Plants, herbivores and carnivores are connected through an intricate array of chemical linkages. The plant kingdom is by far the most efficient producer of chemical compounds, synthesising many products that are used in defence against herbivore attack.
Extracts prepared from plants have a variety of properties including insecticidal, such as toxicity to nematodes, mites and other agricultural pests. They also have antifungal, antiviral and antibacterial properties against pathogens. Many farmers in developing countries do not have the resources to purchase and apply expensive synthetic pesticides. On the other hand, biological control, in the form of locally abundant natural enemies, together with the preparation of plant extracts from trees growing naturally in the surrounding area, have little to no cost, and are therefore uniquely suited to low-input integrated pest management systems.
A collaborative project is underway between Wageningen University (The Netherlands), the International Foundation of Science (Sweden) and the Plant Protection Research Institute of the Agricultural Research Commission in South Africa to look into the possibility of combining biological control with the use of botanical extracts for the Integrated Pest Management (IPM) of the diamondback moth (DBM). The diamond back moth, Plutella xylostella is much cause for concern as a pest of crucifers, that is plants such as cabbage, cauliflower, broccoli and canola.
The diamond back moth
DBM was recorded as a pest on cabbage in South Africa as early as 1917. A variety of indigenous natural enemies have been found to attack DBM in the field. However, control of crucifer pests in South Africa still remains heavily dependent on synthetic insecticides, despite an abundance of natural enemies found in the crop. Largely because of the negative impact of pesticides and the increasing difficulty encountered in controlling DBM populations by means of chemicals, much effort is being devoted to finding alternative control solutions for this pest problem.
Biological control is widely recognised today as a major component of DBM management strategies, particularly where chemical control has failed. However, as the sole method of pest control in a specific target crop, biological control is seldom sufficient. Therefore, biological control must be integrated with other control tactics to obtain a successful outcome.
Plant extracts for alternative control of DBM
Of the 1 800 plant species reported to possess pest control properties, 82 species have been found to be active against DBM. Extracts from the neem tree, Azadirachta indica (Meliaceae), have been made from seeds and kernels and have been found to give good control of DBM. A closely related species, the syringa tree, Melia azedarach (Meliaceae), also has insecticidal properties and has been tested against a number of insect species including DBM. These botanical pesticides are thought to be compatible with biological control as they have little to no impact on natural enemies. Although the neem tree does not grow in South Africa, the closely related and exotic syringa tree is common throughout the country.
The contrasting effects of certain chemical compounds
Members of the plant family Cruciferae are chemically linked by the almost universal presence of glucosinolates. These compounds are considered the first line of defence in crucifers against herbivore attack. However, some insects, such as DBM, have adapted to this strategy and are crucifer specialists that actually utilize the glucosinolates and their volatiles to recognise and locate suitable host plants. Secondary chemical substances, such as glucosides and volatile mustard oils, stimulate feeding by these crucifer specialists.
In contrast, the complex tetranortriterpenoids found within plants from the Meliaceae are thought to be feeding deterrents. Such chemicals play an important, if not major, role in host plant selection by herbivores. The Meliaceae are best characterised by the production of limonoids, a group of modified triterpenes. The neem tree, for example, contains upwards of 100 different limonoids in its different tissues. Many of these are biologically active against insects and act as anti-feedants. There is ample anecdotal evidence as to the insecticidal, repellent or deterrent properties of neem and the closely related syringa trees.
The infochemical signal
Plants also release chemical signals, or infochemicals, in response to herbivore attack. Natural enemies make use of these infochemicals to locate their herbivore hosts. If a cabbage plant is treated with extracts from neem and syringa trees the infochemical signal of the cabbage plant may be altered. It is not known how the herbivores and their natural enemies will then respond to these new complex chemical signals. These aspects are currently being investigated. Initial experiments have been carried out looking at the impact of botanical extracts on the survival of DBM and its natural enemies in South Africa.
Results thus far indicate that neem and syringa extracts are effective against DBM, significantly reducing the survival of larvae feeding on cabbage leaves treated with these extracts. In addition, if DBM larvae are given a choice they prefer to remain on the untreated side of the leaf. Once the cabbage is treated with neem or syringa extracts, it no longer acts as a feeding stimulant for the crucifer specialist. Neem and syringa extracts appear to mask the inherent attractive properties of the cabbage plant to the larvae.
Biologically active substances such as botanical pesticides have an impact on the target organism, but they also have side effects on non-targets such as natural enemies. These effects are, as a rule, relatively minor and therefore tolerable, especially in an IPM situation. Experiments carried out to test the impact of the extracts on two of the hymenopterous parasitoids most commonly found in the field, Cotesia plutellae (Braconidae) and Diadromus collaris (Ichneumonidae) showed that these extracts did not have a negative impact on parasitoid survival.
Initial results appear to suggest that biological control and the use of botanical pesticides can be integrated. However, it is still unclear whether these botanicals can actually enhance biological control. This important aspect is currently being examined.
Results from this research will help in understanding the relationships that are important in the integrated pest management of DBM. It is hoped that the results will be used to benefit resource-poor farmers in South Africa, and elsewhere in the region, through the improvement of biological control and a reduction in the use of chemical pesticides.