HYPOXIA REDUCES REPRODUCTIVE SUSCEPTIBILITY OF PLUM CURCULIO (COLEOPTERA: CURCULIONIDAE) TO IONIZING RADIATION

Abstract

Ionizing irradiation is used in Florida and Hawaii to disinfest several fruits and sweetpotatoes of fruit flies (Diptera: Tephritidae) or other insects (Hallman 2004a). Importation of fruit irradiated against 11 fruit fly species and the mango seed weevil, Cryptorhynchus mangiferae (F.) (Coleoptera: Curculionidae), has been approved (APHIS 2002). The treatment shows promise for widespread implementation, as it is safe, broadly efficacious, accepted by consumers, cost-effective, may be applied after packing, and widely tolerated by fresh agricultural commodities (Hallman 2002). A large body of research has determined minimum absorbed doses of ionizing radiation to prevent development or reproduction for many different species of arthropods . Some of this research has shown that hypoxia can reduce some of the detrimental effects of irradiation to insects used in sterile release programs. The effect of hypoxia on irradiation disinfestation treatment efficacy has not been studied until recently, although some agricultural commodities are stored under hypoxic conditions, a strategy that is increasing in application. Hallman (2004a) found that while no oriental fruit moth, Grapholita molesta (Busck) (Lepidoptera: Tortricidae), fifth instars developed to the adult stage when irradiated at 200 Gy in ambient atmosphere, 5.3% of those irradiated in hypoxic atmospheres developed to the adult. Hypoxia caused a small increase in the ability of apple maggot, Rhagoletis pomonella (Walsh) (Diptera: Tephritidae), third instars to emerge as adults after irradiation (Hallman 2004b). Plum curculio, Conotrachelus nenuphar (Herbst) (Coleoptera: Curculionidae), is native to the eastern Neartic and is a quarantine pest of stone and pome fruits exported from the United States and Canada. Hallman (2003) determined that a minimum absorbed dose of 92 Gy (the maximum recorded dose when 80 Gy was sought) prevented reproduction of adults, the most radiotolerant stage of the insect. The objective of this research was to determine the effect of hypoxia on reproduction of the plum curculio. Plum curculios originally collected near Gainesville, Florida, were obtained from a colony at the United States Department of Agriculture, Agricultural Research Service facility in Byron, Georgia. The insects were reared at about 25EC, 70% RH, 12: 12h (L: D), on immature apples that were picked when about 3 cm in diameter. Larvae emerging from the apples were placed on sterilized potting soil until adult emergence. A radiation source of 137Cs (Husman Model 521A, Isomedix, Inc., Whippany, NJ) that delivered a gamma ray dose rate of about 40 Gy min-' was used in this research. Routine dosimetry was done with radiochromic film (Gafchromic MD-55, ISP Technologies, Inc., Wayne, NJ) and read with a spectrophotometer at 510 nm (Milton Roy Spectronic 401, Ivyland, PA). Adult plum curculios were irradiated in ambient atmospheres and in atmospheres of mostly nitrogen. Cylinders (polyvinyl chloride, 37.5 cm inside length, 10 cm inside diameter) fitted on one end with a screw cap sealed with vacuum grease and on the other end with 2 brass, barbed-nipple compression hose fittings (25 mm long, 4-mm inside diameter) were constructed. Two-week-old plum curculio adults were placed inside the cylinder with a few immature apples, and the atmosphere was purged through the hose fittings with nitrogen at a pressure of about 3 kPa for 2 minutes 20, 16, and 2 h before irradiation with an absorbed dose of 40 Gy. After purging, the hose fittings were sealed with rubber septa and the cylinders held at about 24?C. About 1.5 h after irradiation, the cylinders were opened to return the insects to ambient atmosphere. There were 6 replicates of 300-600 each for adults irradiated in ambient or hypoxic atmospheres. Controls consisted of 6 replicates of 30-100 insects each in a cylinder under ambient atmosphere that were not irradiated. After irradiation, adults were maintained on immature apples at about 25?C, and mortality was determined every week. The apples were replaced every week and maintained at 25?C for development of any immatures inside. Larvae emerging from apples were collected every 1-3 days and placed on potting soil for pupation and adult emergence. After larvae were no longer emerging, the apples were opened and any remaining insects collected. Analyses of variance were done with Prism 4 (www.graphpad.com). The mortality rate for the irradiated plum curculios was greater than that for the control until 15 weeks after irradiation when the rate for the control accelerated (Fig. 1). There was no significant difference among the treatments for time to reach 95% mortality (overall mean = 27.4 + 2.6 weeks, F = 1.56, P = 0.26, df: 2,15). Reproduction under irradiation and hypoxia was increased by over 20-fold compared with irradiation in ambient atmosphere (Table 1). Reproduction was greater than the 34.4 Gy for 4th instars per female reported by Hallman (2003) for