In South Africa, this is a relatively new pest in cabbage. The western flower thrip is the problem and apparently was accidentally introduced from the US in chrysanthemum cuttings. After spotting it on cabbage, I mentioned it to Prof Mark Laing, at the time at the University of KwaZulu-Natal, and sent him photos.
It has subsequently become a huge pest of many crops and is also responsible for tomato spotted wilt virus becoming a major disease in crops where it was almost unknown.
T here are still some farmers who don’t recognise this pest and attribute the lack of plant performance to other factors. Unless searched for, the pest is not easily visible. When insects confine themselves to the underside of leaves, they rasp away at the leaf surface, drink the sap and cause typical lesions.
Thrips are fond of newly planted cabbage. They fly in and colonise the underside of leaves, causing a dull appearance and curling slightly downwards. At a glance, the plants may appear to have water stress.
At this stage, thrips are particularly harmful as they stunt the plants and eventually prevent the plants from making good-sized lower leaves, required to develop a large frame. Inspect plants for thrips a week after transplanting by turning the leaves. They are 1mm to 2mm long, a dark colour, elongated and are easily spotted when they start forming silvery patches. Sometimes there are hundreds on one little leaf.
Although thrips may not damage larger cabbages that are close to maturity, monitor them closely as they may migrate to later plantings. Where a high thrip population is present, it may help to use a soil-applied insecticide to protect the young transplants for a few weeks. This pest thrives in hot, dry conditions and is rarely a problem in winter. Populations tend to drop in rainy weather.
A few systemic products are effective in controlling thrips, and control other cabbage pests at the same time.
Organic growers and those practising integrated pest management can now spray Biocure which is “soft” on parasitic wasps that control diamond back moth and aphids. Always try to wet as much of the surface under the leaves with the correct spray pressure and droplet size.
Thrips are part of the cabbage growing process in summer, and they are natural enemies as well. For this reason don’t eliminate all thrips, but try to keep the population down to a level where there is no damage. If the leaves look normal and healthy from above, the population is usually tolerable. – Bill Kerr Contact Bill Kerr on (016) 366 0616 or e-mail [email protected]. |fw