Effect of Temperature on Egg Development of Diaprepes abbreviatus (Coleoptera: Curculionidae)

Abstract

The Diaprepes root weevil, Diaprepes abbreviatus (L.), has become a key pest of citrus and ornamental plants in Florida since its introduction to the state in 1964. Adult D. abbreviatus feed and oviposit on leaves of many plant species in addition to citrus including crops such as cassava, cotton, peppers, potatoes, sugar cane, and sweet potatoes. As neonate larvae emerge, they drop to the ground and crawl on the surface for a period of up to three hours (Jones & Schroeder 1983) before burrowing into the soil to feed on roots. Eggs are subject to predation by ants and earwigs (Tryon 1986) and parasitization by wasps such as the eulophid Quadrastichus haitiensis Gahan. Recent efforts to establish exotic egg parasitoids in Florida have met with some initial success (Hall et al. 2000), but a more complete understanding of host biology may contribute to effective rearing and release. Lapointe and Shapiro (1999) and Lapointe (2000) described the effects of humidity and temperature on larval and pupal development of D. abbreviatus. To complete the description of the thermal requirements for development of D. abbreviatus, I report here the response of eggs of D. abbreviatus to a range of temperatures and describe the thermal upper and lower limits of egg development. Male and female adult D. abbreviatus were obtained from a laboratory colony at the U.S. Horticultural Research Laboratory, Orlando, FL, now located at Ft. Pierce, FL. Approximately 20 male! female pairs were placed in a screened cage (30 by 30 by 30 cm) and provided with bouquets of citrus foliage as food and strips of wax paper for oviposition (Wolcott 1933). Eggs on wax paper strips were collected from cages twice daily and placed in plastic vials with plastic caps in temperaturecontrolled growth chambers with a photoperiod of 12:12 L:D and constant temperatures of 12, 15, 18, 19, 21, 22, 26, 30, and 32?C. Vials were misted periodically with sterile water and re-capped to avoid desiccation of the eggs. The wax paper strips were gently separated 2 to 3 d after oviposition to facilitate emergence of the neonate larvae. Vials were checked daily for neonate larvae and time to hatch was recorded. After egg hatch commenced, the number of eggs that failed to hatch after 7 consecutive days of no neonate emergence in the vial was recorded. The mean and median number of days to egg eclosion and 95% confidence intervals were calculated (Snedecor & Cochran 1967). The developmental rate (inverse of median no. days to eclosion) was plotted against temperature and a linear regression was calculated (Legg et al. 2000). Wolcott (1936) reported that eggs of D. abbreuiatus hatched 7 d after oviposition and suggested that this was unaffected by temperature. In this study, no eggs hatched when held at constant temperatures of 12 or 32?C. The percentage of unhatched eggs ranged from 6% at 18?C to 43% at 300C (Table 1). For the constant temperatures regimes tested here, there was a positive linear relationship (y = 0.009x 0.108, r2 = 0.99) between developmental rate based on median values and temperature between 15 and 30?C (Fig. 1). Extrapolation based on the regression equation yielded a median functional lower developmental threshold of 12.0?C. The upper thermal limit for