Biological Control of Insect Pests: Southeast Asian Prospects - EcoPort

48 Biological Control of Insect Pests: Southeast Asian Prospects
Natural enemies
Two groups of hymenopterous parasitoids attack (but are restricted to)
aphids, a larger one consisting of species belonging to the family Aphidiidae
and a smaller group belonging to the family Aphelinidae. Both groups occur
worldwide as solitary endoparasitoids. Although many of the species are
recorded as having an extensive host range, there is almost always a
significant degree of host restriction. Hosts are frequently some (but not all)
of the species in a particular aphid genus or several closely related genera.
There is good evidence that there are biotypes within some parasitoid
species, since populations from some hosts or some areas parasitise a
narrower range of hosts than the species as a whole. There may also be
differences between biotypes in their preference for the host aphid when
feeding on a particular host plant or in a particular habitat. When a parasitoid
is abundant on a preferred host it may occasionally attack a nearby nonpreferred
host, as with Diaeretiella rapae,
a major parasitoid of the cabbage
aphid Brevicoryne brassicae,
which has occasionally been recorded from
both A. craccivora and A. gossypii,
but for which it exhibits a low preference
(Dhiman et al. 1983). There are some species (or biotypes of species) that
have been found capable of generally causing high levels of parasitisation of
A. craccivora.
Those selected by Starù (1967a, b) are shown in bold italics in
table 4.4.1 and might be considered first as potential species for biological
control introductions to areas where they do not already occur. Valuable
reviews of the effectiveness of aphid parasitoids are provided by Carver
(1989), Hagen and van den Bosch (1968) and Hughes (1989).
Although many coccinellids, syrphids, chrysopids, hemerobiids and a
few predator species from other insect families attack aphids, their impact in
regulating populations is generally regarded as disappointing, although they
must certainly at times limit economic damage. The efficiency of a predator
depends upon its searching ability and effectiveness in capturing prey. The
numbers of predators seem to be greatest when aphid numbers are already
declining after a peak in abundance and, thus, their apparently great impact
at that time may actually have little significance in population regulation.
Predators can increase rapidly in numbers only after their prey has become
sufficiently abundant, so there is an important time lag between prey and
predator numbers (Hemptinne and Dixon 1991).
Coccinellids have been used successfully for the biological control of
several, relatively sessile, coccid pests, whereas results have generally been
poor against aphids. One of the reasons is that adult coccinellids and their
larvae are poor at capturing other than first instar aphids (Dixon 1989).
Indeed, the survival of newly-hatched beetle larvae is very dependent upon