Developing an IPM outlook

As we’ve said before, integrated pest management (IPM), or using the natural enemies of crop pests to help control those pests, has a long way to go in this country.

Leaf miner can be controlled by parasitoids. Photo: Bill Kerr

Yet there are so many natural enemies of pests we’re not even aware of. As a youngster, I wanted to be an entomologist, and all my birthday and Christmas presents would comprise insect books. And I remain ‘bug conscious’ and am still able to recognise many insects most wouldn’t even notice.

A valuable parasitoid

In my early days of farming, I once saw that an almost mature semi-looper feeding on a cabbage had a small, dark insect on its back. The insect appeared to be injecting eggs into the caterpillar. I waited for the parasitoid to complete its task, then picked up the caterpillar and placed it in a shoebox. After a time the caterpillar developed a granular appearance from all the small pupae in its body.

One day when I opened the box, a swarm of hatched parasitoid wasps was ready to go into action. All of this from one little wasp and one caterpillar host! It became clear to me that this was why this pest was so sporadic and rarely required control. 

At the time, there were also no registered products effective against this pest, making the parasitoids even more valuable.

Leaf miners

Then there was the time that my very young pumpkin plants started dying off. I couldn’t understand why this was happening until I had a closer look. To my amazement, I noticed leaf miner tracks going from the dicotyledonous leaves down the stems. A systemic insecticide saved the crop. A land of beans planted a little later was also attacked, and would not have made it if I hadn’t applied an Abamectin spray.

Flies were still active, but now I noticed a number of different species of minute wasps. It was the last time I had to spray for this pest, and that was many years ago. Were I to use an insecticide which kills parasitoids, I would probably cause the pest to build up again.

Slow process

South Africa will have to follow the route of many other countries which have been using biological control measures for about 20 years. And it will be a slow process, as many farmers are totally unaware this is even an option. It also doesn’t help that some parasitoids are so small they can barely be seen with the naked eye.

However, some farmers are looking at IPM. One showed me an outbreak of caterpillars in his celery crop. He then showed me parasitoid cocoons among these and also showed me some parasitoids which he had found. These were able to completely control the pest. Most farmers would have sprayed an insecticide which would have also killed the parasitoid wasps, and possibly meant they would have to do repeat sprays.

Old habits 

Old habits die hard and IPM requires farmers to develop a whole new approach to pest control. It will mean learning to recognise the beneficial insects and determining when the beneficial insect has built up enough to take over as the pest control agent. (Waiting too long before spraying may cause crop damage.) Farmers will also have to determine what products can reduce the pest numbers without killing off the beneficial insects.

Here there’s a trade-off, as the product may kill off some of the beneficial insects but do sufficient damage to the pest to allow the beneficial insects to increase and take over. This is what happened with the leaf miner on my beans. All of this requires a new understanding and practical experience. But the sooner farmers learn about IPM, the sooner they’ll cut their costs and conform to the ever-increasing legislation on crop residues.