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

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).