Parasites can hang around and make life miserable but parasitoids have a mission to kill their host. That’s not what you want to hear as a host, but it could be very good news if you are a graingrower and the host is Helicoverpa, popularly known as heliothis.
Farm consultants and advisers attending a Grains Research Update in Goondiwindi earlier this year were urged to get to know tiny parasitoids like the native trichogramma and microplitis wasps which can play a part in controlling heliothis.
Queensland Department of Primary Industries (QDPI) entomologist Brad Scholz urged growers to recognise parasitoids and maximise their use.
“Parasitoids are hard to see because they are so tiny,” Dr Scholz added. “Sometimes you can’t see them at all, but you can see signs of their activity.”
He said QDPI Farming Systems Institute research trials had found 56 per cent of sweet corn cobs in a spring trial were infected by heliothis. In summer, when trichogramma numbers had built up, only 5 per cent of cobs were infected, even though no spraying had been carried out against heliothis.
These findings begged the question, ‘Why risk killing the valuable parasitoids by spraying for heliothis in summer, when heliothis numbers are so low in the presence of trichogramma?’
Trichogramma attacks heliothis eggs and turns them black. By finding and counting the discoloured eggs, it is relatively easy to assess trichogramma’s impact.
A number of other local wasps can help attack heliothis and, for this reason, Dr Scholz recommended using selective rather than broad spectrum insecticides.
Biopesticides like Dipel® and Gemstar® effectively killed heliothis but not trichogramma, according to Dr Scholz.
Dr Scholz also reported that other research supported by the Farming Systems Institute and the GRDC had found sorghum and maize to be good host crops for parasitoids, while chickpea, pigeon pea and lablab were poor.
Contact: Dr Brad Scholz 07 4688 1312