The Effect Of Low Temperature On Feeding Of Plutella Xylostella(Lepidoptera: Plutellidae)

Plutella xylostella(Lepidoptera: Plutellidae) is a very important pest that can cause huge damage to Brassica plants in China. The overwintering stage and overwintering geographical distribution of P. xylostella in China remains unclear. To clarify the effects of low temperatures and food abundance on overwintering survival, we tested the critical low temperatures to stop feeding and start feeding; clarified the effect of starvation and low temperatures on larvae cold tolerance; explored the effect of periodical feeding on larvae survival under different low temperatures and tested the effects of cold spells in early spring on the life traits. The results are as follows:1 We tested the critical low temperatures of P. xylostella to stop feeding and start feeding. We exposed larvae to different constant low temperatures from 1°C to 13°C for 4h, and counted the number of larvae in feeding. We found that the 2nd instar larvae stopped feeding at 2°C, and 3rd and 4th instar larvae stopped feeding at 1°C. We also simulated dynamic cooling and heating process, by decreasing temperatures from 20°C to-9°C(0.2°C/min decrease rate) with 15 min balance time at-9°C, then increasing temperature to 20°C( 0.2°C/min increase rate). The 3rd instar larvae stopped their feeding at 4.7°C and start feeding at 12.2°C; the 4th instar larvae stopped feeding at 4.6°C and start feeding at 12.6°C.2 We clarified the effect of starvation and low temperatures on cold tolerance of the larvae of P. xylostella. No 1st and 2nd instar larvae could survive at 6°C, but 16% of the 3rd or 43% of 4th instar larvae could survive to pupa stage when they were provided with food at 6°C. We also tested the effect of low temperatures and exposure time on the survival of the larvae of P. xylostella. We found that 1) the larvae previously supplied food before cold treaments had a higher survival rate at-5°C or-10°C than those without food; 2) under cold treaments of 0°C, 2nd instar larvae which were previously supplied food before had a higher survival rate than those without food; but the 3rd or 4th instar larvae without food had a higher survival rate.3 We explored the effect of periodical feeding on survival of P. xylostella under different low temperatures. We found that 1) the 2nd, 3rd, 4th instar larvae periodically supplied food had a higher survival rate than those without food when exposing them at 0°C and 5°C for 4d; 2) the 2nd, 3rd instar larvae periodically supplied food had a higher survival rate than those without food when exposing them at 0°C, 5°C or-5°C for 6d, but the 4th instar larvae had a lower survival rate at-5°C when they were periodically supplied food than those without food; 3) The survival rate decreased with the increase of exposure time at low temperatures.4 We tested the effects of cold spells in early spring on the life traits of P. xylostella. To simulate cold spells commonly occurred in the early spring of Wuhan, we exposed the 4th instar larvae or firstly eclosed adults reared at 20°C to 1°C, 4°C or 7°C for 3d or 6d. We found low temperature experience on larval stage had no effect on larval survival; however, low temperature treatments significantly extened the development time from 4th-instar to pupal stage and the duration time increased as the decrease of treated temperautres. The pupae weight and fecundity increased when exposing larvae to lowtemperatures. We also found that low temperatures experience on adult stage increased adult longevity, but depressed fecundity.