Managing the environmental componentGiven the release of parasitoids of adequate
quality into the habitat or the presence of
wild parasitoids of proper quality, we can
then manage the environment side so as to
maximize performance. Two basic objectives
must be achieved: retention of the parasitoids
in the target area and efficient host-search
and attack behaviour. As a requirement for
this to occur, the host plant or other substrate
must be suitable and there must be a suffi-
cient host density, otherwise the parasitoids
will not remain in the area. The economic
damaging threshold of the host pest can be at
a density below that needed to sustain effec-
tive parasitoid foraging behaviour.
Semiochemicals offer good prospects as a
tool for managing the parasitoid behaviour
independent of these variables.
Hagen et al. (1970) first manipulated a
bollworm natural enemy with a semiochemi-
cal by using an artificial honeydew to attract
adults of Chrysoperla carnea (Stephens). These
chemicals provided a kairomone and food
supplement, both of which served to
increase predator density. Indole-acetalde-
hyde, a breakdown product of tryptophan
found in the yeast hydrolysate of the artifi-
cial honeydew, operated as a kairomone by
attracting adult lacewings into target fields
(van Emden and Hagen, 1976). Other com-
ponents of the artificial honeydew (sugar,
water, whey–yeast hydrolysate) arrested
movement of the lacewings and served as a
nutritional supplement, thereby promoting
oviposition. One week following an applica-
tion of such a food spray to cotton,
Chrysoperlaegg density increased from one
to three per plant, and the density of boll-
worm eggs and the number of damaged
bolls declined (Hagen et al., 1970).
The use of semiochemicals from plants
and the host, Heliothis zea, has been shown to
increase rates of egg parasitism by
Trichogrammain the field. For example, para-
sitism of eggs of H. zeaby Trichogrammaspp.
increased from 13% in control plots to 22% in
soybeans treated with an extract of scales
collected from H. zea (Lewis et al., 1975a).
Similarly, the release of a synthetic blend of
the sex pheromone of H. zea in cotton
increased parasitism of eggs from 21% in
control to 36% in treated plots (Lewis et al.,
1982). Altieri et al.(1981) demonstrated that
spraying various plant extracts on crops can
stimulate increased rates of parasitism. For
example, parasitism of eggs of H. zea by
Trichogrammaspp. was 21% on soybeans
treated with an extract of Amaranthuscom-
pared with 13% on plants sprayed with
water. The behaviours that lead to increased
parasitism by Trichogrammain the presence
of either plant extracts or sex pheromones
are not yet fully understood, but are sup-
posed to be based on arrestment responses of
the parasitoid (e.g. Noldus et al., 1988, 1990).
Application of semiochemicals to crops has
not been universally successful in stimulating
increased rates of mortality. For example, the
use of a uniform spray of moth-scale extracts
may reduce egg parasitism by Trichogramma
spp. at low host densities, apparently by stim-
ulating females to search too intensively
where no hosts are present, thereby lowering
their efficiency (Lewis et al., 1979). This prob-
lem can be partly overcome by impregnating
particles of diatomaceous earth with the moth-
scale extract to mimic natural scales (Lewis et
al., 1979). When dispersed through a field, the
treated particles intermittently stimulate
searching by Trichogramma, rather than doing
so continuously.
From these and other studies, it is obvi-
ously important to understand the respective
roles of the various cues in the host-selection
sequence and to apply the long- and close-
range cues in proper proportions and distrib-
ution so as to retain the parasitoids effectively
in the desired habitat without interfering with
efficiency. The vacuum in understanding
long-range foraging behaviour is a particular
obstacle in achieving these needs.
A discovery with the larval parasitoid M.
croceipes opened a new avenue for potential
application. Lewis and Tumlinson (1988)
found that, when contacting and antennat-
ing faeces of their Heliothis larval host, the
female links plant and other volatile odours
to a host-recognition kairomone in the fae-
ces and associatively learns to fly to those
volatile odours in search of hosts. Actual
contact with hosts and oviposition are not a
necessary part of this process. We visualizeVariations in Foraging Behaviour 53