Farmer's Weekly (South Africa)
Parasitoids vs chemicals
When the leaf miner first hit South Africa, the results were nothing short of devastating. Farmers spent thousands of rands per hectare on chemicals, but gained only a semblance of control over the pest. The reason was that this particular species, the pea leaf miner ( Liriomyza huidobrensis), had already been exposed to pesticides in other countries, and this, combined with its rapid life cycle, meant it had developed resistance to most chemicals by the time it arrived in South Africa.
The adult stage of L. huidobrensis is a very small fly, comparable in size with the familiar vinegar flies ( Drosophila spp) attracted to overripe fruit.
L. huidobrensis lays its eggs in the leaves of the host plant, lettuce being much favoured. It also causes lesions in the leaves to feed on the sap, and these are seen as small, white spots.
The hatched larvae make unsightly serpentine tunnels through the leaves, making the lettuce unmarketable.
‘ predatorfriendly’ on the label may not mean 100% safe for parasitoids
range of parasitoids
The chemical abamectin was found to reduce leaf miners to manageable levels, while having a minimal effect on beneficial insects. I started using this product on my cucurbits, beans and beet, and found that while it helped a great deal, it did not eliminate the pest.
At the same time, however, I noticed that parasitoid wasps were arriving and attacking the leaf miners. Intrigued, I caught a number of these parasitoids and took them to the Agricultural Research Council, where the head of entomology photographed them through his microscope.
There were many species in my sample, and the good news was that some were better adapted to certain seasons and climates, so there would always be at least one species around to control the leaf miners on my lands.
imbalance
Parasitoids build up much faster than leaf miners and can get the pest completely under control. On occasion, though, parasitoids do such a thorough job that they cannot find enough leaf miners in which to breed, and their numbers then drop drastically. If leaf miners subsequently show up, their population will increase greatly and they will do much damage to the crop until the parasitoids catch up again.
This is when it’s necessary to apply a parasitoid-friendly insecticide. There are a number of suitable options, but caution is advised; most are not 100%
us since humanity first started trading fruit and vegetables, and the basic principle has never changed: quality sells!
Our technological savvy has found many ways to define, measure and categorise quality in fruit and vegetables. This fits in with the demanding environment in which we operate, but could also tempt us to overlook quality aspects because we’re so focused on that very same technology we rely on.
We cannot go back in time, and anybody who so wishes is not living on this planet. All we can do is move forward with the aid of the technology and all its benefits. But we should use it wisely and ensure that quality retains its focus.
• Michael Cordes is an agricultural journalist, consultant, trainer and former farmer. Email him at farmersweekly@caxton.co.za. Subject line: Market floor.