Trichogramma pretiosum

Description

Trichogramma pretiosum

Trichogramma are minute wasps, less than 0.5 mm long. The adult female lays her eggs into moth eggs. When the wasp eggs hatch, the larvae devour the developing caterpillar inside the moth egg. The Trichogramma larvae pupate and grow into fully formed wasps inside the moth eggs. These turn charachteristically black as the wasps develop inside. Wasps emerge by chewing a hole in the moth egg and are then ready to parasitise other moth eggs. This process takes from seven to ten days, depending on temperature. A female wasp can parasitise over fifty moth eggs during her lifespan of up to 2 weeks. Mated female wasps will produce both male and female offspring. Unmated females can parasitise eggs but will produce only male offspring.

Releases of mass-reared Trichogramma in combination with reduced pesticide use complement the natural build-up of beneficials. Trichogramma wasps have been used to control caterpillar pests in a wide range of horticultural and field crops throughout Australia. They can also be used to target household pests such as pantry moths and clothes moths.

Target pests

• Heliothis Helicoverpa armigera and H. punctigera
• Loopers Chrysodeixis spp.
• Carob moth Ectomyelois ceratoniae
• Cabbage moth (diamondback moth) Plutella xylostella

Trichogramma wasps are very sensitive to insecticides and are best suited to those crops grown organically or under Integrated Pest Management. Trichogramma pretiosum have been successfully used in crops such as tomatoes, sweet corn, capsicum, french beans, cut flowers, lettuce, cotton, strawberries and such tree crops as citrus and avocados.

Before release

Be ready to release at the optimum time for your crop. Make sure there are no harmful chemical residues in the crop before releasing Trichogramma. Some chemicals may be toxic for up to four weeks after they are applied.

Monitoring regularly for moth eggs is essential in order to determine the best time for release and the release rate. Trichogramma should be released when moths are active and laying eggs. Refer to your local history for a guide when ordering. Peak egg lay usually occurs just after a peak in the number of moths caught in pheromone traps. Heliothis or looper eggs are easily seen in most crops. They are about 1 mm in diameter and creamy white, usually laid on the tops of young leaves or near flowers.

At release

Trichogramma are despatched in the form of parasitised moth eggs. The eggs are distributed in capsules and are sold by the sheet. One sheet comprises 60 capsules, each containing 1000 parasitised moth eggs.

Each capsule should be stapled through one corner to a leaf or post (tomatoes) or dropped into a whorl (sweet corn). In hot weather, a shady spot should be selected. The wasps will gradually move downwind, so place any extra capsules along the windward boundary of the crop.

The more Trichogramma applied, the quicker the control of the pest. However, economics will also dictate the number released. Experience overseas and recent work in Australia suggest that regular release of Trichogramma at variable rates according to pest pressure and plant development is appropriate.

To establish a continuous Trichogramma population in the field, it is necessary to make two releases between five and seven days apart. The wasps only live for that period in the field and it takes approximately nine days for any attacked eggs to produce a wasp. Thus seven days after the original release, all the original wasps will have died. It will take a further two to five days before their offspring emerge. A second release will ensure that there are still adults present while the first generation completes its development.

Wait until close to emergence before distributing in the field. This reduces the chances of predation and loss due to rain or heat.

If field conditions are unfavourable for release (raining or very hot), the capsules can be stored at 8 to 10°C for up to three days, depending on previous storage and how close they are to emergence. Do not allow the temperature to drop below 3°C.

Recommended release rates

Release rates will range from 25,000 parasitised eggs per hectare for low pest levels to over 200,000 per hectare for high pest levels.

• Tomato and capsicum: Where the target pest is heliothis, weekly releases are recommended starting as soon as moths or moth eggs appear. Release at 50,000 to 150,000 wasps per hectare.
• Sweet corn and tree crops: Minimum 50,000 to 200,000 wasps per hectare.
• Enclosed areas: Minimum 10,000 to 20,000 wasps per 1,000 m2.

A note on release rates: Unlike chemicals which generally exhibit a clearly defined dose response curve, with beneficial insects, more is always better. However, they are costly to produce and the goal should be to achieve the best results at minimal cost. We are constantly trying to strike a balance between cost and efficacy. There are many factors that should be considered including the value of the crop, the magnitude of the pest population and the activity (or otherwise) of naturally occurring beneficial species. Also unlike chemicals, where it is common to respond to pest populations that have already exceeded some ‘economic threshold’, we recommend establishing beneficials early in the life of the crop before pest populations reach threatening levels. In most cases our releases are inoculative and we anticipate that our beneficials will establish and breed up within the crop to give long term control. As a general principle, 2-3 releases of modest numbers is better than a single large release – this reduces risk, improves establishment and accelerates the development of multiple overlapping generations of the beneficial species.

After release

The hatched female wasps will seek out suitable host eggs, preferably freshly laid ones, in which to lay their own eggs. It may take a number of releases and generations before parasitism increases to high levels.

Although the level of parasitism is a useful indicator of Trichogramma activity, overall control of the target pest is better determined by the number of developed caterpillars present, and actual damage to the crop. The level of parasitism required for control will vary with the crop, with the level of pest pressure and with the impact of other beneficials.

Cultural practices to aid establishment

The persistence of Trichogramma from one season to the next will be influenced by the presence of other crops in the area which can host moth pests. Winter and summer legumes, lucerne, grain and forage sorghum and maize provide good refuges for moths and Trichogramma. Strip cropping is more likely than large areas of fallow to favour persistence of Trichogramma.

Chemical use

Trichogramma wasps are extremely susceptible to most chemical insecticides and their residues.

Using Trichogramma for household pests

Trichogramma can be used to target household pests such as pantry moth and clothes moth. Several releases at weekly/fortnightly intervals are required to break the lifecycle of these pests.

Other natural enemies of moth pests

• Green lacewings Mallada signata
• Ladybird beetles
• Predatory bugs