Bug strategies beat plain old spraying

The is the first year of a three year evaluation by Plant & Food Research as part of a Ministry for Primary Industries Sustainable Farming Fund (SFF) project titled 'IPM strategy development and demonstration for forage and seed brassicas'.

An IPM approach to pest control uses beneficial insects (such as natural predators and parasitoids), management methods and, when necessary, selective insecticides to kill pests in crops. IPM has been successful in the New Zealand horticultural industry for many seasons, Dupont says.

The study aims to look at the benefits of an IPM programme in brassica crops versus the use of traditional broad spectrum insecticide. This latter affects target pests and beneficial insects and it can lead to spikes in populations of unwanted insects at various times of the season.

IPM uses chemistry such as DuPont Exirel insecticide which selectively targets pests such as Cabbage white butterfly (Pieris rapae), Diamondback moth (Plutella xylostella), Soybean looper (Thysanoplusia orichalcea), European leaf miner (Scaptomyza flava) and Grey cabbage aphid (Brevicoryne brassica).

It also leaves key beneficial insects in the crop where they can keep killing pests that reappear in the brassica crops.

In trials in Canterbury in forage and seed brassica crops, a farmer's standard programme was compared alongside an IPM strategy. In all sites predators such as lacewings and ladybirds were active, although it was the parasitic wasps that seemed to have the biggest impact on reducing aphid and diamondback moth pressure.

Results of the first year showed the importance of relying on predators and parasitoids in conjunction with selective products from chemical companies for an overall IPM strategy.

Farmers who took part in the first year were positive and better understood the role beneficial insects can play in brassica production.

Results from the year showed IPM could kill pests as effectively as the conventional approach and with fewer insecticides. The arrival of pests such as Diamondback moth into a crop was generally followed by parasitic wasps, and the use of an insecticide such as Exirel could help kill the pests while beneficial numbers were increasing.

At an irrigated kale paddock on a sheep and beef farm Diamondback moth caterpillars were present at the beginning of December. Numbers kept rising and at the end of December the broad spectrum insecticide Lorsban was applied to the conventional side and a selective Bt insecticide was applied to the IPM side.

Caterpillar numbers declined on the conventional side and, after a small decline of the IPM side and some feeding damage, a decision was made to apply Exirel insecticide across the whole paddock on January 9. On the IPM side, as a result of the Bt spray, high rates of parasitism developed and the reduction in caterpillars from the Exirel insecticide in combination with the presence of the parasitic wasps meant that a good kill was maintained for the rest of the season.

High rates of parasitism were noted on the caterpillars in the weeks that followed and numbers remained similarly low across the whole paddock from mid-February onwards.

An additional spray of Ampligo insecticide on February 18 on the conventional side did not visibly alter the diamondback moth abundance compared to that on the IPM side where parasitic wasps were killing the caterpillars.

Aphids were not found in the crop until mid-January, but the numbers did not build too rapidly (possibly because there were already lacewings and other predators present in the crop). A month later, aphid-associated parasitic wasps were present and these maintained good kill levels in the rest of the crop.

The project will continue to June 2017; yield results and gross margins will be examined.

Expanding IPM systems could raise the financial and environmental performance of these crops, Dupont says. There is growing interest from farmers wanting to adopt new pest management practices, and agronomists wanting to offer a wider range of services to their clients.