Egg parasitoids kill their hosts before larval hatch, thus preventing crop damage by emerging caterpillars. Egg parasitoids are difficult to see because of their small size, even smaller than the size of a moth egg. The best way to find and identify them is to collect brown Helicoverpa eggs and store them in a clear container. If parasitised, these eggs will turn black and adult wasps will emerge in about 10 days. Unparasitised eggs will produce Helicoverpa larvae in 3 5 days.

The three main Helicoverpa egg parasitoids found in and around the cotton farm include Trichogramma spp., Trichogrammatoidea spp. and a species of Telenomus.

Identification: Trichogramma and Trichogrammatoidea wasps are very tiny.  They are less than 0.5mm long - that is smaller than a pin head. These wasps are brown or yellowish with red eyes.  There are two species commonly found in cotton - Trichogramma pretiosum and Trichogrammatoidea bactrae. These species are difficult to distinguish without the aid of a good microscope and taxonomic skills. 

The Telenomus wasp is also tiny (about 0.8mm long) and has a black body and black eyes. This wasp is not abundant in cotton.

Lifecycles: Typically two to four Trichogramma or Trichogrammatoidea wasps develop within one Helicoverpa egg. Trichogramma uses its antennae to measure the size of the host egg in order to determine the number of eggs it will lay in it. In contrast, only one Telenomus wasp develops per Helicoverpa egg. For all species, parasitised eggs turn black after 3 days and fail to hatch caterpillars but will eventually produce one or more wasps. Adult wasps emerge from parasitised Helicoverpa eggs after 8-10 days development during summer.

Habitat:  Helicoverpa egg parasitoids occur in all cotton districts, but are reduced in regions which are sprayed regularly with broad spectrum insecticides. These egg parasitoids are crop specialists, are mainly found in the crop, but also on the edge habitat of crops and remnant vegetation, making them even more susceptible to broad spectrum insecticides. Small populations are found in native vegetation which is some areas serve as an overwintering habitat. The ecology of egg parasitoids is poorly understood. However, unsprayed sorghum and maize are good nursey crops for egg parasitoids in summer, particularly if selective insecticides, such as GemStar, are used to manage Helicoverpa caterpillars. Crops such as faba beans and canola are the best nursery crops in winter/spring, although will not be found in chickpeas.  Early season build up of parasitoids are enhanced by having good unsprayed habitats such as native vegetation and soft selective sprays on early Helicoverpa pressure.  Heavy grazing in native vegetation also reduces Trichogramma numbers in those habitats.

Targeted prey: Both species primarily attack Helicoverpa eggs.

 
“This Trichogramma wasp is laying up to 4 eggs in a Helicoverpa egg.  The egg will eventually produce 1 or more wasps instead of a Helicoverpa larva.” 0.4mm Photo: B. Scholz

 
“Telenomus wasps are not as common as Trichogramma. This wasp will only lay one egg inside a Helicoverpa egg.” 0.8mm Photo: B. Scholz

 
Normal eggs (left) and parasitised eggs (right). Photo: B. Scholtz

 
Unsprayed sorghum is a good nursery crop for egg parasitoids. Photo: M. Dillon