Beyond Economic Thresholds: Incorporating Proactive Pest Management Strategies in Alfalfa Pest Management Programs for Potato Leafhopper

Final Report for GNC13-167

Project Type: Graduate Student
Funds awarded in 2013: $8,509.00
Projected End Date: 12/31/2014
Grant Recipient: University of Wisconsin-Madison
Region: North Central
State: Wisconsin
Graduate Student:
Faculty Advisor:
Dr. Carrie Laboski
University of Wisconsin-Madison
Faculty Advisor:
Dan Undersander
University of Wisconsin-Madison
Dr. Eileen Cullen
UW-Madison Dept. of Entomology
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Project Information

Summary:

The efficacy of an integrated pest management program for potato leafhoppers in alfalfa was studied. It included host plant resistance, the cultural control of orchardgrass intercrops, and an economic threshold model. Results show that potato leafhoppers are significantly reduced in plots grown with resistant alfalfa throughout much of the study but especially during the seeding year and when populations peak in the production years. Potato leafhoppers show a range of responses to orchardgrass intercrop from suppression to no response, but in general the trend is not consistant. Low ambient potato leafhopper populations through the duration of the experiment made studying the economic threshold difficult so caged field trials were installed in which potato leafhopper populations were manipulated. Results from these trials suggest that the economic threshold should be reevaluated as the market value of alfalfa is increasing and especially with regards to drought conditions.
Concurrent field trials were established to follow up on farmer observations that liquid dairy manure can suppress potato leafhoppers. The results from this study were inconclusive; in year one, potato leafhoppers were significantly suppressed in plots that had been amended with manure while in year two, there was no difference in potato leafhopper populations between treatments.

Introduction:

Wisconsin’s $4.1 billion dairy industry (USDA ERS 2012) depends on high quality alfalfa as an important source of energy, protein and fiber for dairy cattle (Jennings 2006); lactating cattle in Wisconsin consume an average of 25% of their diet as alfalfa forage (R. Shaver, personal communication). Alfalfa is the second largest Wisconsin crop, harvested on over 13 million acres in 2012 (USDA NASS 2012).The production of dry matter alfalfa hay (roughly half of the acreage of alfalfa production) in WI in 2010 was valued at over $4 million (USDA NASS 2012).

Along with the valuable role alfalfa plays in livestock feed, it also provides ecological services. As a perennial crop with deep roots, alfalfa helps stabilize soil and protect it from erosion. Additionally, due to alfalfa’s symbiotic relationship with nitrogen fixing soil bacteria, it replenishes nitrogen supply to the soil. Alfalfa fields can harbor nearly 1,000 arthropod species, providing habitat and prey for hundreds of natural enemies (Summers 1998).

However, not all arthropods inhabiting alfalfa are beneficial; some are pests. The potato leafhopper (Empoasca fabae) (Harris) is the most economically damaging pest of alfalfa in the North Central U.S.; in most years, one or two insecticide treatments are applied (DeGooyer et al. 1998). Potato leafhopper feeding damage significantly reduces yield, stand longevity and forage quality resulting in economic losses for farmers (Cuperus et al. 1983; Hutchins 1987; Hower 1989; Vough et al. 1992; Lamp et al. 2001).

The current pest management paradigm in alfalfa for the potato leafhopper is to scout for the pest throughout the season and treat with an insecticide when economic thresholds, developed over 30 years ago (Cuperus et al. 1983), are reached (Degooyer et al. 1998, Cullen et al. 2012). Briefly, economic thresholds are the insect density at which insecticide sprays are recommended so that farmers can avoid losing economic returns on their crop yield. They are based on economic injury levels, which take into account the cost of the insecticide treatment, the value of the crop, and the crop yield loss response to insect damage. In 2007, alfalfa market value increased sharply and has continued to rise (Gould 2012) which has sparked farmer interest in lowering the economic threshold for potato leafhoppers (Mintert 2008). UW Extension county agents have reported a trend in which some farmers are applying insecticide treatments to alfalfa stubble before regrowth and omitting the important practice of scouting (Proost 2007). It is not advisable for farmers to apply insecticides at insect pest densities below the economic threshold because there will be no measurable yield return and there can be environmental and economic costs (Pedigo 2011). Entomologists have been solicited to revisit the economic threshold in response to the high hay prices (Holin 2008). Annual improvements of alfalfa varieties, increasing alfalfa market value and changing farmer practices necessitate up to date research regarding the relationship between potato leafhopper densities and alfalfa yield response.

