Effects of heat hardening on life parameters and thermostability of Bradysia odoriphaga larva and adults

High temperature in summer is a key barrier for Bradysia odoriphaga
(Diptera: Sciaridae), a serious pest of vegetables in China. Because mild
heat hardening can enhance the thermotolerance of many insects, we
determined if heat hardening was beneficial for B. odoriphaga adaptation
to high summer temperatures. We studied the biological performance,
antioxidant responses and hsp70 and hsp90 expression of B. odoriphaga 4th
instar larvae and adults after pre-exposure to mild heat hardening
temperatures (30-36°C). The tolerance of larvae to severe heat stress was
enhanced after mild heat hardening (32-36°C). However, life parameters
were significantly inhibited, development was delayed, longevity was
shortened and fecundity was decreased. Mild heat hardening (30-34°C)
caused serious adverse effects on life parameters and less enhancement of
heat resistance for 4th larvae. Mild heat hardening at 36°C weakened the
heat tolerance of adults. The activity of four antioxidant enzymes in B.
odoriphaga changed dramatically after heat hardening at 32-36°C. CAT and
GST maintained successive higher activities, and POD also had higher
activities in larvae after 48 h recovery at 25°C, pupae, and adults.
Simultaneously, the expression levels of hsp70 and hsp90 increased
significantly upon thermal hardening. Higher expression levels were also
detected in larvae after 48 h recovery at 25°C, pupae, and adults. Mild
heat hardening played an important role in heat tolerance plasticity of B.
odoriphaga larvae. The antioxidant system and hsp70 and hsp90 genes were
relevant to this adaptation process.