Biological Control of Insect Pests: Southeast Asian Prospects - EcoPort

238 Biological Control of Insect Pests: Southeast Asian Prospects
Damage
max (soybean: southern China); Lablab niger (= Dolichos lablab)
(hyacinth
bean: India); Pisum sativum (pea: southwestern Asia); and Vigna
unguiculata (cowpea: Africa) (Purseglove 1968; Spencer 1973). Wild hosts
include Canavalia ensiformis,
Crotalaria juncea,
C. laburnifolia,
C. mucronata,
Macroptilium atropurpureum,
M. lathryoides,
Phaseolus
panduratus,
P. semierectus and Vigna radiata (Goot 1930; Kleinschmidt
1970; Spencer 1973; Abate 1991).
There is a large variation in susceptibility between different cultivars of
susceptible species. This variation affords an important opportunity to select
cultivars that suffer comparatively little damage and is being extensively
investigated (e.g. Annappan et al. 1984, Gill and Singh 1988; AVRDC 1990,
Talekar and Hu 1993; Talekar and Tengkano 1993; Gupta et al. 1995). Both
morphological and chemical characteristics are involved (Chiang and Norris
1983).
O. phaseoli can be a limiting factor in the cultivation of susceptible legumes
in Southeast Asia and most other regions where it occurs. Spencer (1973)
and many other authors consider it to be one of the most serious of all
agromyzid pests. Losses of 50% to 100% of crops are reported from many
parts of the world, and are particularly heavy under dry conditions.
Although some leaves may wilt as a result of mining and petiole
tunnelling, most damage results from destruction of tissue at the junction of
the stem and root. When damage is limited, plants may survive by forming
adventitious roots (e.g. with P. vulgaris and soybeans) and produce a limited
crop. However, seedlings most frequently die. Plants that do not respond
rapidly to root damage and develop adventitious roots are liable to break off
at ground level during windy periods. When infestations are heavy, the
aggregation of puparia within the stem results in it swelling, splitting open
and rotting. De Meijere (1922) reported that young plants in Sumatra
generally died when they contained 10 to 20 larvae. In Egypt, 25 larvae and
pupae have been found in a single bean plant (Hassan 1947) and as many as
320 in a cowpea plant (Abul-Nasr and Assem 1966).
Seed treatments or post emergence sprays with broad spectrum
insecticides have been used to control O. phaseoli but, inter alia,
they
undoubtedly have serious adverse effects on its parasitoids. Useful control
has been obtained by intercropping, the use of resistant varieties, adjusting
planting dates, crop rotation and other cultural methods such as planting into
rice stubble or covering the newly sown areas with rice straw. It is clear,
however, that, in areas where effective parasitoids are already present, these
can play an important role in minimising bean fly damage if not interfered
with by insecticides.