A fully developed integrated pest management (IPM) system is comprised of multiple strategies incorporating host plant resistance, biological, cultural and physical controls and chemical control when necessary (Pedigo 1999). Several management strategies have been developed and observed for the potato leafhopper in alfalfa. For example, alfalfa varieties bred for resistance to the potato leafhopper became available to farmers in 1997 (Miller 2000). Some farmers have also employed the practice of intercropping grasses in their alfalfa stands to suppress leafhopper populations. In Part 1 (IPM trial) of this multi-year, multi-site study, alfalfa host plant resistance to potato leafhopper and orchard grass intercropping are integrated and evaluated within the framework of an economic threshold model, allowing for farmer flexibility to optimize leafhopper management costs, benefits and risks.

Wisconsin alfalfa growers have reported anecdotal experience with another potential pest management tactic. They have communicated to county agents a connection between the application of liquid dairy manure to their fields and a decreased incidence of economically damaging potato leafhopper populations (M. Rankin, personal communication). In part 2 (nutrient trial) of this study, research is conducted to follow up on this farmer observation.

Potato leafhopper response in both parts 1 and 2 of this research provides valuable insight for integrating management strategies. Alfalfa yield and quality response to the potato leafhopper population densities among the IPM systems tested here will provide current research-based data in response to farmer practice of spraying below established economic thresholds. Judicious insecticide use improves environmental quality by minimizing impacts on non-target organisms (beneficial insects, pollinators) present in alfalfa fields and improves farmer profitability by protecting yield and decreasing input costs.

After the initial release of glandular-haired potato leafhopper resistant alfalfa in 1997, Lefko et al. (2000) validated the need for differential economic thresholds between resistant and susceptible varieties. An established resistant alfalfa stand may be able to tolerate up to 2.5 times the potato leafhopper economic threshold densities established for susceptible varieties (Lefko et al. 2000). Other studies report resistant alfalfa stands demonstrate no yield advantage over susceptible varieties when potato leafhopper pressure is low (Hogg et al. 1998). At the conclusion of a previously funded SARE Northeast Farm Research/Partnership project, Hall (2005) reports that the majority of observations over two seasons showed no difference in potato leafhopper abundance between resistant and susceptible alfalfa stands. When the economic threshold was reached in susceptible stands, an insecticide treatment was applied. However, the economic return was equivalent on both the susceptible and resistant stands. Their study shows that resistant alfalfa can increase farmer profitability and environmental stewardship in times of high potato leafhopper pressure. An in-progress Northeast SARE research and education project also aims to address the ability of resistant alfalfa varieties to suppress potato leafhoppers in 2013 (Karsten 2012).

Several studies have revealed the potential for grass intercropped into alfalfa stands to suppress potato leafhopper populations. Population reductions have been observed in mixed alfalfa-grass stands between 25 and 90% compared to pure alfalfa stands. However, this grass effect is not apparent at each harvest (Roda et al, 1997; Degooyer et al. 1999). Both past and present SARE research grants have shown interest in this pest management tactic and also highlight the inconsistency in potato leafhopper response. Maravell (1991) of the Maryland Organic Food and Farming Association did not see any response in potato leafhopper populations to the alfalfa-grass mixtures compared to pure alfalfa stands, but he also observed low potato leafhopper populations even in the pure alfalfa stands. Karsten (2012) documents that in the first research season, potato leafhoppers were suppressed in the alfalfa-grass mixtures but that in the second research season, the effect reversed and potato leafhoppers were more abundant in plots with grass.

Degooyer et al. (1999) suggest that orchard grass (Dactylis glomerata L.) intercropped in alfalfa along with proactive management may aid in reducing potato leafhopper population below the economic threshold. The work outlined in this proposal is the first research we know of to combine the practice of grass intercrops with resistant alfalfa varieties. It will also be the first that we know of to research these IPM systems in the context of an economic threshold to support insecticide treatment decisions in a broader IPM context. 

No previous work has been done examining the putative relationship between manure amendments on alfalfa fields and PLH response, but a growing body of literature supports the hypothesis that organic soil amendments decrease herbivore pest incidence when compared to use of mineral fertilizers. A recent meta-analysis of peer-reviewed literature examining pest response to fertilizer concludes that pests are less abundant when manure is used compared to conventional fertilizer, perhaps because manure enhances soil environment for plant growth, which can increase their resistance to pests (Garrett et al. 2011).

