Heliothis armigera (Hübner) is the most common pest attacking maize on the Darling Downs and in some seasons can be present in economic numbers, particularly on the newly emerged silks of the maize cob. In spite of the size of the population on infested cobs, the damage caused by its presence seems to be consistent from season to season. During the study it was established that, irrespective of the initial egg density recorded on the silks, the population of final instar larvae per infested cob rarely exceeded unity. The mortality factors associated with an infestation on maize were monitored and the extent of egg and larval parasites and diseases was established as being insignificant in causing this decline. These results would be expected in view of the protection afforded by the tight husks around the cob. The decline in population within the cob has therefore been associated with the activity of
intraspecific competition (namely cannibalism) with the major proportion of the total population decline taking place during the first and second instars. This time period coincides with the stage of larval feeding when larvae begin to enter the cob by feeding along the silks. For most lines of maize, this area represents a restricted "arena" where larval density increases towards the cob and, as such, provides a greater opportunity for conflict between larvae.
Laboratory studies of the nature of larval growth were undertaken in order to prepare a suitable method of sampling and identifying field populations of H. armigera.
The relationship of development time to temperature was examined using the Thermal Summation method. From the analysis of these data the Threshold of Development was estimated to be 10.7° and the development time 260 day degrees (in excess of
10.7°) for development from hatching to final instar.
In order to identify the stage of development of the larvae, head capsule measurements were made of laboratory bred larvae. However, it was established that the use of head capsule widths did not give a consistent indication of the stage of larval development due to variation recorded in the number of larval moults. The cause of this variation was outside the scope of this study but was considered to be due to the nutritional qualities of the larval food. This nutritional deficiency increased development time and resulted in more moults.
For field collected material the recording of head capsule widths indicated that there were commonly six larval instars but these were not precisely defined. There was a large degree of overlap in the head widths of third, fourth and fifth instar larvae.
A method of rearing Heliothis
sp. on an artificial diet is discussed but its usefulness is considered as limited to four consecutive generations, after which larval and pupal development times become more variable, the incidence of malformed adults increases and the pupal weight decreases.