Efficient Release Strategies for Aphid Natural Enemies in Flower Crops Progress Report — March 2000
Final Report Deadline:
Title of Project
-Efficient Release Strategies of Natural Enemies into Flower Crops
Running Title: (3 word title)
- Efficient Release Strategies
Institution(s) Where Work Was Conducted
-Texas A&M University
Total Endowment Funds - $
122,500, Grant Period-
Jan. 1996 to July 1999
Kevin M. Heinz ,Texas A&M University
Title - Associate Professor
Telephone Number - 409-862-3408
Fax Number - 409-845-7977
E-Mail Address - KMHeinz@tamu.edu
Revised 12/99
INTRODUCTION
Aphids are serious pests of almost every
floricultural crop produced worldwide. Current control measures depend almost
completely on the prophylactic use of insecticides. While this practice is
understandable in view of the explosive potential population growth within
aphids, this repeated heavy use of insecticides is clearly inconsistent with
integrated pest management, a concern for the environment, and a concern for the
health and safety of agricultural workers. This project examined the potential
of an aphid predator and an aphid parasitoid to control aphid outbreaks in
floricultural crops. The use of regular releases of commercially available
natural enemies has been demonstrated to be an effective alternative to
insecticide-based control. However, the substantially higher direct cost
associated with biological control compared to conventional insecticide control
has hindered widespread acceptance of biological control. Several methods have
been proposed to reduce the cost of biological control; and while each of these
techniques has helped to reduce the cost structure associated with biological
control, none of them has reduced the cost structure to levels generally
accepted by greenhouse growers. Using the aphid system as a model, this project
evaluated natural enemy release strategies based upon aphid distributions and
natural enemy behavior as a mechanism that has not been explored in sufficient
detail. As a result, the goal of the project was to produce a success and
cost-effective method for biological aphid control. In addition, the method will
be applicable for natural enemy releases targeted at other pests of
floricultural crops.
Aphids (esp. the green peach aphid, Myzus persicae,
and the melon aphid, Aphis gossypii) rank as one of the most serious
pests of greenhouse floricultural crops. Difficulty in controlling these pests
could be due to the development of insecticide resistance or it could be due to
the development of various biotypes or races. Hence, complete reliance on
chemical control is a risky proposition. In addition, it is becoming necessary
to find aphid control strategies that are compatible with control measures for
other key pests of ornamentals.
Under the optimal conditions of the greenhouse
environment, aphid population growth can be explosive. Aphid young are born
fully formed and are able to feed immediately. They grow rapidly, molting 4
times before they mature, often within a week or less. Because fertilization is
not required, eggs can start developing within an aphid when, or even before, it
is born. By the time a female matures, several young are fully developed in her
reproductive system and are ready to be born. Young are then produced at a rate
of 3 or even 6 a day for several weeks.
Aphids damage plants directly, wilting and
distorting leaves and flowers as they feed, but they also cause a number of
other problems. The physical presence of a large number of aphids can be a cause
for concern, and the honeydew they excrete as they feed promotes the growth of
black sooty molds, which in turn reduce the crop’s photosynthesis as well as
its aesthetic value. Dust, dirt, and skins shed in molting adhere to the viscous
substance, making plants unsightly. Furthermore, aphids transmit several plant
viruses.
Because most floricultural crops have little
tolerance for damage, growers must identify pest populations early and take
appropriate control measures. The need for early control has resulted in the
sometimes-unnecessary prophylactic use of chemicals for aphid control in
greenhouses. The repeated use of insecticides is clearly inconsistent with
integrated pest management, a concern for the environment, and a concern for the
health and safety of agricultural workers.
Inundative and augmentative biological control,
or repeated releases of large numbers of predators and parasitoids, have been
proposed as possible methods for controlling insect pests of floricultural
crops. This practice is extremely effective across a variety of pest-crop
systems. Regardless, the 3-10 fold difference in the monetary costs typically
associated with implementing biological control compared to conventional
insecticide practices prevents most growers from embracing it as a regular
practice.
Although many natural enemies of aphid exhibit
potential to provide control under greenhouse conditions, previous research
efforts have concentrated on Aphidoletes aphidimyza (the aphid midge), Aphidius
matricariae (and other parasitoids), Chrysoperla carnea (the green
lacewing, and Verticillium lecanii. The studies described below
concentrate on C. carnea and Aphidius colemani for several
reasons. (1) C. carnea is readily available and relatively inexpensive
($2-4 per 1,000) from numerous commercial insectaries throughout the U.S. (2)
Previous research illustrated that successful biological control of green peach
aphids infesting potted chrysanthemums could be obtained by releasing C.
carnea. (3) A comparative study of several aphid parasitoids discovered that
A. colemani parasitized significantly more green peach and melon aphids
than did A. matricariae and Lysiphlebus testaceipes. Based on this
result, it was concluded that A. colemani may be the most suitable
species (of the three parasitoid species) for use in aphid control.
Chrysoperla carnea
does equally well feeding on green peach and melon aphids. At 24oC,
the duration of the various developmental stages are 5.3 days as eggs, 5.8 days
as 1st instar larvae, 3.5 days as 2nd instars, 5.2 days as
3rd instars, and 13.4 days as pupae. The threshold temperature for
development is 9.3oC. Laboratory studies report that adult mating
occurs within the first few days of emergence and almost immediately after they
begin to lay 400-500 eggs over their 2-3 month adult lifespan. These aspects of
adult reproduction have yet to be verified in the greenhouse setting.
