Endocrine changes associated with the starvation-induced premature metamorphosis in the yellow-spotted longicorn beetle, Psacothea hilaris

Abstract

Under 25 °C and a long day photoperiod, about half of the fourth instar Psacothea hilaris larvae molt to the fifth instar on day 13 and pupate about 18 days later; the rest pupate without a further larval molt with a mean fourth instar period of 24 days. However, starvation of fourth instar larvae exceeding a threshold weight induces premature pupation, resulting in the formation of small but morphologically normal adults. To clarify the endocrine basis for this premature pupation, hemolymph juvenile hormone (JH) and ecdysteroid titers were quantified during the fed and the starved periods. Normally fed fourth instar larvae exhibited two populations with regard to JH and ecdysteroid titers, one having JH titers ranging from 1.2 to 2.1 ng/ml through to day 13, the other, similarly high titers in the early part of the instar but low titers reaching 0.1 ng/ml on day 13. One population had ecdysteroid titers with a peak of 43 ng/ml on day 10, coinciding well with the period when some larvae normally molt to the fifth instar (day 13), the other, a small peak of 14 ng/ml on day 14 and a large peak of 70 ng/ml on day 17 coinciding well with the period just before the prepupa stage. When fourth instar larvae were starved after 4 days of feeding, JH titers decreased sharply in the next 24 h and never recovered, and a small but significant increase (to 21 ng/ml) in ecdysteroid titers was observed on day 6, followed by a large peak of 63 ng/ml on day 11. Altogether, these results suggest that starvation induces a rapid decline in the JH titer, and this cues the early occurrence of a small ecdysteroid peak that commits larvae to early metamorphosis.