In vegetable crops where release rates are 60 capsules per hectare or more the level of parasitism immediately following the release is likely to be around 50%. This will increase over the following weeks to high levels if egg pressure is steady. If egg pressure is very low at the time of release then the subsequent build up of wasps is likely to be slower.
In field crops where release rates are generally lower, parasitism after the first week is likely to be between 10% and 30%. After the 2nd generation this is likely to increase to around 40-50% and (if egg pressure is steady) in the 3rd generation to high levels. If egg pressure is very low at the time of release then the subsequent build up of wasps is likely to be slower.
The more eggs laid in the crop, the more the wasps can breed. So if "low toxicity" products are used to target larvae during this period, then Trichgramma numbers can increase very quickly.
Ten eggs per meter translates to around 100,000 eggs per hectare. So, if half of these eggs are parasitised then the numbers of wasps emerging from those eggs in the next week will be around 150,000 per hectare (3 wasps from each parasitised egg). When wasp numbers reach such levels, parasitism rates can be very high even under heavy pressure. Its amazing to see a big egg lay and very few larvae coming through.
Often spray decisions are based on the numbers of eggs per meter. But to get the most benefit from Trichogramma, its best to collect eggs and grow them out to see how many actually yield larvae before making your spray decisions.
Two days after collecting the eggs all larvae have hatched and the rate of parasitism and unviable eggs can be estimated. This information will have a bearing on your spray decisons.
You don't have to wait for the wasps to come out.
See Monitoring for parasitism for details.
When Trichogramma become established problems with heliothis are greatly reduced. It is easy to let it go and ease up on the monitoring. But watch out for "secondary" pests which may have been suppressed by broad spectrum insecticides. Refer to your crop guides for details e.g. "Sweet corn pests and their natural enemies" and The Cotton Pests and Beneficial Guide.
On the other hand, we have received many reports of a decline in secondary pests when a "soft" insecticide program is adopted - natural enemies for mites, aphids and thrips are able to move into the crop.
The message is: keep up the monitoring and be able to identify a wide range of natural enemies.
See the ABC Web Page for an introduction to other beneficials.
