www.goodbugs.org.au

Introduction to chemical use

Many chemicals are hazardous to beneficial insects and predatory mites. Care must be taken in the choice of products and the timing and frequency of application.

Good populations of beneficials typically enable reductions in chemical use with all the benefits that this brings - less environmental impacts, less exposure for the farmer, fewer problems with withholding periods and residues and less disruption to neighbours.

In recent times a number of "soft" biological and chemical insecticides have become available and these have made it much easier to incorporate beneficial insects into IPM programs. In this way, natural enemies can make a useful and often major contribution to pest management. They also reduce reliance on insecticides and become an important tool in resistence management.

Broad spectrum pyrethroid, organophosphate and carbamate insecticides are generally "highly" or "very highly" toxic to beneficial insects (although there are some exceptions) and should be avoided if possible. Applications of these products may severely reduce numbers which can take many weeks to recover.

Although beneficial insects are sensitive to many chemicals they are often found in crops that have been sprayed with "hazardous" chemicals. How have they survived? Many beneficials have a phase where they are more protected - for example, predatory mite eggs may be laid on the underside of leaves against a leaf vein where spray contact may be reduced. Many developing parasitioids are protected by the body of their host. And ladybirds in the pupal phase may also be protected to various degrees. By the time of emergence the insecticide residues may have dispersed.

If spraying is not done repeatedly then recovery does occur and sometimes quickly - depending on the situation. For instance, residues from chemicals in protected crops may take months to disperse while those in fast growing outdoor vegetable crops may be gone in a week. Ladybirds may take longer to recover than Trichogramma wasps if there was lots of parasitism before the spray.

Applied = toxicity of chemicals when sprayed on the beneficials in the field. The rating is a general overall toxicity allowing for the impact on the population of the beneficial.

L = Low toxicity = nil or low impact on beneficials.

M = Moderate toxicity = beneficial activity significantly reduced but can recover in a week or so.

H = High toxicity = a high proportion of the beneficial population is killed and re-establishment will not be possible for several weeks.

( ) = estimated toxicity, based on anecdotal information and known impact on similar organisms.

Residual = suggested waiting time (in weeks) after application of the product before introducing beneficials. In protected crops this period may be much longer.

Some IPM issues for vegetable and field crop growers

Heliothis

If Trichogramma wasps are not reducing your viable egg pressure below thresholds (determined from a parasitism check, eggs per meter x % parasitism) then, consider the numbers of other natural enemies in the system before spraying. Next to trichogramma, the major natural enemies of heliothis are predatory bugs and beetles which attack the egg and/or larval stages. If there are more than 3 predators per meter this is likely to further reduce the numbers of eggs and larvae (but by how much is not clear).

B.t products (Dipel® etc) and NPV products (Gemstar®, Vivus®) have no impact on Trichogramma and can be used anytime. If Trichogramma activity is moderate to high then either of these products is likely to reduce heliothis below damaging levels. Various additives to these products (e.g. AminoFeed, milk powder) will improve their effectiveness. See the manufacturers recommendations.

In recent times a number of selective chemical insecticides have become available. The safest chemical products to Trichogramma are Prodigee® and Avatar®/Steward®. Be aware that some selectivie insecticides may impact on some natural enemies e.g. Avatar/Steward® is hazardous to some predatory bugs while Success/Tracer® is hazardous to adult micro wasps.

Aphids

Aphids have many natural enemies including predatory lady beetles and bugs, green and brown lacewings, hover flies and parasitic wasps. Using soft options for aphids will enable these beneficials to move into the crop. Small numbers of aphids can be useful in this regard by luring natural enemies into the crop. Aphids also produce honey dew which is a food for parasitic wasps.

If aphids increase to damaging levels use the soft options if possible. These include soap and oil sprays. Softer insecticides include: Pegasus®, Chess® and Pirimor®. If aphids occur late in the crop, for instance in sweet corn after silks have browned, then the use of broad spectrum insecticides at this stage will not be an issue for Trichogramma as they have already done their job by this time.

Mirids and green vegetable bug

Insecticide sprays for plant feeding bugs can be very disruptive to Trichogramma and other natural enemies. Recent work by the QDPI has shown that by adding salt to the insecticide mix at 750 gram per hectare the concentration of the active ingredient can be reduced by up to 75% and still get a good kill. The lower concentration of the active ingredient has a much reduced impact on beneficial populations. The amount of salt is low. Much more comes down in rainfall.

Powdery Mildew

Powdery mildew is an issue for some vegetable growers. Sulphur is hazardous to Trichogramma. Mancozeb used as a protectorant is okay if rotated with a "softer" fungicide (see table below). Likewise, carbenzim is okay if used in rotation with softer options. Alternative control measures like milk (at 10% full strength) and baking soda (0.5% plus 0.5% spray oil) are relatively safe to Trichogramma.