Performance of Agasicles hygrophila (Coleoptera: Chrysomelidae), a biological control agent of invasive alligator weed, at low non-freezing temperatures

Abstract

The flea beetle, Agasicles hygrophila, was introduced to control the alligator weed, Alternanthera philoxeroides, in southern China and redistributed for over 20 years. The beetle has succeeded in establishing local field populations. Temperature, especially extreme low temperature in winter, is hypothesized to be a key factor determining the distribution of A. hygrophila. We studied the adult reproduction and leaf consumption, egg hatching rate, larval and pupal survival and development of A. hygrophila in response to low non-freezing temperatures. Female and male adults of A. hygrophila survived at 4°C for 11.4 and 14.2 days, respectively, and adult longevity increased with increasing temperature from 4 to 12°C. Adult longevity was significantly longer at 12°C than at 25°C, and the fecundity at 12°C was approximately 10% of that at 25°C. When A. hygrophila eggs, first instar larvae and pupae were kept at 4–12°C for 1, 4, 7 or 10 days, respectively, and then transferred to 25°C, over one third of eggs hatched after cold treatment at 4°C for 7 days, with an average egg development duration of 3.6 days excluding the cold treatment period. Egg hatching rate increased as temperature during the cold treatment increased and the cold treatment duration reduced. Eggs pretreated at 12°C and those kept constantly at 25°C did not differ in their hatching rates. The first instar larvae of A. hygrophila could not survive 12°C or lower, and exposure to these low temperatures resulted in 100% mortality and a lifespan shorter than 1 day. Eclosion rate of A. hygrophila pupae was 71, 60, 24 and 15% after cold treatment at 4–12°C for 1, 4, 7 and 10 days, respectively, which was lower than that at constant 25°C (over 93%) but did not differ among the cold treatments. Comparing with the prediction in 1980s that A. hygrophila could not overwinter further north than the approximate position of the 9°C isotherm, our recent survey showed that A. hygrophila has now distributed in the regions with January isotherms of 0–9°C in China. These results indicated that A. hygrophila has the capacity to stand relatively low non-freezing temperatures for short durations, which would help it to overwinter and establish natural populations in some areas, especially in areas where protected cultivations are extensive and ambient temperatures are not as low as those in the open field.