By Diedrich Visser, ARC Vegetable and Ornamental Plants, Pretoria

Most vegetable pests prefer warm, dry conditions to thrive. The optimum temperatures under which the important pests inflict the most damage usually range from 23 to 33°C. There are, however, a few pests that are adapted to develop below these temperatures, and they prefer to attack winter crops.

These pests increase their metabolism when the temperature drops, and stop feeding under hot and dry conditions. The latter is referred to as aestivation, also known as oversummering. The term “overwintering” is used for the opposite phenomenon.

Vegetable pests that are more active during the cooler months include the vegetable weevil, the bean seed maggot, and sawflies.

Vegetable weevil

The vegetable weevil (Figure 1) originated in Brazil. The adult beetles are 8 to 10 mm in length, greyish brown (mostly dull in colour) with a pale, short “V” on the elytra. The body may also be covered with tan or grey scales and scattered hairs. Adults may be gregarious at times, hiding or feeding in small groups.

They only feed at night and hide under debris or clods during the day. They seldom fly, even if provoked, but rather feign death or walk away to safety.

The vegetable weevil is a cool-weather pest, being active from late autumn until early summer. They are dormant in the warm summer months (aestivation). Females lay eggs on plants or the soil near plants. They are legless, but manage to move up on stems using ventral and lateral ridges or protrusions on their bodies.

Young larvae have a creamy-white colour, while older larvae may be yellowish or green and can grow up to 14 mm in length. When mature, larvae pupate in the soil, down to a depth of five centimetres.

Only a single generation occurs per year. The beetles are parthenogenetic; only females are known, and mating is therefore not needed to reproduce. Adults and larvae both feed on foliage and roots. They may move down to the root zone in crops like carrot and beet. Adults also feed on stem tissue, sometimes causing damage similar to that of cutworms. Most vegetables that are grown during the cooler months are attacked.

Bean seed maggot

The bean seed maggot (Delia platura) is the larva of a small fly, five millimetres in length (Figure 2). Crops planted in freshly worked and well-composted soils are especially vulnerable. It is a cosmopolitan species also known as the seedcorn maggot. Bean seed maggots usually only infest young seedlings and sprouting seeds; they do not attack plants past the seedling stage. Crops grown in the cooler months may be attacked by the maggots.

Females lay eggs in groups of up to ten near decaying or sprouting seeds. Poor quality seed and seedbeds with rotting seeds and rotting plant material attract females to lay their eggs nearby. Larvae may burrow into the soil down to a depth of eight centimetres.

When seeds are in short supply, they may develop on other rotting materials, including compost. The flies and maggots prefer cool weather; the optimal ambient temperatures for the growth of larvae are between 21 and 23°C. Development of the maggots stops during the summer months.

Damage to crops is inflicted by the maggots only; the fly feeds on liquids like nectar and honeydew. One of the challenges to identify the bean seed maggot as the culprit of seedling loss is the fact that damage occurs under the soil, usually before the seedlings emerge. Plant density may therefore be reduced, and usually, the quality of the seed is questioned.

When seeds are planted in freshly worked soils or soils with fresh compost, the bean seed maggot must always be considered in pest control strategies. When planting under cool conditions, the problem will be exaggerated.

Sawflies

Sawflies are not flies, they are stingless wasps. Its name is derived from the saw-like ovipositor of the female auwith which she lays her eggs inside plant material. The best-known local sawfly is the pear slug, attacking plum and pear trees. However, Athalia marginipennis is also known to attack cabbage and turnips in Southern Africa.

All Athalia adults are similar in colouration: head and thorax blackish, abdomen yellow, and legs yellow with black rings at each joint (Figure 3). Larvae are darkly coloured, but this may change to a paler colour in older individuals (Figure 4). Although sawfly larvae look remarkably similar to caterpillars, they can be distinguished by the number of abdominal legs; they have seven pairs, while caterpillars have a maximum of five.

Larvae go through five or six instars and then spin a silken cocoon in the soil under plants. Athalia species are cold adapted, requiring ambient temperatures of between 10 and 25 °C for optimal development. Development time is also indirectly proportional to day length. In some species, a summer diapause may be induced when day lengths increase. Sawfly larvae damage cabbage plants by consuming leaf material from the margins. However, they are usually not encountered in large enough numbers to cause crop losses in Southern Africa.

We thank the Agricultural Research Council (ARC) Vegetable and Ornamental Plants of South Africa for the material they provided for the readers of ProAgri BNZ. For more information, visit their website www.arc.agric.za.