Chrysodeixis acuta
Common names: Green semi-looper, Tunbridge Wells gem
Family: Noctuidae (subfamily Plusiinae)
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The tomato semi-looper has a broad pantropical and subtropical distribution, occurring across much of Africa (including South Africa, Kenya, and Angola), parts of Asia such as India, Japan, and Oman, the Canary Islands, parts of southern Europe, and Oceania, including Australia and various Pacific islands. It is not a recent arrival to southern Africa but rather a long-established, widely distributed pest of vegetable and field crops across the region.
The larva is the life stage growers most often encounter, and its "looping" style of movement gives the group its common name. The caterpillar is glassy green and has only three pairs of abdominal prolegs rather than the usual five, which forces it to arch its body into a loop as it crawls. Adult moths are brown, with a wingspan of around 35–37 mm, and are readily identified by two prominent silvery-white spots on each forewing set against a mottled dark-brown pattern. As with most noctuid moths, development proceeds through egg, several larval instars, pupa, and adult, with multiple generations possible during the warmer months.
This is a highly polyphagous pest. While tomato is a major host, it also attacks a wide range of other vegetables, field, and ornamental crops, and larvae are known to shift readily between host plants when preferred foliage becomes scarce.
Larvae feed on foliage, chewing ragged holes in leaves and, in tomato, occasionally damaging young fruit as well. Because the caterpillars tend to concentrate low in the plant canopy or on the underside of leaves, they can be easily overlooked during casual inspection, allowing populations to build before damage becomes obvious. Heavy infestations cause significant defoliation, reducing the plant's ability to photosynthesise and, in severe cases, affecting yield and fruit quality.
A number of general predators (including various true bugs) and parasitic wasps attack semi-looper eggs and larvae in the field, along with naturally occurring fungal pathogens. Conserving these natural enemies by minimising broad-spectrum insecticide use is an important part of keeping semi-looper populations below damaging levels.
Regular field scouting is central to managing this pest, since larvae are easily missed until defoliation is already underway. Bacillus thuringiensis (Bt)-based biological insecticides are widely used against semi-loopers and other lepidopteran defoliators because they are effective on young larvae while sparing many beneficial insects. Growers should also rotate insecticide modes of action where chemical control is required, as related looper species elsewhere have shown a tendency to develop resistance under repeated use of the same product class.
