Radiographic studies of Sitophilus zeamais Mots. in wheat kernels

Abstract

Ninety-nine wheat kernels infested by Sitophilus zeamais Mots., were radiographed daily, to study development from egg to adult and behavior of the insects within the kernels. Enlarged photographic prints were prepared from these radiographs to facilitate interpretation. Instars were identified by examining prints and by measuring the widths of tunnels on radiographs and also by measuring the widths of the head capsules of the larvae dissected from kernels. On hatching, each larva was observed to increase the width of the tunnel for a few days, then for 1–3 days the width of the tunnel remained static after which it was again widened. There were four periods of tunnel widening. Since there were four corresponding distinct sizes of head capsules for larvae dissected from kernels, we conclude that the four periods of tunnel widening represent the periods of larval growth and that the periods of static tunnel width define the time of moulting. The average widths of tunnels were successively 0·33, 0·55, 0·90 and 1·49 mm. The mean developmental periods for stadia 1–4 at 27°C and 69 ± 3% r.h. were 3·6, 4·7, 4·8 and 5 days, respectively. Average developmental periods for prepupa, pupa and pre-emerged adult stages were 1, 5·3 and 5·3 days. The average length of the developmental cycle from the start of a 3-day oviposition period to emergence of the adult was 36·3 days. Of the eggs in dissected kernels, 42, 37 and 21 per cent were deposited in endosperm, germ perimeter, and germ center of the kernels, respectively. Of 26 emerged adults, 6 prepared the pupal chamber in one side of the kernel; 8 diagonally across the crease; and 12 parallel to the crease but occupying portions of both kernel halves. Data obtained from these studies are compared with data from previous radiographic studies of Sitophilus oryzae (L.) and S. granarius (L.).