Efficient Release Strategies for Aphid Natural Enemies in Flower Crops Progress Report — June 1998
Date:7/1/98
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: 7/97 to 7/98
Anticipated Date of Project Completion /Final Report:
6/30/99
Individual(s) Conducting Project:
(List Project Leader First)
Dr. Kevin Heinz
Telephone Number:
enemies in flower crops
Report Content:
Documentation of the Buildup and Distribution of Aphid Outbreaks in
Chrysanthemums. During the
early stages of an outbreak, aphids form small clumps on individual
plants within chrysanthemum greenhouses. These initial outbreaks
quickly become serious problems because aphid populations quadruple in
size daily when occurring on healthy plants. In addition, results
conducted by my laboratory in Texas A&M University greenhouses demonstrated
that green peach aphids can spread over an area of 120 ft’ per day after
infesting an single potted chrysanthemum. Hence, aphids may be present
for a long period of time within a greenhouse and have the opportunity
to reproduce and spread throughout the crop before being noticed.
These abundance and distribution data formed the basis for the development
of optimal natural enemy releases strategies for biological aphid control
(outlined below), and have been used to develop an aphid sampling plan
that can be used in conjuction with existing and developing sampling plans
for leafminers, spider mites, whitefly, and thrips.
Basic Studies on Aphid Natural Enemies. For predators and parasitoids
to provide successful aphid biological, these natural enemies must locate
and consume aphid patches when they are relatively scarce before the aphids
infest the entire greenhouse. Therefore, growers should only use
the most effective predators and parasitoids, and release them in such
a manner whereby they can locate and kill aphids at a sufficiently high
rate.
Determining exactly how natural enemies respond to aphid patches as
they change in time and space requires long and tedious experiments.
After completing a set of intensive studies, my laboratory discovered the
limitations to obtaining biological aphid control by releasing predators
and parasitoids.Green lacewing larvae, used as a model predator, were found
to be incapable of navigating between potted chrysanthmums placed atop
solid benches. Although lacewing larvae voraciously consume aphids
once discovered, successful biological control requires placement of lacewing
larvae onto each individual plant infested with aphids.
By comparison, studies with the parasitoid wasp A. colemani demonstrated
that it could spread over an area of 147 ft’ per day after being released
from a single potted chrysanthemum. From these results, my lab determined
that the most effective biological aphid control could be obtained by releasing
A. colemani from points no greater than 12 feet apart within a potted chrysanthemum
greenhouse.
Knowing How to Release Natural Enemies Makes a Difference. Greenhouse
trials were conducted to test the influence of A. colemani release strategies
on their ability to biologically control green peach aphids in research
greenhouses. Wasps were released at the rate of three per pot per
week from 4 points, 12 feet apart or from one central point within 1200
ft’ greenhouses filled with potted chrysanthemums. Additionally,
each greenhouse contained a screened cage, which covered a bench of chrysanthemums,
and prevented wasps from accessing aphid infested plants. Comparisons
between aphids densities within the cages to those outside the cages (into
which A. colemani were released) provided an experimental method for assessing
the impact of parasitoid releases. At the beginning of the trial,
every third pot within the greenhouse was infested with 3 green peach aphids.
Both A. colemani treatments (released from I or 4 points) yielded significant
aphid suppression. Densities in the cages from which wasps were excluded
exceeded 5,000 aphids per plant by week 6 of each of the trials.
Aphid densities climbed to 27.5 per plant at week 10 in greenhouses where
wasps were released from one central point. By comparison, aphid
densities reached a maximum of 5.8 per plant at week 3 in greenhouses where
wasps were released from four points.
Harvest quality of the potted chrysanthemums at the completion of each
trial were determined by 14 horticulture and entomology faculty, staff,
and students who judged representative samples of pots from each treatment.
Pots were ranked in terms of quality with I representing the highest quality
and 6 the lowest quality. In addition, judges identified pots viewed
acceptable for purchase as a gift. Because plants were not pinched,
grown under shade cloth, or treated with growth regulators, quality estimates
probably represent a minimum for each treatment.
Pots from the aphid infested cages were judged completely unacceptable,
always ranking the poorest in quality and never being acceptable for purchase
as a gift. Pots from greenhouses where wasps were released from four
sites always ranked higher in quality than pots from greenhouses where
wasps were released from a single location.
Further, the percentages of pots deemed acceptable for gift-giving
from the 4-release point greenhouses were greater than the percentages
of gift quality pots never infested with aphids. Thus, biological
control is not only an effect method of aphid control, but facilitates
production of high quality potted crysanthemums. This biological
control releases technology, and the sampling program developed coincidentally,
are ready for final evaluation. During the 1998/99 fiscal year, sampling
validity and aphid biological control will be evaluated in commercial chrysanthemum
greenhouses in terms of pest control, economic feasibility, and grower
acceptance. Trials will be conducted in two sites and at two different
times of the year to assess the influence of cultural practices on the
sampling and biological control programs. The sampling program is
compatible with existing monitoring programs targeted at other key pests
and the biological control methods in two sites and at two different times
of the year to assess the influence of cultural practices on the sampling
and biological control programs. The sampling program is compatible
with existing monitoring programs targeted at other key pests and the biological
control methods evaluated will be applicable for natural enemy releases
targeted at other pests of floricultural crops.
