Efficient Release Strategies for Aphid Natural Enemies in Flower Crops Progress Report — June 1997
Date:5/30/97
Title of Project: Efficient release strategies of natural enemies
in flower crops.
Institution where work is being conducted: Texas A&M
Amount of Endowment Grant: $35,000
Covering Period: 7/96 to 7/97
Anticipated Date of Project Completion /Final Report:
Individual(s) Conducting Project:
(List Project Leader First)
Dr. Kevin Heinz
Telephone Number:
introduction of novel natural enemies for a particular insect pest. Although
biological control has yielded excellent control of several insect pests
of greenhouse crops, this technique has not been widely embraced by growers
primarily due to its prohibitive costs. In response to this problem, many
researchers continue to search for natural enemies that may be more effective
than their present counterparts in hopes that a more effective natural
enemy will reduce costs. While this approach may eventually be successful,
it has not yet proven to be an effective approach to biological control
of greenhouse pests, or more specifically, for control of pests of floricultural
crops. The research I have conducted over the last several years has taken
another approach, one that assumes that at least one of the many commercially
available natural enemies will be economical and effective if it is produced
and utilized in the optimal manner. The approach of my research funded
by the American Floral Endowment has been to develop efficient release
strategies to bring about economically feasible aphid biological control
in potted chrysanthemums.
Project Objectives:
1. Compare rates that natural enemies locate and exploit patches of
aphids on chrysanthemums (Completed and reported previously).
2. Determine how plant spacing and plant architecture influence the
ability of natural enemies to locate and kill aphids (Completed and reported
previously).
3. Develop and assess release strategies necessary for successful biological
control for a particular crop spacing and stage of the crop (Discussed
in this progress report).
4. Conduct greenhouse trials to compare efficacy of each release strategy
(To be completed during year 3).
Summary of Results to Date:
Using Myzus persicae (green peach aphid) and Aphis gossypii (cotton
aphid) as our model pests, we have been studying how the commercially available
parasitoid, Aphidius colemani, should be released into greenhouse floricultural
crops to maximize the probability of biological aphid control. Research
conducted by European scientists has identified A. colemani as the best
candidate for release based upon its life history and search characteristics,
yet control in greenhouse floricultural and nursery crops remains illusive.
Results from our laboratory research (previously reported to the Endowment)
demonstrated that the number of release points in a greenhouse may be more
important to obtaining successful biological control than is the numbers
of wasps released. Regardless of aphid distributions or densities, the
greater the number of release points, the greater the level of aphid suppression
that should be obtained. Parasitoids find patches of aphids more quickly
and hence can suppress aphid densities more quickly when there are many
release points compared to when there are few release points. When parasitoids
find aphid patches too late, aphid population growth rates greatly exceed
parasitoid parasitism rates, and thus prevent successful population suppression.
During the past year, we have studied this phenomenon using large
research greenhouses on the Texas A&M campus. Each 600-ft’ greenhouse
holds 192, 6-inch pots spread across 8 benches. Planted into each pot are
4 chrysanthemums, var. Potomac. Two extremes (– two experimental regimes)
of A. colemani efficiency are being examined. In the first, distributions
of aphids are clumped within benches and wasps are released from 4 points
within the greenhouse. In the second regime, aphids are distributed randomly
among plants within benches, and wasps are released from a single point
in the middle of the greenhouse. Based upon previously documented searching
efficiencies of the parasitoids, aphid suppression is predicted to be the
greatest in the first regime (clumped aphids, multiple release point) and
suppression in the second regime (random aphids, single release point)
is predicted to less than the first.
Three treatments within an experimental regime were used to assess the
impact of A. colemani on aphid populations: (1) a complete exclusion of
A. colemani cage containing 24 chrysanthemum pots, (2) an identical exclusion
cage receiving parasitoid releases as a control for cage effects, and (3)
releases on natural enemies into the greenhouse but outside either cage.
One third of all pots used in each treatment were inoculated with 6 MY=
persicae, and pots within benches were arranged in a clumped or random
fashion. Wasps were released weekly into all treatments at the rate of
I wasp per plant. Whole plant samples of 3 plants per cage and 18 plants
per greenhouse were censused weekly for live and parasitised aphids.
While both release strategies suppressed aphid populations (relative
to the complete exclusions cages) by more than 90% by weeks 9 and 1 0,
A. colemani released from multiple sites within the greenhouse provided
significantly better control that wasps released from a single site (see
graph below). Parasitoid releases made directly into the cages eliminated
the aphid population by no later than week 6.
(1) the distribution of the target pest, and (2) the natural enemy release
strategy utilized. Knowledge of the pest distribution can be obtained by
implementing monitoring strategies, and selection of an appropriate natural
release strategy is obtained by understanding the host searching
and dispersal behavior of the natural enemy in question. Previous funding
by the Endowment facilitated research on aphid natural enemy searching
and dispersal behavior.
Future Plans Covered by the Endowment Grant:
The information obtained from this research yields a clear picture
as to the methodologies necessary to obtain successful biological aphid
control. Future plans include (1) development of an acceptable aphid monitoring
program and (2) testing our aphid biological control program at a commercial
scale. We will conduct commercial scale trials at Ellison’s Greenhouses
(Brenham, TX) or Powell Plant Farms (Troop, TX).
Anticipated Benefits for the Floral Industry:
The development of non-chemical methods of arthropod pest control continually
ranks as a top research priority for the floral industry. In response to
this demand, research on the use of parasitoids, predators and pathogens
for arthropod pest control has spurred a tremendous growth spurt in the
commercial availability of biological control agents. Unfortunately, scientific
data necessary to develop effective and economically feasible biological
control programs has not kept pace with the growth of the industry. Simply
stated, the research community is currently unable to provide scientifically
based recommendations to the floral industry as to the efficacious and
cost effective use of most, if not all, of the commercially available natural
enemies for use in floricultural crops. Biological control remains very
much an art form rather than a reliable technology. At the completion of
my anticipated 3-year program, I will have made significant advances in
correcting this problem for at least one major pest of floricultural crops.
Current benefits already include, (1) delineation of maximum aphid densities
at which biological control can be expected to be effective, (2) quantification
of the role of chrysanthemum crop phenology on biological control, (3)
comparison of the relative abilities of natural enemy species to bring
about successful biological control, and (4) demonstration how natural
enemy release strategies may be manipulated to improve the level of biological
control obtained.