Previous findings supported the need to study C.
carnea biology more thoroughly in order to develop effective biological
control strategies. Based on results from these studies, it was concluded that
at least four releases of lacewings at the rate of 1 predator to 50 aphids were
needed to provide satisfactory control. In addition, if there were four or fewer
aphids per plant, these were not discovered by the searching predator and no
control was achieved. It was estimated that 1st instar lacewings
search an area no more than 6 inches from the release site suggesting that
predator movement patterns, plant spacing and predator release strategies are
essential components to controlling aphid populations.
Laboratory studies have demonstrated the
potential for achieving biological control of green peach and melon aphid
infesting greenhouse crops with releases of parasitic wasps 11.
In these studies, host suitability of the melon, chrysanthemum (Macrosiphum
euphorbiae), and green peach aphid for the parasitoids A. colemani, A.
matricariae, and L. testaceipes were tested. In these petri dish
tests, none of the parasitoids successfully parasitized chrysanthemum aphids. A.
colemani parasitized 80% of the melon aphids and 57% of the green peach
aphids. A. matricariae parasitized 43% and 7% and L. testaceipes
parasitized 27% and 7% of the melon and green peach aphids, respectively.
Specific Objectives
:
1. Assess How Natural Enemies May Be Optimally
Utilized in Greenhouses Characterized by Variation in the Spatial Distribution
and Densities of Aphids.
Completion of
this objective led to (1) an understanding of how spatial distributions of
aphid outbreaks influence natural enemy foraging behavior, (2) comprehension
of natural enemy foraging behavior under the most realistic situations as
possible, and (3) measurements of the impacts of natural enemies on aphid
outbreaks in chrysanthemum.
2. Conduct Trials in Research and Commercial
Greenhouses to Test the Efficacy of Release Strategies.
Tests
will be conducted in greenhouses provided by Texas A&M University and by
commercial cooperators to evaluate the dependence of successful biological
control on the spatial distribution and timing of natural enemy release
points. Providing a useful pest management tool to the greenhouse and
floriculture industries is the goal of this project.
RESULTS
1. Assess How Natural Enemies May Be Optimally
Utilized in Greenhouses Characterized by Variation in the Spatial Distribution
and Densities of Aphids.
A. 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. Aphid population growth on a newly infested pot
is greatest aphid densities are low and decreased when aphid infestations
became more dense. In addition, results conducted in Texas A&M University
greenhouses demonstrated that green peach aphids can spread over an area of
120 ft2 per day after infesting an single potted chrysanthemum.
Mean daily movements did not differ significantly between M. persicae
and A. gossypii. Hence, aphids may be present for a long 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.
The occurrence and rate of aphid clumping significantly affected
B. 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 and 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, results demonstrated
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 chrysanthemums 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 ft2
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.
C. Measurements of the Impacts of Natural
Enemies on Aphid Outbreaks in Chrysanthemum.
Observations recorded during natural enemy movement studies indicated the
percentage of A. colemani actively searching for aphids on potted plants
to be significantly greater than the percentage of Chrysoperla sp.
actively searching for aphids. Coupled with differences in dispersal
capabilities, A. colemani located 97% of aphid-infested plants compared
to Chrysoperla sp. ability to locate only 49% of aphid-infested plants.
Given an aphid-infested plant was located by A. colemani or Chrysoperla
sp., there were no significant differences in parasitism (by A. colemani)
compared with predation levels (by Chrysoperla sp.). Thus the abilities
of these natural enemies to effect successful biological control depends greatly
on their ability to find aphid-infested plants quickly and not the ability to
attack aphids once the locate a aphid infestations.
3. Conduct Trials in Research and Commercial
Greenhouses to Test the Efficacy of Release Strategies.
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 ft2
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.
Research greenhouse
containing potted chrysanthemums and a cage used to exclude wasps from
aphid-infested plants.
Both A. colemani treatments (released from
1 or from 4 points) yielded significant suppression of green peach aphids.
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.
Aphid densities in each of the
3 experimental treatments. After week 3, aphids in the “no
release” treatment exceeded 500/plant and are not shown on the graph.
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 1 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 chrysanthemums.
| Plant Quality At Harvest | ||||
| Rank | Acceptable for purchase as a gift | |||
| Wasp Release Points | 1 | 4 | 1 | 4 |
| Treatment | ||||
| Uninfested Plants | 2.0 1.6 | 2.9 2.9 | 30% 30% | 50% 73% |
| Infested Plants No Wasps | 5.5 5.4 | 5.5 5.5 | 0 % 0 % | 0 % 0 % |
| Infested Plants With Wasps | 3.6 3.1 | 2.1 2.1 | 65% 65% | 77% 85% |
Two pots per treatment and from
greenhouses, into which wasps were released from 1 or 4 points, were
ranked in order of quality with 1 representing the highest quality and 6
the lowest quality. The average rank score is provided for each treatment.
In addition, judges were asked to identify pots acceptable for purchase as
a gift. There percentages of pots within each treatment judged of
“gift quality” are illustrated in the table.
Conduct Trials in Commercial Greenhouses to Test
the Efficacy of Release Strategies.
This
biological control releases technology was evaluated in commercial chrysanthemum
greenhouses in terms of pest control, economic feasibility, and grower
acceptance. Trials were conducted at Powell Plant Farms (Troup, TX). Except for
differences in aphid management, the crop will be grown using standard
practices.
The effectiveness of natural enemy releases were
determined by comparing aphid populations in grower treated ranges with aphid
populations in experimental ranges receiving natural enemy releases. Each
release distribution will be replicated 3 times. Prior to initiating natural
enemy releases, all of th