Catching up with a determined pest

Press, 27 February 1981, Page 11

Catching up with a determined pest

GARDENING

M. Lusty

One of the most devastating insects in the home garden usually goes unnoticed until the damage is severe. Even then, some other factors such as dry weather or faulty plants are not . infrequently blamed. Thrips are tiny insects which develop from egg to nymph, then to the adult stage. They do not undergo the more common metamorphosis associated with many other insects, but simply grow in size from nymph to adult. The young are small, creamy-white or orange, cigar-shaped, and minute in size. They are usually found in cavities, or tightly rolled leaves. They are readily found in the curled, puckered leaves of callistemon (bottlebrush) between December and March. The adults are usually dark brown or black, very mobile, and not so commonly seen unless the in-

festation’ is very heavy. They tend to disperse rapidly when disturbed; they are of similar shape, but bigger than the young. Different species of thrips suck quietly at a wide range of plants, including gladioli, onions, citrus, cabbage, rhododendrons, and numerous glasshouse plants. One species, the onion, thrips, is the vector of tomato spotted wilt virus. They feed by rasping the leaf surface of the host plant and then suck up the sap, which is thus exuded.

Plants which ar J badly infested by these insects

tend to develop characteristic symptoms such as leaf distortion, spotting, and dehydration, as well as the silvering of the foliage, so often noticeable on rhododendrons, gladioli and other, bulbous crops. The flower spikes of infested plants such as gladioli may become twisted, and the blooms dwarfed, with the possibility that they may not even open. Once the damage has been done it is too late to carry out remedial measures, so the flowers will not recover. Extremely severe and unchecked infestations can lead to defoliation, and even to the eventual death of the plant concerned. Tomatoes smitten by spotted wilt, the transmission of which is by onion thrips, tend to have a bronzed appearance. As symptoms become more pronounced growth becomes almost negligible; the plant begins to shrivel

up and die. There 5s a wide host range on which this disease both over-win-ters and is destructive. Under warm conditions, thrips are likely to r breed and feed on stored corms; those which are infested should on no account be planted. Before storage, a thorough dusting with an insecticide is recommended; flaked mathelene can also be used —30 g per 100 corms. following which the corms should be aired after three weeks.

Thrips also enjoy feeding on fruit. One of the most obvious places to look for them is on ripening peaches. Timeliness in spraying is the most important aspect in preventing these pests causing debility in the plants which are susceptible to them. Probably the best, most effective spray now available for home garden use is acephate, which is systemic in its action.