Date 9/1/95

Title of Project Efficient Release Strategies for Aphid Natural Enemies in Flower Crops

Institution where work is being conducted Texas A&M University

Amount of Endowment Grant $30,000
Covering Period 1/1/95 to 12/31/95

Anticipated Date of Project Completion/Final Report 12/31/97

Individual(s) Conducting Project:

(List Project Leader First)

Dr. Kevin M. Heinz - Title Assistant Professor

Telephone Number

Efficient Release Strategies of Aphid Natural Enemies in Flower Crops

Kevin M. Heinz

Texas A&M University

Progress Report to the American Floral Endowment, 9/1/95

Since my arrival at the Texas A&M campus on October 15, I have made great

strides in developing a research program aimed at studying methods of biological

control that are both practical and effective. Most of the biological control research

of conducted over the past 100 years has concentrated primarily on the

identification and introduction of novel natural enemies for a particular insect pest.

While this approach has yielded some fantastic results to fruit and nut growers, the

approach has been less successful when applied to row crops and has had minimal

impact on greenhouse growers. As an alternative, annual crop producers have

attempted to utilize inoculative or inundative approaches to biological control.

These approaches require several to many releases of natural enemies to be made

within each crop cycle to achieve the desired level of control. While these

approaches have also yielded some excellent results, they have not been widely

embraced by growers due to their 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 yet proven

to be an effective approach to biological control of greenhouse pests, or more

specifically, for control of pests of floriculture crops. I have elected to take another

approach, one that assumes that at least one of the currently available natural

enemies will be economical and effective if it produced and utilized in the optimal

manner. My approach has been to address:

(1) how various species of natural

enemies may be used in concert to achieve biological control of an entire pest

complex,

(2) how release strategies may be modified to achieve the best possible

level of control for a specific number of natural enemies released, and

(3) mass-

rearing practices may be modified to reduce producer costs that will ultimately

reduce the costs to growers.

The American Floral Endowment, through their generous grant support

during the last year, already plays an important role in achieving my goals. With

support from the Endowment, I am making progress in developing efficient release

strategies to bring about aphid biological control in potted chrysanthemums. I also

recognize the financial limits and the numerous and diverse interests represented

by the Endowment. However, I am pleased to report that I have and will continue

to solicit complementary funding from other sources to speed attainment of my

stated goals. I have received two years of funding from the USDA*National

Research Initiative to study the interactions and compatibility of different natural

enemies necessary to effectively suppress various pests within a cropping system.

Often researchers focus on one pest and one natural enemy within a cropping

system. However, if biological control in to ever be an effective peat management

strategy, it must either be compatible with existing technologies or provide control

for more than one insect pest. Another significant hurdle associated with

implementation of biological control of greenhouse pests is the prohibitive cost

structure associated with its use by comparison to conventional chemical control

strategies. To study how to modify natural enemy production methods to reduce

the price structure associated with inoculative and inundative biological control, I

have solicited funding from the Texas Higher Education Coordinating Board. This

grant proposal has the full support of the Association of Natural Biocontrol

Producers and the USDA-APHIS Biological Control Laboratory in Mission, TX. Use

of various funding sources permits the Endowment to be an active participate in my

research program while at the same time pursuing many of their diverse interests.

The focus of my research funded by the Endowment is to develop efficient

release strategies for aphid natural enemies. A basic assumption associated with

this research is the availability of natural enemies for use by interested growers,

This assumption was recently tested by USDA-APHIS through their attempts to

restrict the use of biological control by a proposed met of restrictive regulations.

Together with the rest of the silent community, we have successfully fended off

the implementation of these regulations, at least for the present time. I authored a

brief news article published in the August issue of GrowerTalks (pg. 10) outlining

the potential onslaught of new regulations on environmentally benign peat

management practices. I will continue to fight against unnecessary regulations

that may severely hinder the ability of growers to produce an economically viable

crop within scientifically defensible guidelines, and I will keep the Endowment

informed of my activities in this area.

Research Progress:

My aphid research is progressing on schedule despite numerous delays,

Because the planning and budget office of the Texas Agricultural Experiment

Station was initially unwilling to sign the standard Material Agreement with the

Endowment, funds for my project did not arrive until early April, 1995. The

University prohibits any expenditures from being charged to a grant until the funds

are in place. Hence, the results we have achieved are truly amazing given the five

months we have been permitted to work on the project. The success we have

experienced to date is also due to the support provided by Yoder Bros. (for providing

rooted chrysanthemum cuttings), Bunting Biological North America (for providing

parasitic wasps), and Buena Biosystems (for providing predators).

