Biological Control of Insect Pests: Southeast Asian Prospects - EcoPort

320 Biological Control of Insect Pests: Southeast Asian Prospects
Damage
families, although cultivated brassicas are those most favoured when
available (Martin et al. 1976a; Sutherland and Greene 1984). Brassicas and
cotton are most frequently cited as being damaged, although the list of
economic crops affected also includes asparagus, beans, sugarbeet,
cantaloupes, capsicum, carrot, celery, maize (silks), cucumber, lettuce,
parsley, pea, potato, soybean, spinach, squash, tobacco, tomato and
watermelon.
At times, serious infestations occur, but T. ni is generally regarded as a
secondary pest whose numbers increase late in the season (Ehler 1977a,b).
Differences in susceptibility to T. ni have been found in cabbage and
related brassicas, in cotton and in lettuce (Cuthbert and Kishaba 1984), in
tomato (Sinha and McLaren 1989) and in soybeans (Luedders et al. 1978;
Khan et al. 1986), but these largely remain to be exploited. Transgenic
cotton lines containing Bacillus thuringiensis toxin genes limited damage to
initial feeding sites, compared with more extensive skeletonisation in 2
control cultivars (Flint et al. 1995). Transgenic Bt canola (rape) showed
excellent resistance to T. ni (Stewart et al. 1996).
Larvae are easily reared on an artificial diet (e.g. Shorey and Hale 1965;
Honda et al. 1996).
The cabbage looper is a major pest of commercial brassicas in North
America and many other areas where it occurs and causes significant
damage also, in particular, to lettuce, tomatoes, celery and cotton. Indeed,
Schwartz (1983) claimed that, if uncontrolled, 92% loss would be sustained
in the cotton yield in USA, compared with 30% if controlled. Larvae chew
large irregular holes, leaving only main veins, in the outer leaves of cabbage,
cauliflower and related plants, often leaving them riddled with holes. Later,
the outer layers of cabbage heads are eaten and masses of faecal pellets
contaminate the feeding sites. So much leaf tissue is eaten that heads of
cabbage and cauliflower are stunted and other leafy vegetables are rendered
unfit to eat.
Damage caused to cotton by the larvae consuming leaves is often
considered less serious, since it frequently occurs late in the growth of the
cotton plant, so that it may not have a major effect on yield. Cabbage looper
larvae are essentially foliage feeders and cause their damage in this way.
Natural enemies
Over the last few decades T. ni has become a very widely used laboratory
insect, particularly in North America. As a result, there are many papers