Hope for the humble cabbage - biological pest management programs
Deidre Charleston
 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.
More Information:
Deidre Charleston
Insect Ecology Division, Rietondale
Tel: +27 (0) 12 329 3269
+27 (0) 82 962 5588
Fax: + 27 (0) 12 329 3278
e-mail: rietdi@plant2.agric.za
Article taken from: Plant Protection News No.60 (Autumn/Winter 2002).
Bulletin of the ARC-Plant Protection Research Institute. pp 7-9.
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