Biological Control of Insect Pests: Southeast Asian Prospects - EcoPort

4.4 Aphis gossypii 81
probing) of single infected aphids on a crop may lead to substantial loss,
reduction in aphid numbers will more than proportionally reduce the
probability of flying aphids migrating to an uninfected plant (Wellings
1991), because reduced crowding of aphids usually results in a lower
number forming wings. As indicated above, there have been a number of
attempts at classical biological control of aphids, and there are at least 7
well-documented successes up to 1988 (Hughes 1989). None of these,
however, involved A. craccivora or A. gossypii as the main target. These
latter species have, however, been subjected to important attack by
parasitoids introduced primarily against another pest aphid in the same
general environment. For example, although Aphidius colemani failed to
establish on Pentalonia nigronervosa in Tonga, it did so very successfully
there on A. gossypii. Although A. craccivora is also present in Tonga,
Aphidius colemani has not yet been recorded from it (Carver et al. 1993), but
monitoring has been minimal.
The best predictor of success in biological control is previous success
with a natural enemy in a similar environment. If this experience is
unavailable, the best chances appear to be with a climatically adapted,
adequately host-specific, natural enemy that is known to attack the pest in its
native or expanded range. Parasitic wasps appear to be the best natural
enemies available for aphids, because they are generally far more host
specific than predators and are often more efficient at searching for hosts at
low aphid densities (Hughes 1989). Predators can be very effective in
reducing aphid numbers at certain times of the year, but are often unable to
prevent damage. Furthermore, their general lack of host specificity makes
most of them unattractive to authorities responsible for approving
introductions, so they have not been dealt with in any detail in this dossier.
A feature that makes it difficult to generaliseÑand even to make clear
recommendationsÑis that many of the identifications of some of the
parasitoids are incorrect, particularly the earlier ones, but even some of the
more recent ones made by non-specialists. The selection of appropriate
natural enemies for an aphid biological control program requires a detailed
knowledge of the ecology of the target aphid and often of other potential
aphid hosts in the environment where it is causing problems; also of where to
obtain parasitoid biotypes with appropriate host specificity and habitat
adaptation. If an apparently good species fails to establish or, if established,
to become effective, it is probably worth seeking the same enemy from a
potentially more appropriate source, such as one with a better climate match;
or a biotype that exhibits a special preference for the target aphid; or the first
generation from field-collected material, rather than employing individuals
bred for many generations in the laboratory (Hughes 1989).