANS RAMLØV1,2,* AND RICHARD E. LEE JR2 1Roskilde University, Department of Life Sciences and Chemistry, Building 16.1, PO Box 260, DK-4000, Roskilde, Denmark and 2Department of Zoology, Miami University, Oxford, OH 45056, USA *Author for correspondence at address 1 (e-mail: [email protected]) Accepted 22 November 1999; published on WWW 26 January 2000 Summary During winter, larvae of the goldenrod gall fly Eurosta transition temperature of 40 °C, the thermal dependence of solidaginis are exposed for extended periods to severe low the permeability increased abruptly to 0.0400 µgh−1 ambient temperatures and low humidities within plant cm−2 Pa−1 °C−1. Larvae treated with hexane and acetone galls. The resistance of these larvae to desiccation at remained remarkably resistant to water loss. However, various temperatures and humidities, the transition treatment with chloroform:methanol increased the water (critical) temperature, and the effects of treatment with loss rate approximately 25-fold. organic solvents on the larval rates of water loss and on During desiccation at 4 °C and 4 % relative humidity changes in osmolality during desiccation were examined. for 21 days, E. solidaginis larvae showed a mass loss of The water loss rates of the flesh fly Sarcophaga crassipalpis 18.5±4.4 % (mean ± S.E.M., N=6). Animals dried under the under desiccating conditions were also measured. same conditions over the same period showed a The water permeability of the cuticle of E. solidaginis haemolymph osmolality of 851±75 mosmol kg−1 (N=4). larvae was very low (0.038 µgh−1 cm−2 Pa−1 at 20 °C and Larvae freshly removed from the galls showed a 4 % relative humidity) compared with that of larvae of haemolymph osmolality of 918±67 mosmol kg−1 (N=3). A other species. The value for E. solidaginis is equivalent to higher osmolality in the dried compared with the fresh that of the very drought-resistant larvae of the tenebrionid larvae would have been expected. The present observation beetle Tenebrio molitor (0.038 µgh−1 cm−2 Pa−1 at 30 °C). In suggests that important ions in the haemolymph may have contrast, the permeability of larvae of the flesh fly been excreted or rendered osmotically inactive during Sarcophaga crassipalpis at 20 °C and 4 % relative humidity desiccation. was 0.331 µgh−1 cm−2 Pa−1. The thermal dependence of the cuticular permeability increased with temperature by approximately 0.0010 µgh−1 Key words: desiccation, cold-hardiness, insect, water balance, cm−2 Pa−1 °C−1 in the interval between 4 and 40 °C. At the winter, Eurosta solidaginis, Sarcophaga crassipalpis. Introduction The emergence of arthropods as the most successful group from Texas into southern Canada (Lee et al., 1995). Third- of land animals required solutions to problems associated with instar larvae of this species overwinter within spherical stem water balance. Their high surface area to body volume ratio galls on several species of goldenrod (Solidago spp.). Although posed particular challenges to life in desiccating habitats. the above-ground portion of their host plant senescences and Comparative physiologists have often turned to arthropods dies in late summer or early autumn each year, the stem and living in deserts to study extreme adaptations for maintaining gall often remain upright and extend above the snowpack water balance. These investigations examined the reduction of throughout the winter. This species has been studied transpiratory losses via cuticular and respiratory pathways as extensively with regard to its freezing tolerance (for references, well as osmoregulatory and excretory mechanisms for water see Storey and Storey, 1988; Baust and Nishino, 1991; Lee et conservation (for reviews, see Edney, 1977; Hadley, 1994). al., 1995). Larval freezing is promoted by the susceptibility of However, alpine and polar arthropods or species overwintering its cuticle to inoculative freezing by external ice and by the ice- in temperate regions have received little attention despite the nucleating activity of calcium phosphate crystals within the fact that they must also sometimes endure severely desiccating Malpighian tubules (Layne et al., 1990; Mugnano et al., 1996). conditions. Survival of freezing to temperatures as low as −55 °C is The goldenrod gall fly Eurosta solidaginis (Diptera, promoted by various responses including the synthesis Tephritidae) is widely distributed in North America, ranging of a multi-component system of low-molecular-mass 784 H. RAMLØV AND R. E. LEE cryoprotectants (glycerol, sorbitol and trehalose), that in glass test tubes fitted with netting over Drierite during collectively may reach levels exceeding 1 mol l−1, and exposure to the various conditions. increases in the unsaturation of membrane fatty acids (Storey The surface area of the larvae was calculated from the initial and Storey, 1992; Bennett et al., 1997). mass of the animals using Meeh’s formula: Previous work on water balance in this species established S=kW0.667 (1) that the body water content remains relatively constant throughout the winter. In a Texas population of this species, (Wigglesworth, 1945; Hadley, 1994), where S is the calculated Rojas et al. (1986) reported that between November and surface area (mm2), k (mm2 mg−1) is a species-specific constant January the larval water content remained constant between 60 and W is the mass (mg) of the animal. A value of and 64 % even though the water content of plant gall tissues 2.56 mm2 mg−1 was calculated for k by measuring the surface decreased from 65 to 20 %. Furthermore, the synthesis and area of E. solidaginis larvae (N=5) of known mass. The surface accumulation of the cryoprotectant glycerol within the larvae area was obtained by cutting open the body of the larva and was strongly correlated with the decrease in the water content expressing the contents before measuring the surface area on of the gall, suggesting that plant senescence triggers glycerol millimetre square graph paper. production. In an Ohio population, two separate studies found The vapour pressure deficit ∆P between the surrounding air that the water content of larvae remained relatively constant and the haemolymph of the animals was calculated using the between 59 and 63 % during the winter (Lee et al., 1995; equation: Bennett and Lee, 1997). Layne and Medwith (1997) reported ∆P = [55.556/(55.556 + O) − RH/100]P * , (2) similar results for a population from Pennsylvania. w Recent work by Layne (1991, 1993) and Layne and where Pw* is the standard vapour pressure of pure water at the Medwith (1997) has demonstrated the extreme variability of given temperature (Lundheim and Zachariassen, 1993) and O temperatures and moisture conditions experienced by larvae is the osmolality of the larvae. All calculations using this during the winter. On a daily basis, gall temperatures often formula were corrected for Pw* (Weast, 1986). The ∆P values vary by 20–30 °C and reach absolute temperatures in excess of were: 4 °C, 4 % RH, 767.8 Pa; 4 °C, 75 % RH, 190.6 Pa; 20 °C, 35 °C during September and October in western New York 4 % RH, 2206 Pa; 20 °C, 75 % RH, 546.5 Pa. The following (Layne, 1991). Furthermore, the water content of gall tissues values were all based upon a relative humidity of 4 %: 30 °C, varied markedly from 10 % to more than 60 % during the 4003 Pa; 37 °C, 5920 Pa; 40 °C, 6960 Pa; 45 °C, 9043 Pa; 50 °C, winter and generally matched precipitation patterns (Layne, 11.64 kPa. 1993). Fluctuations in the water content of the gall tissues Osmolality was determined by drawing haemolymph into a significantly affects the susceptibility of the larvae to capillary from animals freshly removed from the galls and inoculative freezing by external ice that forms in the plant from animals kept at 4 °C at 4 % RH for 21 days. Samples tissues and thus influences the temperature at which the larvae (10 µl) were measured using a Wescor 5500 vapour freeze and the number of freeze–thaw cycles they will pressure osmometer. The osmolality of fresh larvae was − experience seasonally (Layne et al., 1990; Layne, 1993). 918±66 mosmol kg 1 (mean ± S.E.M., N=3). Consequently, overwintering larvae are exposed for months To determine the water loss rate at various times during to variable and low gall water contents and sometimes to exposure, living larvae (N=4–6) were exposed to 4 % and 75 % severe low ambient temperatures within plant galls that offer RH at 4 ° and 20 °C, and the mass loss was measured at little protection against these environmental extremes. Despite intervals during this period. To investigate whether water loss these extreme conditions, larvae maintain a relatively constant rate was under physiological control, larvae (N=6 in each water content. Since surprisingly little is known concerning the treatment) were killed by exposing them to cyanide for 5 h at water relations of this species, we examined the resistance of room temperature. They were then exposed to 20 °C and 4 % larvae to desiccation at different temperatures and humidities, RH. The onset of pupariation was identified by a decrease in the transition (critical) temperature above which transpiration larval activity coupled with an initial smoothing of the cuticle rates increase rapidly and the effects of treatment with various and later by cuticular darkening. solvents on larval rates of water loss. The critical or transition temperature was determined by exposing larvae (N=6) to temperatures of 4, 20, 30, 37, 40 and 45 °C for 24 h and to 50 °C for 7 h over Drierite.