There are a whole host of potential bean pests, but only a few are common and occur regularly. O ne of the most common pests is the African bollworm, sometimes incorrectly referred to as the American bollworm. The problem with this pest is that it is sporadic and therefore likely to catch one off-guard. Unless regular inspections are made, often the first time the farmer knows about the pest is when it is munching holes into the bean pods. It can also tunnel into the pods, making control less successful. The remedy is to keep on the lookout for eggs and young larvae. The night-flying, yellowish to orange moths lay single eggs on young, tender foliage. Because they lay their eggs at random, there is usually a fairly uniform distribution through the land. The caterpillars have distinct stripes down the sides. After hatching, they start eating holes into the leaves. They are not always easy to see, especially when they are the same colour as the foliage. When young, they tend to be darker and more visible. They actually don’t do too much damage when eating the leaves – it is when they tuck into the flowers and pods that the trouble begins. They move around, eating holes into a great many pods. Not only does this make them unmarketable, but there are labour costs involved in removing spoilt pods in the packhouse. The best remedy is to apply an insecticide at flowering as a precaution. At this stage the caterpillars are likely to be on the small side and more easily killed. As the pods have not yet formed, they will not be affected by residual toxin.
There are a number of contact insecticides that have a long residual action which virtually ensures the crop will be intact up to harvest. Red spider mite and loopers In areas where red spider mite is a problem you will have to make a more careful choice of insecticide, as some can cause this pest to flare up by killing off their natural enemies. Red spider mite is usually more of a problem in hot, dry conditions. This pest is fairly easy to detect – small white spots form on the leaves which, when turned over, reveal the small reddish mites on the underside. Bollworms are not the only caterpillars that can attack this crop. Semi-loopers – which are usually called loopers although they aren’t true loopers – are more sporadic than bollworms. They have a looping action, light stripes down their sides and a body that is narrower towards the head. These are usually controlled naturally by an extremely effective, pinhead-sized black parasitic wasp. For this reason it is better not to spray unless their numbers really justify it. The bottom line is, never let the plants go beyond flowering without either spraying or making sure that there are no caterpillars or eggs. – Bill Kerr ([email protected] or call (016) 366 0616). |fw