PAUL Grundy, DPI Biloela, spoke at a recent IPM field day about his work using the native assassin bug in pest management.

Attendees were able to look over a trial where these predator insects had been released. Paul prepared the following notes from his discussions.

The Assassin Bug, Pristhesancus plagipennis (Walker) is a natural enemy that has considerable potential for controlling heliothis (Helicoverpa spp.) and plant-sucking bugs in cotton.

Experiments in cotton have demonstrated that during low to moderate pest pressure years, a single release of these predators was enough to provide season-long control resulting in equivalent yields to conventionally managed cotton.

Assassin bugs feed on many different insects but prefer soft- bodied prey such as caterpillars and bugs. Assassin bug nymphs have large appetites and can consume up to 160 small to medium-sized Heliothis larvae over a 9-12 week period.

Like many predatory insects, assassin bugs are often scarce in crops at critical times. However, this problem can be overcome by mass-rearing and releasing nymphs into crops as required for the control of insect pests.

Pisces Enterprises in Brisbane has commenced commercial production of these predators and intends to increase output according to demand for domestic and agricultural use.

To give an idea of the scale of Pisces' insect production, each week this company produces 0.25 tonnes of mealworms (similar to wire worm) and 400 000 crickets for the aquarium and zoo industry.

Pisces intends to target the domestic market first, but with a commercial supplier on the scene scope exists to release these predators into cotton.

Assassin Bugs and Cotton Pest Management

Experiments have shown that biological control provided by mass-releases of assassin bugs into cotton can result in comparable yields to conventionally managed (sprayed) cotton.

The use of these predators is not likely to suit mainstream cotton production; they are a new alternative with a few niche uses.

I envisage that these predators would best be used in environmentally sensitive areas where it is desirable to minimise spraying due to nearby waterways or populated areas.

The cost of this approach will depend on the number of predators required over a given area. Previous experiments on the Darling Downs suggested that 1 predator per metre could provide effective control of heliothis over a 10 week period during which adjacent conventional cotton was sprayed five times. The cost of this approach was approximately $300 per hectare.

Obviously there would be few growers who would adopt this strategy at this cost even though the insecticide costs on the adjacent cotton that yielded the same were $230 plus application and labour expenses.

However, a lower release rate of 0.5 predators per metre combined with 1 - 2 strategic applications of GemstarŪ or other soft option chemical might produce a similar outcome at a significantly reduced economic and environmental cost.

An interesting observation in the predator plots was that aphid activity was minimal compared to the insecticide treated plots. It is likely that the absence of insecticide applications in the assassin bug plots allowed aphid natural enemies to proliferate.

Further work is being conducted with assassin bugs to develop ways of integrating them into cotton IPM programs that minimise costs whilst maintaining control. At this stage the use of these predators looks encouraging.

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