Optimal Placements of Biocontrol
Agents for Optimal Results
Parasitic wasps are integral to biocontrol programs in greenhouse crops.
Important examples include several Aphidius species and Encarsia
formosa that are used for management of aphids and whiteflies, respectively.
Efficient foraging or location of prey by these wasps contributes to their
success as biocontrol agents. However, in greenhouse crops where initial
pest infestations are isolated or occur in patches, location and suppression
of such areas by parasitic wasps may not occur in a timely fashion. Timeliness
is key for growers, considering, for example, that aphid populations can
increase at a rate of about 12 times each week under greenhouse conditions,
depending on temperatures. Users of biocontrol agents can assist them
for more timely control by placing them as strategically as possible within
the crop. To demonstrate the importance of optimal placements, this article
focuses on dispersal by Aphidius colemani and Trichogramma species.
Dispersal by Aphidius colemani
Currently, many growers place banker plants for release of Aphidius
species along main walkways at the ends of rows (Fig. 1) which are often
90 m long. Although such locations greatly facilitate ease of placement
and maintenance, they may not facilitate efficient searching and location
of aphid patches by the wasps. Field studies with A. colemani in
Germany showed that this wasp can disperse randomly from a central point
with a radius of about 16 m within 24 hr after release. However, the majority
of the wasps moved only 1 to 2 m from the point of release within 24 hr,
and 88% of their eggs were laid within the first 2 days after emergence.
Such data indicate that the majority of parasitization by A. colemani
occurs within a few metres from their points of release. This study also
indicates that the current placement of banker plants along walkways may
not facilitate efficient and timely location of aphids within a crop,
leading to problems with aphid management. Perhaps, more even distribution
of banker plants throughout the greenhouse, with the distance between
banker plants not exceeding 40 m, will provide for better management of
aphid populations.
Fig 1. Banker plants for rearing of aphid parasitoids
situated along walkway at beginning of rows
Dispersal by Trichogramma species*
A trial was undertaken in 2003 to compare the dispersal and parasitism
of egg masses (previously frozen cereal moth eggs) by Trichogramma
ostriniae and T. brassicae in commercial greenhouse peppers
in Ontario. Egg masses on cards were placed in the pepper crop in a grid
pattern with the central point of the grid serving as the site for a single
point release of T. ostriniae or T. brassicae. Egg masses
were located at 3.6 m (12 ft), 7.3 m (24 ft) or 11 m (36 ft) from the
point of release. A total of 48 egg masses were used for each of 3 replicates.
Eggs were placed in the upper third of the plant canopy whereas Trichogramma
sp. were placed about mid-way down the canopy using a rate of 200,000/acre
(500,000/ha). An interval of 96 hr was allowed to elapse following release
of each Trichogramma species prior to collection of egg masses
for determination of incidence of parasitism. The average percentage parasitism
was 61, 19 and 4% for egg masses located 3.6 m, 7.3 m, and 11 m respectively,
for T. ostriniae, and 10, 17, and 42 % respectively, for T.
brassicae. A subsequent trial, in which 3 rates of release (250,000/ha,
500,000/ha, and 1,000,000/ha) were evaluated in a similar experimental
design, indicated that for a given distance from the release point, an
increased rate of release did not result in significantly increased parasitism.
Therefore, distance, not release rate, was key to percentage parasitism.
In conclusion, parasitoids are specialised biocontrol agents that are
good searchers of prey. However, given the low threshold for pests such
as aphids in greenhouse vegetable crops, and the random and isolated pattern
of initial infestations, we need to pay more attention to optimizing release
points for best results.
*Acknowledgements: Assistance provided by the following in these
trials is gratefully acknowledged - Dave Delellis and Xin-Tong Yang of
DelSol Greenhouses, Ontario; Jay Whistlecraft of Southern Crop Protection
and Food Research Centre, Agriculture and Agri-Food Canada, London, Ontario;
François Fournier, formerly of Insecterra, Quebec.
