The regulation of the first peak of haemolymph juvenile hormone esterase activity during the last stadium of the cabbage looper, Trichoplusia ni

Abstract

The role of the head and a juvenoid, epofenonane, in the regulation of the first peak of haemolymph juvenile hormone esterase activity was examined during the fifth (last) stadium of the cabbage looper, Trichoplusia ni. The topical application of epofenonane to unligated day-1 larvae brought about an average 36% increase in the haemolymph juvenile hormone esterase activity. Epofenonane treatment also resulted in a 2-fold increase in the juvenile hormone and α-naphthyl acetate esterase activity of the whole body of unligated larvae suggesting that the juvenoid causes an increase in esterase biosynthesis or activation. Ligation and starvation prevented the appearance of the first peak of haemolymph juvenile hormone esterase activity. Application of epofenonane to newly ligated larvae had no effect whereas it significantly increased the juvenile hormone and α-naphthyl acetate esterase activity of the haemolymph in larvae ligated for 24 h prior to the treatment. In newly starved larvae, epofenonane increased the haemolymph juvenile hormone and α-naphthyl acetate esterase activity above respective controls. In 24 h starved larvae, juvenile hormone esterase activity of the haemolymph increased in response to epofenonane, head extract, or a combination of head extract injection and epofenonane application whereas α-naphthyl acetate esterase activity increased only to head extract or to a combination of head extract injection and epofenonane application. Injection of head extracts of day-1, fifth-instar larvae into the same aged ligated larvae increased both the juvenile hormone and α-naphthyl acetate esterase activity in the haemolymph by about 3-fold 5 h later compared to the respective controls. This increase, however, was not reflected in a similar increase in the whole body esterase levels suggesting that head extract may have caused a release of esterases from tissues into the haemolymph. Injection of head extract from day-1 larvae into the same aged unligated larvae inhibited the normal increase in the haemolymph juvenile hormone esterase activity seen between day 1 and 2 and no inhibition was noted with injections of body extracts or bovine serum albumin. Head extract injection significantly decreased the juvenile hormone esterase activity in the whole body, and haemolymph of unligated and 24 h ligated larvae treated with epofenonane. The head-extract-induced inhibition of haemolymph juvenile hormone esterase activity was dose dependent and the esterase activity in the haemolymph was unaffected when incubated in vitro with head extract for a period of 4 days at 30°C.