Some aspects of the biology and control of Earias huegeli Rog.

Abstract

Descriptions are given of the egg, larva, pupa and moth of Earias huegeli Rog. and of the moth of the other important economic species in Australia, Earias vitella (F.) as well as characters for use in the separation of the two species. The distribution and economic status of the two species are also given.

Studies were carried out on the biology of E. huegeli. These showed that no larvae emerged from eggs held at or below 13.0°C. The minimum temperature for development appeared to lie in the range of 13.0 to 16.2°C. The mean developmental period of eggs varied from 60.0 hours at 37.5°C. and 53 per cent, relative humidity to 372.3 hours at 16.2°C. and 57 per cent, relative humidity. Humidity differences produced large changes in the developmental period of eggs only at 37.5⁰C. where the mean period increased from 70.5 hours at 49.0 per cent, relative humidity to 120 hours at 0 per cent, relative humidity. High egg mortalities occurred when eggs were subjected to extremes of temperature and humidity. These were 0 per cent, relative humidity combined with 16.2 and 37.5⁰ C., and 100 per cent, relative humidity combined with 16.2, 23.5 and 37.5°C.

The five larval instars were differentiated by the use of head capsule size ranges. The mean developmental periods of the larval instars were:- first instar, 3.5 days; second, 2.8 days; third, 3,1 days; fourth, 3.1 days; fifth 4.6 days. The mean maximum and minimum temperatures during the observation period were 24.7 and 22.6°C. respectively.

No pupation occurred below 15.5°C.; however, survival for protracted periods is possible at 12.5°C. The mean developmental period of larvae varied from 12.2 days at 36.0°C.to 53.3 days at 15.5°C.

Studies on larval feeding behaviour showed that squares and young bolls were preferred to the more mature bolls. Larvae fed on all parts of the squares and bolls and normally remained at the one feeding site unless forced to move by adverse conditions. Most squares and young bolls attacked by larvae were shed by the plant while larger damaged bolls were retained. Only a small percentage of the potential yield of the damaged bolls remained available owing to the losses from larval feeding, distortion of the bolls and to the entry of rotting organisms. Larvae were able to change feeding sites, the degree of success depending upon the age of the larvae and the availability of an alternative site. Larvae also fed in the terminals of the cotton plant. This feeding, which normally takes place before the production of squares, resulted in loss of apical dominance and the consequent production of a bushy plant.