agricultural production guidelines
m
Veld in KwaZulu-Natal
Co-ordinated Extension KwaZulu-Natal Veld 11.1 1999
INSECT PESTS
R A Bell
KwaZulu-Natal Department of Agriculture
Army Worm
Harvester Termites
Harvester Ants
Ghost Moth
INTRODUCTION
Insects form an important component of any natural ecosystem. It might be true to say that without insects there would be no veld. One of the most important functions insects have in veld is to provide a vital link in recycling nutrients, thus ensuring sustainable production from the veld. Furthermore, many plant species in the veld would not survive if the insects, which pollinate them, were removed from the veld.
However, several insect species can cause great losses in herbage production. Where this influences forage production the insects are considered to be pests. This Production Guideline outlines the life-cycles of four species of insect which have caused considerable losses to grazing in KwaZulu-Natal, and provides an indication of how outbreaks of these insects may be controlled.
In the high-rainfall areas of KwaZulu-Natal, outbreaks of the army worm (Spodoptera exempta) occur regularly. There are two all-important factors that influence these outbreaks. The first is temperature, and the second is the presence of young, succulent grass. The caterpillars (larvae) can feed only on grass, mainly on sweet grasses such as Cynodon species. They are also partial to Kikuyu and teff. When more mature, they will attack almost any kind of grass, and particularly maize plants, which they strip until there is nothing left but the stalk.
The eggs are laid in masses at night on succulent grass. These hatch after about three days and the tiny larvae scatter and crawl to the top of the grass to feed. There are six instars (larval stages). As the larvae grow older, they become darker in colour until they are almost black. There is a typical inverted "Y" mark on the front of the head of the fully-grown larva. The caterpillar requires temperatures of 24 to 32EC to develop. Within these temperature limits the period of development is about three weeks. The caterpillar then enters the soil, and pupates about 40 mm below the surface, in a fragile cocoon of soil particles tied together with silk. The pupal stage lasts about three weeks, so that the total life-cycle takes about 45 days. The moth is dark in colour, with a wingspan of 40 mm, with the forewings of a blackish-brown colour and the hind wings white.Life-cycle
Control measures
Where the caterpillars are migrating from one field to another, they may be checked with barriers. The cheapest barrier is a plough furrow, with its flat, mouldboard side facing towards the ‘army’. The caterpillars crawl along the bottom of the furrow and fall into pits about 1.5 m deep, which have been dug at intervals. There they can be dusted with poison or buried. In controlling the army worm, it is important to find the worms in time, and to apply measures at once.Mechanical control
Chemical control
In controlling this pest, it is essential that the small caterpillars should be found as soon as possible after they have hatched, and that control measures should be applied while they are still young. The first and second instar larvae are much more susceptible to poisons than the older ones and, in fact, once they are nearly fully-grown the damage is done and they are very hard to kill. A range of insecticides used in the treatment of army worm is presented in Appendix Tables 1 and 2. Pre-grazing intervals in days are given in parentheses after the name of the pesticide, and grazing may not occur within this period after application of the insecticide. Endosulfan may be used only on maize, sorghum and wheat.
Harvester termites cut grass into short lengths and carry it into their nests, where they feed on it. By feeding on grass, direct competition with grazing animals results. In the semi-arid areas of KwaZulu-Natal (Bioresource Groups 21, 22 and 23), in particular, where grass production is relatively lower than in the sourveld regions, the effects of harvester termites become more noticeable. Two groups of termites occur in KwaZulu-Natal.
Snouted harvester termites build the well-known, round ‘ant-heaps’, or termite mounds, which are common in veld. The genus is Trinervitermes. The soldiers have their heads elongated into a snout. They do not bite, but squirt out a sticky secretion which entangles their enemies. The workers gather the grass, tend the young, and feed the king and queen, and do all the nest-building. The workers and soldiers are sterile. The king and queen are not very large, and move freely about the nest, but are usually found in the lower part of it, below ground level. During the winter, winged reproductives develop in anticipation of swarming. At about the time of the summer rains, their wings are fully developed and, when weather conditions are suitable, the workers make special holes in the nests through which the reproductives escape for their nuptial flight. After a short flight, the female settles, usually on some elevated surface, and secretes a pheromone which acts as an attractant to the male. Within a few minutes a male arrives, they shed their wings, and burrow into the soil to establish a new colony. In heavily-infested veld, many of these pairs settle on the tops of termite mounds and, if these mounds have been deserted by previous colonies, the new pair will enter and a new colony will be built up in the old nest.Snouted Harvester Termite
Life-cycle
Control measures
These termites are controlled by injecting 10 to
20 ml of 1,3-dichloropropene fumigant liquid into one or two holes, 70 to
200 mm deep, in the top of the mound. The holes must then be sealed with
mud. The dosage depends on the size of the mound. The proprietary names of
1,3-dichloropropene are Telone and D-D 92 Soil Fumigant.