The first objective of my aphid research is to measure how natural enemies

find and exploit patches of aphids on chrysanthemums. To effectively control

aphids, aimple knowledge of the number of aphids that can be killed by a natural

enemy is not enough. Equally important is the speed with which natural enemies

locate aphids within the greenhouse. Without this information, released of natural

enemies into greenhouse rely completely on guesswork! This trial-and-error

methodology is extremely risky for floricultural crops due their near-zero damage

tolerance levels and extremely high dollar value. Furthermore, such a methodology

will never permit minimization of the costs associated natural enemy releases.

Lastly, aphid populations exhibit explosive population growth and effective natural

enemies must locate aphids prior to significant deposition of honeydew and cast-

skins.

In our studies we are comparing the response of a commercially available

predator (Chrysoperla rufilabris) and parasitoid (Aphidius colemani) to

experimentally manipulated aphid distributions. We have selected these two

natural enemies for several reasons. (1) C. rufilabris is relatively inexpensive ($2-

3 per 1,000) and is available from numerous insectaries throughout the US (2)

Previous research suggests that successful biological control of green peach aphid

infesting potted chrysanthemums could be obtained by releases of Chrysoperla. (3)

A comparative study of several aphid parasitoids discovered that A. colemani

parasitized significantly more green peach and melon aphids than did two other

parasitoids, A. matricariae and Lysiphlebus testaceipes. Considering this result, we

concluded that A. colemani would be the most suitable species for use in our

experiments.

In one of my 600 ft2 greenhouses, I measured the responses of C. rufilabris

and A. colemani to various distributions of aphids within chrysanthemum plants

(var. ‘Pomona’) spaced 12 inches apart. Plants completely free of any aphid

infestation were also used to generate an environmental check or control treatment,

In each trial, one hundred wasps or third instar lacewings were released from a

point source into the greenhouse no later than 0700. At two hour intervals, all

infested plants and 30% of the uninfested plants are examined thoroughly and the

number of natural enemies per plant recorded. Examinations of the plants

continued throughout the release day until dark, at which time all infested plants

are collected. These plants are held in a walkain cold storage room until each plant

can be examined with the aid of a dissecting microscope and the level of natural

enemy activity scored. Our studies pertaining to this objective will be concluded

when we examine each natural enemy species on each host distribution five times.

To prevent cross-contamination (from the two natural enemies) we are only

able to complete approximately two replicates per week. As of this writing, we have

completed approximately half of our work associated with this objective (exactly the

point where weshould be within the cycle of available funds). Results from this

work are summarized in the four following figures.

First, my results suggest that C. rufilabris larvae are capable of moving from

one plant to the next even when the foliage among adjacent plants are not touching

(Figure 1). Although dispersal rates are rather slow [ranging between 1 and 2 cm

of displacement away from the point of release] this result is contrary to popular

belief, a belief that has never previously be tested. A second interesting point is

that rate of dispersal varies with time. The maximum rate of dispersal for C.

rufilabris larvae was measured at 6 hours after their release, and rates were

significantly lower at times earlier and later to the 6 hour post-ralease time, These

results provide some very practical information. First, dispersal by C. rufilabris

larvae from one plant uninfested plant to another is possible, but slow. Hence,

larvae released a few plants away from an aphid-infested chrysanthemum plant

will probably not significantly impact this aphid iufsatation before substantial

damage occurs. Second, ability of C. rufilabris larvae to locate aphids will also vary

with time. Larvae, when first released, do not move substantially from their

release point. Not until approximately 6 hours after their release do these

predators achieve maximal movement. This delay in dispersal may further hinder

the ability of this natural enemy to provide effective biological control in potted

chrysanthemums.

Even though lacewings are able to move from plant to plant, A. colemani are

able to locate aphid outbreaks approximately 1040 times faster on average than C.

rufilabris (Figure 2). Maximum dispersal rates observed were 50 cm per hour (= 20

inches per hour), meaning wasps could easily move from one bench to another

within a 2 hour period. Wasp movement is the greatest immediately after release

and slows with time [this slowing with time is expected as wasps space themselves

in order to minimize negative interactions but yet remain within the confines of the

greenhouse]. Hence, A. colemani are expected to locate aphid outbreaks up to 50

72 times faster than are C. rufilabris. Further, wasps may possibly be released from

fewer points within an infested greenhouse that may represent a significant

saving in labor costs associated with natural enemy releases.

Our research with C. rufilabris is further along than the research with A.

colemani. The reason for this difference is due to the many more plants required to

execute our experiments with A. colemani compared to those with C. rufilabris and

the associated space limitations associated with growing sufficient plant material.

Hence, the remainder of the report will concentrate on results obtained from the C.

rufilabris research.

