Crop environment and weather issues

It is common for egg parasitism to increase to high levels where little if any spraying is required for nutborer. 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 high level of parasitism of say 3 eggs per 100 nuts 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, say below 1 egg per 100 nuts before a jump in pressure, then it will take a week or two for the numbers of wasps to build up to a level where higher parasitism rates can be expected.

Its good to be aware of the density of parasitised eggs in the crop.

If this is low, say less than 1 parasitised egg per 100 nuts, then the wasps can be set back by hazardous sprays or unfavourable weather (extreme heat or several days of rain).

If parasitised eggs are common, say over 5 per 100 nuts then recovery after an adverse event will be much quicker.

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 14 days will severely reduce wasp numbers.
• high predator counts may reduce the number of parasitised eggs yielding wasps.
• incompatible chemical applications.