Mozambique or Northern
Harvester Termite
The common representative of the second type of
harvester termite found in southern Africa is Hodotermes mossambicus.
This species does not make large, hard mounds as does Trinervitermes. The
colonies live entirely underground, and are often quite large, being up to
1 m in diameter, but they may be as much as 8 m below the surface. In
heavily-infested areas all the nests are inter-connected, and really form one
huge colony. The individual termites might go from one nest to another and when
they intermingle there is no fighting such as is found in the case of many
Hymenoptera.
The northern harvester termite is found more commonly in the eastern parts of South Africa. Apart from veld denudation, it is a very serious pest of cultivated crops, particularly of winter wheat which has been sown on new land. Thousands of hectares of wheat are eaten off in the Free State each year. In addition, other cereal crops, beans, lucerne and groundnuts are often attacked. These termites also attack lawns, flowers and vegetables. In overstocked areas, the combined effects of overstocking with sheep and cattle and the attack of harvester termites have resulted in very serious and critical fodder shortages. The grass cover on the veld in these areas cannot be restored by controlled grazing alone. The termites must first be destroyed, and only then can the veld be improved by proper grazing management.
Hodotermes mossambicus is a comparatively large species, with the soldiers reaching a length of almost 25 mm and the workers 18 mm. The soldiers have very large and characteristic mandibles. Most people are familiar with the clouds of flying termites which emerge from the soil soon after rain on late summer afternoons, and flutter about until they break off their wings. These are the winged reproductives. After pairing off, they burrow into the ground to start new colonies. The young queen lays its first egg seven days after swarming. After about four months, the first worker nymphs tunnel up to the surface to collect grass, and begin feeding the king and queen.Life-cycle
For the first two to three years, all the eggs develop into workers or soldiers, the workers being in the majority. When the workers are sufficiently numerous, winged forms begin to develop. In the meantime the queen has enlarged considerably, so that its abdomen might be nearly 40 mm long. The king remains its original size. The winged reproductives develop slowly in the nest during the winter so that their wings are fully developed by the spring, but they do not mate or lay eggs until they have left the nest by swarming.
Control measures
Carbaryl bait, applied at 10 kg/ha, may be
scattered around entrance-holes to nests when the termites are active. This
should be repeated after 2 to 4 days, and again 10 days later if activity has
not ceased. Animals must be kept out of treated camps for 3 days. Aerial
treatment, of not less than 14 ha at a time, may be carried out with an
application-rate of 6 kg/ha. The pregrazing interval is, again, 3 days. The
proprietary names of carbaryl bait are Harvester Termite Bait, Kamikaze,
Termites and Termivlug.
True ants belong to the order Hymenoptera, and the family Formicidae. They must not be confused with termites, which belong to the order Isoptera, and various families, such as Termitidae (snouted harvester termites) and Hodotermitidae (Mozambique or northern harvester termites). Harvester ants (also called harvesting ants) are true ants, and belong to the subfamily Myrmicinae, and the genera Messor, Solenopsis, Pheidole, Pheidologeton, Meranoplus, Monomorium, Tetramorium and Ocymyrmex.
Harvester ants collect the seeds of grasses and carry them into their nests. Here a special caste of "millers", which have large heads and powerful jaws, grind up the seeds into a kind of meal. When the milling season is over the other workers cut off the heads of the millers, as they are no longer required. Harvester ants have affected experiments by removing grass seeds, but it is not known what effect they have on pastures or the veld, on a large scale. It can be inferred from their feeding habits, however, that their presence could have an effect on the seeding of veld grasses where this is an important reproductive strategy (e.g. sweetveld). It is doubtful, though, that their presence is a serious threat to veld condition.Life-cycle
Control measures
No control measures have been registered, or are
necessary, for harvester ants.
A common pest of lawns in the Highveld is the gorgopis lawn moth (Gorgopis libania). It causes large brown patches which appear around the holes made by the larvae. More serious is Dalaca rufescens, a ghost moth found in the moister regions of KwaZulu-Natal. It feeds on veld, and reduces the grass cover in the same way that the harvester termite does in drier areas.
Since adult ghost moths cannot easily fly into, or out of, areas with tall grass, they seek open patches of short veld or pasture in which to lay their eggs. The eggs are laid among the grass-blades. The larva makes a vertical shaft, which might be as much as 12 mm in diameter when the larva is full grown, and up to 300 mm deep. The shaft is lined with silk. A web is spun around the opening of the hole between the grass blades, and well-defined runways radiate from this. The grass is destroyed at the ends of these runways. The larva pupates well below the surface, after closing up the opening from inside.Life-cycle
Control measures
No insecticides are registered for used against Gorgopis
libania or Dalaca rufescens. With correct pasture and veld management
(i.e. preventing the development of short, patch-grazed veld), control is
usually not necessary.
Appendix
Table 1
Appendix Table 2
[ << PREVIOUS CHAPTER | CONTENTS ]