Project Objectives:

Results of this research will be disseminated through extension publications, UW-Extension pest management update (PMU) meetings, and at the Wisconsin Crop Management Conference. These meetings reach approximately 2,000 participants annually comprised of growers, crop consultants and county Extension agents. Short term outcomes include; awareness of various pest management strategies and their efficacy; change in perception of insecticide sprays below established economic thresholds for potato leafhopper in alfalfa; knowledge of updated economic threshold parameters; and awareness of how to integrate a diversity of pest management tactics. These short-term outcomes have the potential to influence alfalfa grower decision-making processes. Farmers will have new information to utilize host plant resistance and/or forage grass intercrops to manage potato leafhopper. They will better understand how manure application at appropriate times and rates to their alfalfa stand may or may not impact potato leafhopper populations. If these outcomes are met, insecticide treatment decisions can be made within a broader IPM context potentially reducing risk of potato leafhopper damage and lessening the perceived need to treat below established thresholds during periods of high forage market value. These results will support an IPM system balance between crop protection, farmer profitability and quality of life, and environmental quality.  

Cooperators

Click linked name(s) to expand
  • Dr. Carrie Laboski
  • Dr. Dan Undersander

Research

Materials and methods:

Part 1 – IPM trial

Field Plot Design, Treatments and Data Collection: Multi-year research experiments were established in two locations: Arlington, WI Agricultural Research Station (AARS) and the US Dairy Forage and Research Center (DFRC) in Prairie du Sac, WI. Trials were arranged in complete randomized block with a 2 x 2 factorial design (4 total whole plot treatments). Factorial treatments were alfalfa variety (PLH-susceptible and PLH-resistant) and orchard grass intercrop (alfalfa intercropped with grass and direct seeded alfalfa).

AARS trial site: This site was established spring of 2010. Each whole plot was divided equally into 3 split plots. Split plot treatments were planned to create a range of potato leafhopper populations in order to measure alfalfa yield response and were composed of insecticide applications at one-half the current economic threshold, the current economic threshold, and an untreated control.

DFRC trial site: A fall seeding was planted August 2011 and a spring seeding was completed April 2012. Split plots were not implemented at this trial site because we implemented a caged field study (see below).

Potato Leafhopper Response

Weekly potato leafhopper counts have been conducted for all cuttings and seasons by 20 sweeps with a 15“ diameter sweep net in each split plot at AARS 2010-2012 and each whole plot at DFRC for 2012.

Yield and Forage Quality Response

AARS trial site:In 2010 and 2011, insecticide treatments were applied when potato leafhopper populations reached half economic threshold and economic threshold, to respective split plots. Yield data were collected from each plot. Alfalfa quality (crude protein and neutral detergent fiber) was analyzed by near-infra red reflectance methods on dried and ground alfalfa samples.

Caged field trials

Low potato leafhopper populations in 2010 and 2011 resulted in only one cutting in which the economic threshold was reached; therefore in 2012 methods were adjusted. To ensure high PLH density, we created a caged-field experiment in the established plots at both AARS and DFRC, methods adapted from Lefko et al. (2000). In brief, cages are placed over newly cut and insecticide treated alfalfa and then infested with a range of PLH densities. The number of PLH in each cage are then correlated to sweep net values as per Degooyer et al. (1998) in which a regression model was determined to relate sweep net values to absolute density of PLH. Yield is collected from each cage and related through linear regression to PLH density. Results from 2012 have been disregarded due to field conditions. Severe drought created variable alfalfa growth and yield within and between field plot replicates.

In 2013, we refined the caged-field trial methods with guidance from a statistician so that the data is best able to detect the yield response to PLH at DFRC for two cuttings. This was accomplished by increasing the number of different potato leafhopper densities from four to 10, giving greater statistical power to analyze a linear regression.

Economic threshold analysis

The regression of insect abundance on yield is fundamental to the economic threshold model. This is determined by creating a linear model for each alfalfa variety. The slope and intercept variables are needed to calculate the economic injury level (EIL). The EIL is the lowest pest density that creates damage in yield loss equivalent to the cost of treatment.

The economic injury level is defined as:

EIL =  C/V*I*D 

Where C is the cost of treatment per acre; V is the value of alfalfa per ton; I is injury per insect unit and; D is the damage per unit of injury. For piercing-sucking insects such as the potato leafhopper, the I and D variables are combined because there is no way to meausre them separately. This combined value is determined from the slope of linear regression of yield and insect density determined from the experiment (Pedigo 1999). The ET is then set at 75% of the EIL to allow time for growers to respond.

Part 2 - Nutrient Trial.

Field Plot Design, Treatments and Data Collection: A randomized complete block experiment with six repetitions was established in 2011 at AARS on a second year alfalfa stand to test the effect of two soil treatments on PLH and alfalfa yield. Treatments were 1) liquid dairy manure, applied by tanker after the first and second harvests at a rate of 5,000 gallons/acre and, 2) mineral N-P-K-S, hand applied at the same time, at rates similar to results from a 2010 manure sample and 3) an untreated control. In 2012, we repeated the experiment on the same field and established an additional field at AARS, arranged on a contour strip of a 3rd year alfalfa stand. Weekly potato leafhopper sampling and alfalfa yield and forage quality analyses are all managed according to methods described in Part 1 (IPM trial) at AARS from 2010-2011.

