When the tomato leaf miner arrived a few years ago, I contacted an agricultural department entomologist to enquire whether there were any parasitoids or other natural enemies in South Africa that could bring it under control, in the same way as certain parasitoids controlled the other leaf miners (Liriomyza spp).
He was surprised to hear about the success of these parasitoids, and asked if I had clients who farmed potatoes, as he constantly received queries from potato farmers about how to control their infestations. I replied that I did, and these farmers had not seen a single leaf miner on their crops for years.
I added that most farmers were encouraged to use spray programmes and, usually, at least one product in the programme would kill off the beneficial insects. Moreover, repeat applications were required, which meant that many pests rapidly built up resistance.
Apart from this, pests often occur in cycles. For example, there might be a bollworm outbreak in one year, and then no infestations at all for a couple of years, and then perhaps an outbreak of loopers or some other pest.
Inspections are crucial
The most economical and practical way to deal with pests is to inspect your crops regularly. Doing this also enables you to become aware of the status and requirements of the crop.
At the first sign of a pest, apply a product that carries out the job effectively but does not harm beneficial insects. Many such products are available today.
Timing is important too; the earlier you treat the pest, the easier it is to get it under control.
Some of the ‘soft’ products are admittedly more expensive than chlorpyrifos and the pyrethroids, but by not harming the beneficial insects, you might not even have to do a repeat spray.
An exception to this is preventive spraying for cutworm prior to planting, when broad-spectrum products should be applied.
The whitefly example
A seed merchant who operates in South Africa’s northern areas recently told me that, from time to time, he comes across millions of whiteflies when driving past the tomato farms of emerging farmers.
There are so many that he has to use his windshield wipers. This pest has a large number of host plants and would limit the production of many crops were it not for natural enemies such as the parasitoid wasp Encarsia formosa.
By using cheap, harsh products, the emerging tomato farmers in this region kill off the beneficial insects.
I often get a small patch of whitefly in my tomato tunnels, but in 20 years I’ve never had to spray to control it; E. formosa quickly deals with the problem.
An underrated way of monitoring potential crop pests is to use pheromone traps, which can be checked to establish when the pest starts to become active. This allows you to spray it before any damage is done.
By using an integrated approach to pest control, you can save a small fortune on chemicals and use your spray tractor for other duties.
Bill Kerr is a vegetable specialist and a breeder of a range of vegetables.