Biological Control of Insect Pests: Southeast Asian Prospects - EcoPort

82 Biological Control of Insect Pests: Southeast Asian Prospects
Both A. craccivora and A. gossypii are now almost cosmopolitan in their
occurrence throughout the temperate, subtropical and tropical regions of the
world and both are polyphagous. At least A. gossypii exists as a series of
biotypes with different spectra of host preferences and both it and
A. craccivora owe a considerable amount of their economic importance to
their ability to transmit an extensive range of important plant viruses.
Both aphids are attacked by a wide range of parasitoids and share a
number of these species. The majority of these parasitoids are polyphagous
and attack many other (but not all) species of the genus Aphis and some
species in related aphid genera. Two important parasitoids are the American
Lysiphlebus testaceipes and the Indian Aphidius colemani. These parasitoids
both exist in a number of biotypes with different host spectra and abilities to
attack A. craccivora and A. gossypii on some plants, but not on others. Thus,
when biological control is attempted, care must be taken to select a
parasitoid biotype that is appropriate for the aphid biotype, the host plant and
the prevailing environmental conditions. Laboratory comparison of the
impact on A. gossypii of Aphidius colemani, Lysiphlebus testaceipes and
Aphidius matricariae indicated that A. colemani was the most and
A. matricariae the least effective (van Steenis 1992). The polyphagous
nature of many effective parasitoids has the advantage that, in any region, a
number of other aphid species will be parasitisedÑand hence serve as
valuable reservoirs of parasitoids when the target pest population falls to a
low level. The aims of aphid biological control are (i) as far as possible to
maintain densities below those at which alates are formed due to crowding
and (ii) if possible, to depress densities still further, so that sap removal,
volume of honeydew produced and plant deformation become unimportant.
Takada (1992) points out that the aphid parasitoid fauna in Far East Asia
is quite different from that in India. Thus, the most important parasitoids of
A. gossypii in the Far East are Trioxys communis, Lysiphlebia japonica and
Aphelinus sp., none of which occurs in India. On the other hand, the
dominant parasitoid of A. gossypii in India is Trioxys indicus, which is
present in Taiwan, but not in Japan or Korea. Another Oriental species,
Lipolexis scutellaris occurs in Hong Kong, Vietnam and Malaysia. It
appears that Indian parasitoids, rather than Far East Asian species are
present in Southeast Asia. However inadequate information is available in
Southeast Asia on the natural enemies of A. craccivora and A. gossypii
present in the many different crop systems and environments in which these
aphids occur. Even within a single country, it is necessary to examine the
aphid population in the particular situations and the crops where they are
causing important problems. This can be illustrated by the parasitoid
complex of A. gossypii in Japan where it occurs in habitats ranging from