AKA American Bollworm
To avoid destroying the natural controls along with the pest you are targeting you need to follow a process of integrated pest management (IPM). In other words you must manage the damage caused by pests in the most environmentally sensitive way possible, minimising the risks to human health and the environment and encouraging natural pest control mechanisms.
As most gardeners know, African Bollworm (Helicoverpa Armigera, also known as American Bollworm) can cause serious damage to a wide variety of vegetables, fruit, ornamental trees and shrubs, roses, bedding flowers and herbs. As with most pests, this Bollworm has quite a few enemies in nature, one of the most important being Dejeania, a parasitic fly species which, as far as we know, only targets this particular Bollworm. Dejeania is also very host specific, occurring on veld grasses found growing in the shade of bushveld trees as well as broad-leaved herbs, so to benefit from their presence one needs to replicate these conditions in the garden.
Adult Dejeania love to visit open daisy flowers; I often see them feeding on the nectar of Euryops flowers in my garden. They also frequent roses that have been damaged by Bollworms. The Dejeania females lay their eggs directly into the African Bollworm larvae or on plants on which the Bollworms feed, such as Daisies, tomato, spinach, lettuce, Roses and Hibiscus. When the young larvae hatch from the eggs they find their way into the host.
If we are to use pesticides to help control Bollworm in our gardens we need to do this in a way that does least harm to the natural controls. Fortunately, formulations that derive from a bacterium – Bacillus Thuringiensis var. Kurstaki – can be used for this purpose. This bacterium kills young, actively-feeding larvae of various caterpillar species when it is ingested by them, and does no harm to the Bollworm’s natural enemies, including Dejeania.
Unfortunately the bacterium is also fatal to the young larvae of various butterflies, because they take the form of caterpillars, so one must avoid spraying it on plants that are butterfly hosts. On the positive side, however, it is harmless to birds, fish and other aquatic life, parasitic and predatory wasps, earthworms, predatory beetles and bugs. These formulations are available commercially to home gardeners – look for Grovida Dipel Caterpillar Wise as well as Margaret Roberts Biological Caterpillar Insecticide.
Typical signs of Bollworm damage are small holes in closed rose buds, vegetable leaves and flowers, or small chunks of leaf tissue removed on the edges of leaves and flowers. The formulation should be applied to plants at the first signs of Bollworm damage, in the same manner as one would use any other synthetic insecticide, and re-applied at intervals varying from seven to 14 days (depending on the re-emergence of Bollworm activity).
In most regions Bollworm activity starts around September (as it gets warmer) and increases towards the summer months, depending on weather conditions, but activity can occur all year round in warmer areas. Other caterpillar species controlled by these formulations include lily borer (amaryllis borer) on bulbs and Clivias, Looper worm, lawn caterpillar and diamond-back moth larvae.
The Good… an adult Dejeania feeding on nectar. Dejeania is species of parasitic fly that helps reduce African Bollworm populations. It is an important enemy of the destructive Bollworm and to benefit from its presence gardeners should ensure that some of its natural habitat is provided within the garden and avoid using pesticides that will affect it.
The Bad… an African Bollworm (Helicoverpa Armigera) larva feeding on a Coreopsis flower. Use of formulations containing Bacillus Thuringiensis var. Kurstaki will curb Bollworm populations but will not harm Dejeania.