Movement to adjoining plants by C. rufilabris differs whether the lacewing

larvae are released on to uninfested plants compared to larvae released onto

infested larvae (Figure 3). Predators first released onto aphid-infested plants first

consume almost all of the aphids on that plant before attempting to disperse to

adjacent plants. Therefore, movement by C. rufilabris larvae is initially slower

when released on aphid infested plants compared to initial movement when

released on uninfested plants. These results suggest that C. rufilabris larvae

released directly onto aphid infested plants may provide excellent biological control.

However, if any infested plants do not receive an innoculum of these predaceous

larvae, biological control may be a failure due to the inability of these larvae to

located neighboring infested plants prior to the onset of significant damage.

Biological control of aphids using this species of natural enemy my be directly

related to the accuracy of the monitoring program utilized by the grower. If the

locations of the each infested pot are known, biological control by C. rufilabris will

likely be a stunning success. Whether such a monitoring program is feasible is

questionable, however.

A basic assumption upon which our research is based is that dispersal rates

are strongly influence the outcome of biological control. Chrysoperla rufilabris

larvae that disperse at greater rates consume significantly more aphids than

lacewing larvae that disperse at slower rates (Figure 4). Hence thesuccess of

biological control by this species is directly related to its ability to move to other

aphid-infested plants and to successfully forage on the new-found aphids.

Lacewing larvae with higher dispersal rates will find aphid-infested plants more

quickly and may prevent the occurrence of significant damage. Further, few release

points may be used for species with higher dispersal rates and hence require less

labor in achieving successful biological control.

The current information provided in this report yields a clear picture as to

ways natural enemies move through potted chrysanthemums to locate aphid

outbreaks. A comparison of the results obtained with C. rufilabris with those to be

obtained with A. colemani should provide further insight in the development of an

optimal release strategy. A next step in this research will be to measure how the

natural enemies are impacted by the variance in the distribution of aphid

populations, and what are the capabilities of these natural enemies (in terms of

suppressing aphids) after they locate the aphid-infested plants. Based upon this

knowledge base, we will design and field test an optimal release strategy for aphid

biological control. Ellen and Jim Ellison of Ellison’s Greenhouses (Brenham, TX)

have consented to permitting us to conduct the field trials in their greenhouses.

Descriptions of Figures:

Figure 1. Chrysoperla rufilabris dispersal through uninvested chrysanthemums,

The results presented in this figure represent movement by third instar lacewing

larvae in the absence of aphids (environmental check or control). Larval dispersal

rates (distance moved from the point of release per hour after release) are plotted

for five time periods after the release of the larvae. The solid circles represent the

calculated dispersal rate for each of the five replicated studies. The solid line

represents the best fit line and the dotted lines represent the 95% confidence

intervals about the best fit line obtained from a nonlinear regression procedure.

Figure 2. Aphidius colemani dispersal through uninfested chrysanthemums.

These results presented in this figure represent movement by adult wasps in the

absence of aphids (environmental check or control). Adult dispersal rates (distance

moved from the point of release per hour after release) are plotted for five time

periods after the release of the wasps. The solid circles represent the calculated

dispersal rate for each of the five replicated studies. The solid line represents the

best fit line and the dotted lines represent the 95% confidence intervals about the

best fit line obtained from a nonlinear regression procedure.

Figure 3. Comparison of Chrysoperla rufilabris dispersal through aphid-infested

chrysanthemums when the lacewing larvae are released onto uninfested plants (top

figure) versus larvae released onto aphid-infested plants, Dispersal by the lacewing

larvae during the first two hours is significantly greater when released onto

uninfested plants versus aphid-infested plants. This difference is due to the time it

takes the predaceous larvae to consume the aphids on the infested plant prior to

attempting to disperse. Larval dispersal rates (distance moved from the point of

release per hour after release) are plotted for five time periods after the release of

the larvae. The solid circles represent the calculated dispersal rate for each of the

five replicated studies. The solid lines represent the best fit line and the dotted

lines represent the 95% confidence intervals about the best fit line obtained from a

nonlinear regression procedure.

Figure 4. The relationship between Chrysoperla rufilabris movement and the level

of aphid biological control achieved through the release of these predators.

Chrysoperla rufilabris larvae with higher dispersal rates consume significantly

more aphids than lacewing larvae with slower dispersal rates. Hence the success of

biological control by this species is directly related to its ability to move to other

aphid-infested plants and to successfully forage on the new-found aphids, The

number of aphids consumed on infested, potted chrysanthemums plants versus

larval dispersal rates (distance moved from the point of release per hour after

release) are plotted for 8 replicated trials. The solid circles represent the observed

numbers of aphids consumed by a population of C. rufilabris larvae exhibiting the

associated average rate of dispersal. The solid line represents the best fit line

obtained from a non-linear regression procedure.

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