Its common for Trichogramma to increase to levels where little if any spraying is required for heliothis. But there are a number of situations which consultants and growers should be ready for that may reduce wasp numbers necessitating help from a "soft" insecticide.

In general terms, a steady egg pressure of 5 or more eggs per meter will sustain a population of Trichogramma that will be able to quickly respond to a sudden jump in egg pressure achieving moderate to high levels of parasitism.

If egg pressure is low, before a jump in pressure, then it will take a week or so for the numbers of wasps to build up to a level where higher parasitism rates can be expected.

Its worth being aware of the number of parasitised eggs per meter (eggs per meter x % parasitism from collected samples). If this is low, say less than 1, then the wasps can be set back by hazardous sprays or unfavourable weather (extreme heat or days of rain). If parasitised eggs per meter is 5 or more then recovery will be much quicker.

Do not just rely on the observation of "black eggs" in the crop to determine parasitism rates. This can be misleading. Parasitised eggs may fall off in wind or rain but still produce wasps. Its not uncommon to measure high levels of parasitism in crops where few "black" eggs are observed.

Some conditions can reduce Trichogramma activity so it is important to be aware of these:

• several days of extreme heat, especially if the crop is water stressed and the air is dry.
• several days of wet weather will reduce adult wasp activity, so if eggs are laid in the crop just before or immediately after the rain many of these may not be parasitised.
• very low egg pressure for more than 10 days will severely reduce wasp numbers unless alternative moth egg hosts are available.
• high predator counts may reduce the number of parasitised eggs yielding wasps.
• incompatible chemical applications.