Microplitis

Wasps are active during the day. They appear:

• small (3mm)
• brown-black with black wings
• orange upper abdomen and legs.

The larval stage grows within the caterpillar. Parasitised caterpillars only grow to about 15mm, which is half the size of healthy caterpillars. The pupal stage occurs in a fawn-coloured cocoon attached to dying helicoverpa caterpillars.

Other parasitoid wasps.

Microplitis is found in all field crops affected by helicoverpa except chickpea, and is abundant in sorghum.

Microplitis wasps prefer second instar (4–7mm) helicoverpa caterpillars. Third and fourth instar caterpillars are also suitable hosts, but may defend themselves, sometimes injuring or killing the wasp.

This preference for second instar caterpillars is important because in the absence of parasitism these caterpillars have a good survival rate and go on to cause crop damage.

Check crop scouting data—eggs and very small caterpillars not developing into larger caterpillars may be due to Microplitis activity.

Use the split test to identify parasitised helicoverpa.

1. Only test caterpillars up to 15mm long.
2. Hold the caterpillar across a forefinger with one thumb on the rear, and the other thumb on the head.
3. Gently stretch the caterpillar until the skin ruptures.
4. Look for a white to brownish maggot up to 4mm long.

Parasitised helicoverpa caterpillars cause about 90% less crop damage than unparasitised caterpillars. Female wasps also play an important role in spreading ascovirus, a disease lethal to helicoverpa.

Together Microplitis and ascovirus can have a significant impact on helicoverpa populations. Caterpillar mortality can exceed 75%, but 30–50% is typical.

Microplitis presence has greater impact on spraying decisions if helicoverpa populations are near-threshold. For example, if the threshold is 2 caterpillars/m², then 50% parasitism will sufficiently reduce an initial population of 3 caterpillars/m² but not a population of 10 caterpillars/m².

Development from egg to adult takes about 10–12 days:

• usually only 1 egg is laid per caterpillar
• 1 wasp can parasitise about 70 helicoverpa caterpillars
• larva feeds internally for a week
• mature larva chews a hole in the side of its host to emerge and pupate externally.

Broad-spectrum insecticides will kill Microplitis. Biological insecticides (e.g. NPV or Bt) will impact Microplitis populations if the host caterpillar is killed before the parasitoid can complete its development.

The cocoon (pupal stage) of Microplitis is less susceptible to insecticides than the adult and larval stages, but timing sprays to preserve Microplitis is not practical when a range of life-stages is present.

In sorghum, helicoverpa egg lay occurs over a relatively short period (10 days) around flowering. Apply NPV 3 days after 50% of heads have completed flowering (i.e. brown anthers to the base of the head) to allow most of the Microplitis larvae to emerge before their caterpillar hosts are killed by the NPV infection.