Female oviposition choice bioassays: Bioassays were designed to study potato leafhopper response to manure in a controlled setting. Each assay begins in the greenhouse when alfalfa clones are cut to 3”, and one plant for each trial receives 80 ml of manure, to simulate the topical field application rate. When alfalfa plants grow to approximately 8”, 10-15 females and 2-5 males (to ensure each female has mated) are placed in a cage with two plants, one treated with manure and one untreated control. One week after the first nymph has emerged (eggs are laid inside the stem), each plant is examined for the total number of nymphs. Each cage contains one experimental unit of a paired choice assay to determine if females preferentially oviposit in plants grown with or without manure. Five paired tests are conducted simultaneously until a total of n=20 replicates are reached.

Research results and discussion:

Results from the multi-year study are summarized here but can be read in full from the attached dissertation.

Part 1 - IPM trial. We found that alfalfa marketed as resistant to the potato leafhopper did suppress potato leafhopper populations, but more so in seeding years than in production years and more so when the population is peaking. However, we did not find any yield advantage from the resistant alfalfa. We believe that this advantage may be expressed if potato leafhopper populations are very high but we did not experience high potato leafhopper pressure through the duration of this study. 

We found that orchardgrass intercroppings did not have a consistant effect on potato lefahopper populations. Though on a few occassions high orchardgrass density did correspond with a statistically significant decrease in potato leafhopper presence.

From our 2013 caged field trials, we found evidence that the economic threshold for potato leafhoppers in alfalfa may be lowered, especially with regards to crops suffering from drought stress. However, these results need to be validated under natural potato leahopper infestation field conditions across multiple sites and years. 

Regarding the 2013 caged field trial results, a publication is in progress.

Part 2 - Nutrient trial

Potato leafhopper response to liquid dairy manure was inconsistant. In 2011, potato leafhopper populations were significantly reduced in plots receiving manure applications and in 2012, there was no difference between treatments. There are many variables that could have led to these inconsistancies such as differences in the actual manure and differences in temperature and precipitation between the years.

Participation Summary

Educational & Outreach Activities

Participation Summary:

Education/outreach description:

Publications

Chasen, E. M. 2014. Integrated Pest Management for the Potato Leafhopper (Hemiptera: Cicadellidae) in Alfalfa. (Doctoral Dissertation). University of WI, Madison.  

Chasen, E. M. C. Dietrich, E.A. Backus and E.M. Cullen. Potato Leafhopper (Hemiptera: Cicadellidae) Ecology and Integrated Pest Management Focused in Alfalfa. 2014. Journal of Integrated Pest Management, 5(1): A1-A8. 

http://www.ingentaconnect.com/content/esa/jipm/2014/00000005/00000001/art00002

Chasen, E. M., E. M. Cullen and D. J. Undersander. 2013. Integrated manageemnt of potato leafhopper in alfalfa. Wisconsin Crop Management Conference, January 15-17, 2013. Madison WI. 

Chasen, E. M., D. J. Undersander, and E. M. Cullen. In Progress. Revisiting the economic injury level and economic threshold model for potato leafhopper in alfalfa.

Presentations

Economic Thresholds for Potato Leafhopper in Susceptible and Resistant Alfalfa in an Era of Rising Alfalfa Hay Market Value. Agronomy Society of America, November 3-6, 2013. Tampa, FL.

Insect Pest Suppression and Manure Application: Potato Leafhopper Response in Alfalfa (Poster). Midwest Organic Farming Conference, February 21-23, 2013. La Crosse, WI.

Integrated Pest Management for Potato Leafhopper in Alfalfa. Wisconsin Crop Management Conference, January 15-17, 2013. Madison, WI.

Integrating Potato Leafhopper (Empoasca fabae) Management in Alfalfa (Poster). Entomological Society of America, November 13-16, 2011. Reno, NV.

Project Outcomes

Project outcomes:

While completing this research, I presented findings at the Wisconsin Crop Manager Conference in Jan. 2013 to roughly 300 farmers and crop consultants. Since the completion of this research project, we have one published manuscript in the Journal of Integrated Pest Management, "Potato Leafhopper (Hemiptera: Cicadellidae) Ecology and Integrated Pest Management Focused on Alfalfa" (http://esa.publisher.ingentaconnect.com/content/esa/jipm/2014/00000005/00000001/art00002). Because of these outreach efforts, there is an increased awareness and knowledge among farmers, pest control advisors, and extension personnal of pest management tactics for the control of the potato leafhopper in alfalfa including host plant resistance, orchardgrass intercroppings and economic thresholds and how to integrate them.

