Heliothis punctigera Wallengren and H. armigera (Hubner) are major insect pests in Australia. Larvae of both species feed on a wide range of economically important agricultural and horticultural crops as well as a number of native and exotic weeds. Control of Heliothis outbreaks relies heavily on insecticides, a tactic which is associated with increasing problems of insecticide resistance, environmental contamination and human health hazards. Approaches to pest management which are less reliant on insecticides require a basic understanding of the Heliothis life system. In this study I aimed to improve our understanding of dormancy strategies of Heliothis spp. Pupae in temperate Australia, in order to examine how these strategies influence survival in a region and how they relate to population change.
As any arrest in pupal development will alter the time when moths emerge, I initially investigated the development of non-diapausing pupae under natural fluctuating temperatures in an open-air insectary at Toowoomba. Various algorithms were fitted to the development rate- temperature data to allow comparison between sexes of the same species and between species. For each species, development rate of males was ca 92% that of females at any given temperature in the range 15-25°C. H. punctigera and H. armigera differed significantly in their development response to temperature. As the differences between species were small, a single function for both species may be satisfactory for most generalized pupal development subroutines in population dynamics models.
Diapause is an important strategy in temperate ecosystems as it allows Heliothis sup. to survive the winter months when temperatures are too low for reproduction and development to take place, and host plants are unavailable. Temporal patterns of diapause in H.punctigera and H.armigera were investigated in an open-air insectary and in field cages. The incidence of over-wintering diapause increased from low levels in March to high levels in May. The transition from low to high diapause levels was more rapid in H.armigera than in H.punctigera, and the peak diapause levels were highest in H.armigera. Multiple linear regression models were derived to predict the incidence of diapause and make comparisons with published information.
H.punctigera pupae also entered diapause in spring (October-November). As moths emerged from spring diapausing pupae during summer, spring diapause can be interpreted as a mechanism allowing a portion of the population to survive adverse late spring-early summer conditions which may otherwise be “locally” catastrophic for the second generation after winder.
High temperatures experienced during the prepupal and early pupal stages arrested development in some H.punctigera and H.armigera pupae. Quiescent pupae resumed development when placed at lower temperatures, but the relationship between the resumption of development and field conditions warrants further investigation.
Highly variable mortality was recorded in pupal populations sampled under crops on the Darling Downs. Pupal mortality resulting from biotic factors (mainly predators and parasitoids) sometimes exceeded 60%, but their effect on population change was not assessed.
As non-diapausing pupae were able to survive more than 24 hours totally submerged in water at 25°C, direct mortality from drowning may not be important in the field except when prolonged waterlogging occurs. Soil moisture conditions when prepupae burrowed into the soil did not directly influence pupal survival. Simulated rainfall after pupation, but before moth emergence, reduced survival by disrupting emergence tunnels and trapping moths in their tunnels. This effect was greater where prepupae had tunnelled into dry soil than where they had tunnelled into wet soil.
Field cage studies indicated that pupae of H.punctigera and H.armigera could successfully over-winter on the Darling Downs. In the absence of cultivation and with most predators and parasitoids excluded, survival of autumn-formed pupae under field cages was high (ca 80%). Moth emergence from over-wintering pupae under field cages was compared with the spring phenology of male moth catches in pheromone traps. For both species, asynchrony between the emergence of the local overwintering population and pheromone trap catches indicated that immigration was important in some years.
The emergence of over-wintering pupae placed in artificial burrows was examined on a physiological time scale of heat units. Simple thermal accumulation models were derived from these data and used to predict the spring emergence of over-wintering H.punctigera and H.armigera pupae.
The phenology of Heliothis spp. On the Darling Downs is discussed in relation to dormancy strategies. Immigrants from distant breeding areas are also considered as they may influence population levels of H.punctigera and H.armigera on the Darling Downs. The potential for area-wide management of Heliothis spp. Is considered in relation to these findings.