Friendly fighter conquers foe

Microplitis demolitor is just one of many friendly fighters that battle to contain numbers of one of our most important pests, the corn earworm, Helicoverpa armigera.

Corn earworm on grain sorghum is making its presence felt and many crops are being sprayed with Helicoverpa nucleopolyhedrovirus (NPV) to control above-threshold infestations of caterpillars.

The current high value of grain sorghum (over $250 per tonne) means that it is economic to control caterpillars at lower numbers (density) than growers may have sprayed previously when grain value was lower.

Low caterpillar numbers is a perfect situation for Microplitis to chip in a helping hand. It is not uncommon to find 30-40% of small caterpillars on grain sorghum parasitised by Microplitis. In many cases, this level of parasitism may be sufficient to sway a decision to not spray.

What is Microplitis?
Microplitis is a small native wasp that lays it eggs in (parasitises) small helicoverpa caterpillars. The life cycle from egg to adult takes about 12 days. This is made up of 7 days from egg laying to forming a pupa, and then 5 days for pupal development.

Adult Microplitis are small black-brown wasps. They are often seen flying slowly above the crop canopy in search of caterpillars (hosts). A female wasp will parasitise as many as 70 helicoverpa caterpillars. The parasite develops inside the host caterpillar. When fully developed, the Microplitis larva chews a hole in the side of the caterpillar and spins a fawn-coloured cocoon around itself and pupates. The caterpillar that was parasitised may still be alive, but it will soon die.

Clues to identify Microplitis activity include

• Adult wasps foraging on sorghum heads
  
• Split test of caterpillars to reveal internal parasites
  
• Distinctive fawn cocoons next to dead or dying caterpillars
  

Identifying parasitised caterpillars
In the field, parasitised caterpillars can be identified by performing a simple split test. Parasitised caterpillars will only grow to about 15 mm in length, so caterpillars smaller than this are potentially Microplitis hosts. Hold a caterpillar across a forefinger with one thumb and forefinger on the rear end of the caterpillar, and with the other thumb on the head. Gently stretch the caterpillar until the skin ruptures. A Microplitis larva developing within the caterpillar looks like a white maggot up to 4 mm long.

Interactions between Microplitis and NPV
Caterpillars infected with NPV within 3 days of parasitisation by Microplitis will die from NPV. The immature Microplitis will also die because of the death of its host.

When NPV is applied to control corn earworm, it is not unusual for some parasitised caterpillars to survive the treatment. Caterpillars parasitised more than 3 days prior to the NPV treatment will produce healthy Microplitis. Parasitised caterpillars feed less and may not ingest NPV.

In shaking sorghum heads to make post-treatment assessments, parasitised larvae may be dislodged free of the pupal cocoon attached to them. Careful inspection of these caterpillars may reveal a hole in the side of some of these caterpillars, indicating prior parasitisation. These larvae will eventually die.

Microplitis is an important natural enemy of the corn earworm and they need to be considered when making decisions about when to manage corn earworm.

For more information on Microplitis, follow the link to the brochure ‘Microplitis demolitor and ascovirus: Important natural enemies of helicoverpa’ http://www2.dpi.qld.gov.au/fieldcrops/17576.html