Temporal Distribution of Egg Hatch for TwoHomalodiscaSpp. (Hemiptera: Cicadellidae) Under Constant Temperatures

Abstract

The glassy-winged sharpshooter (GWSS), Homalodisca vitripennis (Germar) (=H. coagulata (Say)) (Takiya et al. 2006), is an exotic pest in California and an important vector of the xylem-limited bacteria, Xylella fastidiosa Wells et al., the causal agent of several plant diseases including Pierce's disease of grapes, phony peach disease, almond leaf scorch, alfalfa dwarf, and oleander leaf scorch (Blua et al. 1999; UCOP 2000; Varela et al. 2001). The closely related smoke-tree sharpshooter (STSS), H. liturata Ball (=H. lacerta (Fowler)) (Burks & Redak 2003), is native to California and also a vector of Pierce's disease and oleander leaf scorch (Freitag et al. 1952; Purcell et al. 1999). Eggs of both Homalodisca spp. are laid just below the epidermis of leaves and other plant parts including stems and fruits in clusters, with eggs oriented nearly parallel to one another. Al-Wahaibi & Morse (2003, 2009) described the relationship between the rate of embryonic development of if. vitripennis and temperature in the range of 16.7-35.0°C. We present here a follow-up study aimed to elucidate the distribution of emergence of first instars of if. vitripennis and if. liturata across time, and to determine whether different constant egg incubation temperatures would modify that distribution. In order to satisfy these objectives, 2 experiments were conducted. The first examined the daily distribution of egg hatch of H. vitripennis and H. liturata at 8 constant temperatures. The second evaluated the diurnal distribution of hatching of H. vitripennis at 3 constant temperatures. For the first experiment, egg masses were collected by caging field-collected adults ofH. vitripennis on potted, rooted cuttings of chrysanthemum, Dendranthema grandiflorum (Kitam) (White Diamond cultivar, Growlink Co., Ventura, CA), in sleeve cages which were held at 23°C, 5070% RH, and 14:10 L:D photoperiod inside an insectary room. Separate plants were exposed to colonies of the 2 leafhopper species for 24 h. Plants with egg masses of the 2 species were then incubated at the same time in situ (inside leaves intact on plant) in growth chambers (Percival Scientific, Inc., Perry, LA) at 13.0, 16.7, 19.7, 25.6, 31.2, 32.9, 33.4, and 35.0°C (mean temperatures based on HOBO data loggers, Onset Computer Co., Bourne, MA). For all temperature treatments, relative humidity varied between 50-70%, and the light regime was set at 14:10 L:D photoperiod. When hatching was imminent (presence of large dark eye-spots), leaves containing egg masses were excised and placed inside 100-mm diameter Petri dishes on top of moist tissue paper. This allowed easier and more accurate observation of emergence of nymphs from individual eggs. Egg masses inside Petri dishes were retained at the same incubation temperatures as the egg masses were exposed to prior to excision of leaves. Thereafter, egg hatch was monitored twice daily, in the morning (8-10 AM) and afternoon (4-6 PM). The proportion of eggs hatching during the first day of hatch and during the following 6 d (i.e., 7 d in total) was calculated (out of the total number of hatched eggs per egg mass). Each egg mass was used as a replicate. Following this, the proportion of eggs hatching during each of d 1, 2, 3, and 4-7 were compared among temperature treatments by ANOVA. Means shown to be significantly different by ANOVA were further compared to each other and separated with the Tukey-Kramer HSD procedu e. In addition, the proportion of eggs hatching (pooled across temperature treatments) during ach of dl, 2, 3, and 4-7 were compared between the 2 Homalodisca spp. by £-tests. All statis ical tests were performed with JMP IN (SAS Institute 1996). Most hatching occurred during the first day (79%). The percentage of eggs hatching during the second day averaged 19%, with 2% on the third day. Combined egg hatch from d 4 to 7 eggs averaged 0.2%. There was no significant difference beween the 2 Homalodisca species for the proportion of hatched eggs on each of d 1, 2, 3, or 4-7 (all £-tests, P > 0.05). Temperature had a slightly significant effect on first day hatching (ANOVA, P = 0.0247) (Fig. 1). Generally, greater first day hatch was observed at higher temperatures, with hatching at 33.4°C being significantly higher than at 19.7°C. Temperature had a marginally significant effect on the proportion of second day-egg hatch (P = 0.0502), with slightly higher hatching at 16.7 and 19.7°C than at the higher temperatures. Third day hatching was significantly higher at 13.0°C than at all other temperatures (P = 0.0003). There was no significant effect of temperature on egg hatch over d 4-7 (P = 0.1553).