Soyabean Semi Loopers

Pests of Field Crops in Southern Africa

SOYABEAN SEMI-LOOPERS

(Lepidoptera: Noctuidae)

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Text Box: Well-known to soyabean farmers are the semi-looper caterpillars which can occasionally defoliate this crop almost completely. Although they are often collectively referred to as “Plusia” caterpillars, more than one species is involved. The most common of these are Thysanoplusia (= Trichoplusia) orichalcea, Chrysodeixis acuta and C. chalcites, with the first usually being the most common in the field. Semi-loopers are the most serious pests of soyabeans in Southern Africa. The moths of all three species have mottled brown forewings with paler straw-coloured hindwings, and a wingspan of 16-19 mm. T. orichalcea moths can be recognised by an attractive large wedge of gold colour on the forewings, while the two Chrysodeixis spp. both have a much smaller patch of gold colour in the form of a figure-of-eight pattern on the forewings. These moths lay very pale green, spherical, ridged eggs singly on the leaves. The tiny caterpillars that hatch from them are at first white with black spots at the bases of the hairs, but soon take on a pale green colour with faint white and darker lines running along the body. They have only three pairs of prolegs, instead of the normal complement of four pairs, and this results in a looping gait, and gives them the common name of “semi-loopers”. TOP

Text Box: All three species are usually found feeding on soyabeans (Glycine max) where they do much damage. They have, however, a fairly wide range of hosts in addition to this. T. orichalcea has been recorded on foliage of “blackjack” (Bidens pilosa), sunflower, cowpea, cotton, lucerne, tobacco, mint, potato, sunnhemp, rape, velvet bean, chicory, radish, groundnut, Coreopsis and Phyllopodium. C. chalcites has been found on foliage of tomato, cotton, potato, coffee, Echium, Ficus, Marrubium, Phaseolus, Salvia and Urtica. C. acuta has been recorded on foliage of Canna lillies, banana, potato, celery, cotton and maize. TOP

Text Box: The pest is widespread in soyabeans, but seasonally variable. The newly hatched caterpillars tend to feed on the lower surfaces of upper canopy soyabean leaves in a characteristic manner, leaving the top epidermis intact. If the damage is considerable, this gives the crop a greyish appearance from a distance, and should be taken as the first warning that there may be a problem within the crop. As the semi-loopers grow larger, they feed on the entire leaf surface between the veins, which results in very lacy-looking, or tattered, leaves. In heavy infestations, there is also a tendency for young developing pods to be consumed. The effect of foliar damage on soyabean yields is obviously an indirect one. Much trial work has shown that yield losses depend on a number of factors: the level of damage (the size of the infestation), the health of the crop, the growth stage and whether the soyabean variety is of determinate or indeterminate growth form. Healthy, leafy plants, especially in the vegetative stages of the plant (as compared to reproductive) can actually withstand fairly heavy defoliation (possibly 30-40% leaf loss). Once pod development starts, however, this tolerance drops considerably. Plants of indeterminate growth form, providing the season is not limiting, usually have a greater ability to recover from foliar damage because of their ability to continue putting up some new foliage after pods appear. TOP

Text Box: The tiny eggs are usually laid on the underside of the leaves or young stems. After three or four days they hatch into first-instar larvae which reach about 3 mm long. After four to five days, these hatch into the second instar which is passed very quickly (about one day), and then into the remaining instars, each lasting between three and five days. In general, T.orichalcea has only five instars, while C. chalcites has six. By the time they have reached their last instar, the semi-loopers are usually about 35 mm long, and it is in the final stage that they do most of the damage, consuming about 85% of the total leaf area eaten during the caterpillar stage. They then cease feeding and enter a pre-pupal stage, selecting the underside of a leaf on which to spin a flimsy, white silken cocoon. The pupa is dark brown and will wriggle if disturbed. This stage lasts seven to ten days. The entire life cycle then, lasts a minimum of about 23 days, but takes longer in the cool months. There are definitely several generations per year, usually two of these being on the soyabean crop, and the remainder on alternate hosts. TOP

Text Box: As mentioned above, soyabeans of indeterminate growth form may have a greater chance of withstanding foliar damage than determinate ones, so that this is a factor that should be taken into account when selecting seed types. However, if the season is limiting in terms of rainfall, then there would be little difference between the two types. As with most caterpillar pests, the younger larvae are much easier to control than fully-grown ones. Thus scouting and general awareness of what is happening in the crop are essential. Several organophosphate and synthetic pyrethroid insecticides are registered for control on soyabeans, and if sprayed on young instars, are effective. Carbaryl has also been used effectively, but in some cases has been shown to be phytotoxic to soyabeans, and is less effective if the infestation comprises of C. chalcites. A significant occurrence in semi-looper infestations is the regular appearance of a naturally-occurring nuclear polyhedrosis virus (NPV) which can have a devastating effect on the population. So common is this that many farmers never spray semi-looper infestations, preferring to await the arrival of the disease. As with all naturally occurring insect diseases, there is a lag period between ingestion of the virus by the caterpillars and death, during which time they will continue to feed and cause damage. In some seasons, the virus scarcely appears. This is probably weather-related, or it may be due to the fact that soyabeans have never been grown in that region before. For many years, farmers have been encouraged to recognise and collect diseased caterpillars, and preserve them by freezing until the next season. In this way, they can be (rather crudely) introduced early into the crop once semi-looper damage begins to appear the next year. The virus spreads naturally by rain splash and by birds catching and eating the caterpillars, and subsequently spreading infection by their droppings. Indeed, rainy and overcast weather is essential for the virus to take hold, while hot, dry weather is detrimental to it. A fair amount of research has been done with the disease, collecting and infesting laboratory-reared semi-loopers and crudely refining the crystalline virus particles. It has been dose-tested, applied with additives to give protection against ultra violet sunrays, or to stimulate the semi-loopers to feed more readily. However, lack of funds has precluded a full research effort, and still the most common use of it is a very crude one, or reliance on a natural outbreak. Infected semi-loopers (the virus is entirely specific to these caterpillars) first take on a sickly yellow appearance. Before long, as the virus takes hold of the body they move to the underside of the leaves and take up a characteristic stance, hanging on by their hind legs, the body now brown. Soon the body bursts, and the resulting fluids drip down over lower leaves. The spread of the virus is thus aided by rain splash. Farmers are encouraged to collect the sickly yellow and the hanging brown bodies and keep them frozen in glass jars for the next season. These can simply be added to water and splashed or sprayed into the crop. If a heavy infestation of semi-loopers is left unsprayed, one can soon observe a multitude of parasites and predators which attack the population. Amongst the parasitoids are various Tachinid flies and a number of parasitic wasps from the Chalcid and Braconid families. Predators include assassin bugs (Reduviidae), dark stinkbugs (Pentatomidae) and spiders. TOP

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Soya Bean semi-looper caterpillar.

Soya Bean semi-looper moth.

Damage from feeding of young caterpillars leaves a grey appearance on leaves.

Heavy infestation results in almost complete loss of leaf.

Dead caterpillar leaking virus.