As part of the long term impact, Undersander and Hogg are conducting 2014 (and beyond) field validation of my EIL and ET models from your research. This will help farmers to apply chemical pest control only when it is economically justified.

Economic Analysis

The economic analysis of integrated pest management is addressed in in the economic threshold model. While the economic threshold is an approximation, it is based on the EIL derived from field research for a particular pest-crop combination. The EIL is defined as the lowest number of insects that will cause economic damage (Pedigo and Rice 2009). The EIL is calculated as follows: EIL = C/VIDK, where C = cost of the control action (price per area unit), V = market value of the crop (price per unit weight), I = injury per insect unit, D = damage or yield loss associated with each injury unit, and K = the proportion of yield loss that is reduced when treatment is applied. Identification of incremental damage to a host plant from increasing pest injury (the I and D variables) is vital in order to create a useful model. For piercing-sucking pests such as the potato leafhopper, the I and D variables are combined because there is no way to measure them separately. Instead, experiments are conducted to relate yield loss to insect density and the slope value from the resulting linear regression, representing yield loss per insect, is used in place of the combined I*D values (Pedigo et al. 1986).

A range of alfalfa market values, V, for high quality alfalfa hay (Barnett 2013) and without regard to quality (USDA NASS 2013a) was used in the EIL equations to explore EIL model sensitivity to alfalfa price. The costs of insecticides recommended for use on potato leafhopper in alfalfa (Cullen et al. 2012) were found at USDA NASS (2013b), and obtained from informal telephone surveys of Midwestern US farm supply cooperatives. Application costs were estimated for 2013 from either the cost of operating and owning insecticide application equipment or the cost of hiring out the work (USDA NASS 2011). Cost of control, C, was then determined by summing insecticide and application costs and a range of values were used to assess EIL model sensitivity. When an effective insecticide is used to suppress an insect population, the proportion of yield loss reduced from treatment, K, is assumed to be 100% because the insect population is reduced to a density below the damage boundary, or the population at which measurable yield loss occurs (Pedigo et al. 1986). The ET was then calculated at 75% of the EIL (Lefko et al. 2000).  

The higher market value of alfalfa in table 1 ($0.25 kg-1) most closely represents the current alfalfa prices regardless of forage quality throughout the United States (Gould 2013). At this alfalfa hay price and the cheapest control cost ($23.76 ha-1), economic thresholds calculated in the present study suggest that the current economic threshold could be cut roughly in half. However, this may be a result of drought conditions.  The results of this study need to be tested in a multi-year and –site experiment in order to validate the potato leafhopper economic injury level and economic thresholds. Our results provide a first step to working towards having a documented yield-loss relationship for potato leafhoppers in alfalfa that can be used in EIL and ET calculations. 

Table 1. ET for potato leafhopper control in alfalfa calculated for a range of alfalfa market values (V) and costs of control (C) with the combined yield loss per insect from trial 1 and trial 2.

Alfalfa market value (V)

($ kg-1)

Insecticide Cost (C)

($ ha-1)

ET

 30+ cm

(12+ in)

(PLH sweep-1)

ET

20-30 cm

(8-12 in)

(PLH sweep-1)

ET

 10-20 cm

(4-8 in)

(PLH sweep-1)

ET

0-10 cm

(0.4 in)

(PLH sweep-1)

0.13

23.76

1.9

1.0

0.5

0.2

 

56.58

5.8

2.9

1.4

0.6

0.25

23.76

0.6

0.3

0.1

0.1

 

56.58

2.6

1.3

0.6

0.3

 

Farmer Adoption

I presented results from my research at the WI Crop Management Conference (Alliant Energy center) in which  ~300 people attended, consisting of crop consultants, extension agents and farmers. Results from the economic analysis of my research were presented in 2013, at Pest Management Update meetings at 7 locations throughout WI to almost 400 people including producers, crop consultants and county agents.

At this point, it is too early to discuss farmer adoption.

Recommendations:

Areas needing additional study

It will be very important to follow up on results from the economic threshold work. These studies will be conducted at the field level by applying insecticides at two different economic thresholds (the current recommended practice) and the threshold as calculated from my research. Yields will be compared between the two thresholds. The first attempt at this experiment was conducted this summer but ambient potato leafhopper populations did not reach either economic thresholds. This work will likely be continued in summer 2015. 

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.