Egg Stage of Homalodisca vitripennis and Homalodisca liturata (Hemiptera: Cicadellidae): Ovometrics, Embryonic Development, and Nonparasitic Mortality
Morphometric and developmental aspects of the egg stage of glassy-winged sharpshooter, Homalodisca vitripennis (Germar), and Homalodisca liturata Ball were investigated. Considerable overlap exists between the two species with respect to egg length, width, and clutch size, making these variables unreliable in identifying their eggs to species, Embryonic development of the two species was observed at constant temperatures ranging from 8.7 to 40.4 degrees C. No signs of development were observed at 8.7 degrees C, development was aborted early in development at 11.5 degrees C, and egg masses were desiccated at 40.4 degrees C. For both species, rates of embryonic development increased linearly with increasing temperatures from 16.7 to 25.6 degrees C, peaked at 31.2 degrees C, and decreased between 32.9 and 35.0 degrees C. Based on linear regression, the minimum thresholds for embryonic development were 12.0 and 12.6 degrees C, and heat unit requirements were 111.4 and 104.5 degree-days for H, vitripennis and H. liturata, respectively. We quantified four physiological states of embryonic development, based on eye spots being normally developed, centrally developed, reversed, or undeveloped. The prevalence of these states did not differ significantly between the two species and comparing data from the laboratory versus the field. However, higher experimental temperatures tended to increase the prevalence of abnormally developed eye spots. Nonparasitic mortality of Homalodisca eggs in five field host plants was attributed to undeveloped-abnormally developed embryos and to the inability of first-instar nymphs to emerge from eggs. These causes of mortality varied among host plants. Eggs on mule fat tended to have a higher proportion of undeveloped-abnormally developed eggs, whereas on jojoba, Simmondsia chinensis (Link) Schneider, a higher prevalence of dead but fully developed embryos was observed inside eggs.