Pollution and Pollution Control - Environmental Biotechnology: Theory and Application - page 82

soil insects and many carry pathogenic bacteria in their guts. Once ingested,
these bacteria pass out of the nematode and multiply inside the insect, typically
causing death within a few days. Five species of nematode were originally made
available on the US agricultural market, namely, Steinernema carpocapsae ,
S. riobravis , S. feltiae , Heterorhabditis bacteriophora and H. megidis , each
being effective against different groups of insects. They have subsequently been
joined by two more - S. glaseri and H. marelatus . Despite much research and
development effort, the initial results were largely unpredictable, with success
against many of the target species, such as wireworms and root maggots, proving
elusive. Clearly, farmers and commercial growers will not adopt a biological
control which does not reliably offer at least comparable efficacy with traditional
insecticides, and consequently for a time uptake was understandably poor.
However, technological improvements in organism production, dosage, quality
control and product delivery, coupled with increased understanding of optimal
timing and greater target application specificity have done much to overcome
this. The gap between biological control and conventional agro-chemicals
has been significantly narrowed as a result, allowing nematodes to compete
effectively in a variety of market sectors.
To date, arguably the most successful application for this technology has arisen
not, as many had predicted, in the control of cockroaches, which have been
found to be the most vulnerable species to augmented nematode attack (Georgis,
1996), but in controlling garden slugs. Widely available in garden centres and
from specialist suppliers, Phasmarhabditis hermaphrodita has proven itself to be
a highly effective, and entirely specific, method of control on the domestic scale,
though here too routine treatment and accurate dosage assessment is the key to
success. Augmentation is, obviously, a highly interventionist approach and relies
on a regime of continual management to ensure its effectiveness.
There is also a role for the engineered application of biologically derived chem-
icals in this sector. One example of this is the growing interest in Azadirachta
indica , the neem, a plant which is found naturally in over 50 countries around
the world including India, where its medicinal and agricultural value has been
known for centuries. The compound azadirachtin has been identified and isolated
from the plant and it has been shown to have broad spectrum insecticidal proper-
ties, acting to disturb larval moults and preventing metamorphosis to the imago.
Additionally, it also seems to repel many leaf-eating species, and trials involv-
ing the direct foliar application of azadirachtin has shown it to be an effective
way of protecting crop plants (Georgis, 1996). This duality of action makes it a
particularly appealing prospect for wide scale applications, if suitable methods
for its production can be made commercially viable.
Semiochemical agents
However, perhaps one of the best examples of the use of such biological technolo-
gies in pest control is the development of isolated or synthesised semiochemical
agents.