HortFACT
Identification
Adults have a predominantly black body with some yellow colouring on the abdomen and legs, and are 2.0-2.5mm long. Females have a short, pointed ovipositor through which eggs are injected ('stung') into host caterpillars.
Eggs are elongate and translucent, with a stalk at the posterior end, and are about 0.3mm long. Each female lays about 250 eggs during a lifespan of approximately 2 weeks, depositing them in batches of about thirty, though up to 130 Apanteles can develop in a single host caterpillar if it is 'stung' several times.
Larvae hatch from eggs within 3 days of oviposition, and pass through three larval stages (instars), feeding on the body fluids of the host. During summer, 15 days after eggs are laid, third instar larvae force their way through the host's body wall, emerging to spin cocoons which are grouped in masses. Host caterpillars die within 24 hours of parasite emergence.
Pupae begin to develop in cocoons within about 2 days of cocoon formation, and 5 days later the adult cuts a circular hole in the cocoon with its mouthparts and emerges. Mating occurs almost immediately after emergence, and females are then able to parasitise new hosts.
Host Insects
Apanteles ruficrus attacks the caterpillars of more than fifty different moth species. Some of the more notable hosts in New Zealand are the cosmopolitan armyworm Mythimna (= Pseudaletia) separata, the greasy cutworm Agrotis ipsilon, and the silver Y moth Chrysodeixis eriosoma.
Beneficial effects
Prior to the introduction of new strains of Apanteles ruficrus from Pakistan into New Zealand in 1972, cosmopolitan armyworm was a serious pest of pasture, maize, sweetcorn, and cereal crops, often requiring several insecticide applications each year to achieve adequate control. The release and establishment of the new parasite strains resulted in extremely high levels of parasitism in field populations of armyworm (up to 95% in some instances), and reduced the status of armyworm from a serious pest to an occasional one very rarely requiring any artificial control. The reduced level of damage is at least in part due to the fact that parasitised host larvae &e before reaching their final instar, the instar in which they do 80% of their feeding.
Reduced insecticide requirements and increased profitability of crops save New Zealand millions of dollars each year. The effect of A. ruticrus on pest species has not been assessed, but natural control of these species may be enhanced by its presence.
Distribution
Apanteles ruficrus is a cosmopolitan species, recorded in Britain, Europe, Russia, Asia, China, Africa and Australasia. Within New Zealand it is common and widely distributed throughout both islands.
Life cycle
| Egg |  |
| Larvae* | |
| Pupae | |
| Adult | |
| Days | |
| * Red=Larvae in hosts Grey=Larval Emergence Teal=Larvae in cocoons | |
Adults are present throughout the year, but are most abundant between December and June. At least six generations develop each year, most of them during the warmer months. Development from egg to adult takes 3 weeks in summer, about half the time taken for armyworm to pass through one generation. The shorter generation time of the parasite, coupled with its high fecundity (number of offspring per individual), enables parasite populations to increase rapidly in spring in relation to armyworm populations. This leads to a high proportion of parasitised individuals in armyworm populations during summer, and a reduction in their ability to damage pasture and field crops.
E.P.J. BURGESS