Good Bug, Bad Bug: A Novel Approach to IPM of Solanceous Weeds

Martin M. Williams II
Washington State University
24106 N. Bunn Road
Prosser, WA 99350-8694
509-786-9390
509-786-9370
miwilliams@wsu.edu

Executive Summary

Potatoes are grown on a 3- to 4-year crop rotation primarily to break the life cycle of serious potato pests such as nematodes, however, Solanaceous weeds in non-potato years serve as alternate hosts and viral vectors. IPM of Solanceous weeds is limited and many growers use handweeding crews to uproot escaped weeds. On a daily basis, handweeding crews nearly one-half a year in direct contact with plant and soil material recently treated with numerous applications of pesticides. Native to North America, the Colorado potato beetle (CPB) defoliates several Solanaceous weeds of the PNW, is effectively controlled in potato, and may offer a unique approach to Solanaceous weed management in non-potato crops. The goal of this project is to reduce farm worker exposure to pesticide residues by developing a new Solanaceous weed IPM system that would decrease the need for handweeding. This work will identify the extent to which pesticide use can be reduced by integrating multiple fitness-reducing stresses, including CPB defoliation, for management of Solanaceous weeds. A model study system will focus on suppression of volunteer potato (a Solanaceous weed) with biologically effective herbicide dose, competition from carrots, and CPB herbivory. The general concept of linking chemical- and biological- induced stresses will benefit the scientific community because the work will have numerous applications for IPM. Measurable environmental results from this model study system will include 1) identifying the extent to which volunteer potato IPM can supplant handweeding in carrots, 2) enhancement of IPM strategies currently available to growers in the PNW, 3) change in growers’ perceptions towards pests, and 4) degree to which grower-cooperators would modify their pesticide use patterns and use of handweeding.

Objectives

Identify alternatives to insecticides that currently suppress Colorado potato beetle populations in non-potato crops
Develop functional relationships between the effects of herbicide dose and beetle density on Solanaceous weed fitness
Demonstrate the effects of minimal herbicide use, herbivory, and crop competition on weed fitness on-farm

Justification

This objective comes at the request of local growers who are interested in modifying their pesticide use practices in order to utilize biological weed control. The Colorado potato beetle (CPB) is not a pest in non-potato years in the PNW and would provide partial suppression of Solanaceous weeds. However, many of the rotation crops are treated with insecticides that inadvertently kill CPB larvae and adults. For each rotation crop, this objective will rank strategies for managing primary arthropod pests, according to their potential impact on CPB, impact on target pest, and cost. Those strategies that have minimal adverse effects on CPB populations will be highlighted and recommended for those growers interested in incorporating biological control of Solanaceous weeds, such as volunteer potato, in their rotation crops.
Biologically effective herbicide dose is defined as the minimal amount of herbicide required to result in an acceptable level of weed suppression. Preliminary investigations by the project coordinator have demonstrated that the biologically effective herbicide dose depends greatly on herbivore load. As an example, Williams et al. (unpublished data) found that fluroxypyr use could be reduced 70 to 95% for volunteer potato suppression with a modest level of CPB feeding. The purpose of this objective is to quantify interactions between herbicide dose and herbivore load on weed fitness. Results would identify the extent to which herbicide use could be reduced for a range of beetle densities, while maintaining weed suppression. Such results would be one measure of the potential contribution biological weed control offers for row crop IPM.
Acting in concert with biological, mechanical, and cultural weed management tactics, it is very possible that low doses of a herbicide can eliminate the need for handweeding and the pesticide exposure those crews face on a daily basis. Field testing proposed in this objective will be used to develop a novel IPM framework. The model study system will demonstrate the extent to which herbicide use can be minimized for volunteer potato control by integrating CPB feeding and carrot competition. By conducting the research on-farm, grower-cooperators will have direct ownership in the results and awareness of potential benefits in their operation.

Literature Review

Potato rotation systems of Washington

Washington produces some of the highest potato yields in the world. Production is almost exclusive to the irrigated Columbia Basin of eastern Washington and acreage has varied between 165,000 to 180,000 in recent years (Anonymous 2000). Potatoes are rotated with other crops on a 3- to 4- year rotation primarily to break life cycles of serious pests and diseases of potato. Some 500,000 acres in the Columbia Basin are planted to potatoes a given summer. Rotation crops (statewide acreage) include: alfalfa (470,000 A), irrigated wheat (255,000 A), field corn (155,000 A), sweet corn (99,400 A), carrots (9,800 A), and onions (18,800) (Anonymous 2000).

Solanaceous weeds

Solanaceous weeds infest nearly all annual cropping systems in the PNW (Callihan et al. 1990, Ogg et al. 1981). Hairy nightshade (Solanum sarrachoides), black nightshade (Solanum nigrum), cutleaf nightshade (Solanum triflorum), and volunteer potato (Solanum tuberosum) are the main Solanaceous weeds ubiquitous in annual cropping systems of the PNW. Hairy nightshade occurs throughout the United States and southern Canada, but is most troublesome in the irrigated farmlands of the West. Black nightshade is less prevalent but can be locally abundant in the far western states. Cutleaf nightshade, a native to North America, contains larger fruit than hairy and black nightshade and is found mainly in arid and semi arid areas of the PNW. Due to harvest inefficiencies, potato tubers following harvest can exceed planting density by an order of magnitude and these tubers frequently survive the relatively mild winters in the PNW, giving rise to a serious weed problem in crop rotations (Bond 1993, Boydston 2001, Lutman 1997). Volunteer potatoes are one of the most serious Solanaceous weeds due to large food reserves in the tuber and the ability to resprout after various control tactics. Volunteer potato is extremely competitive especially in crops that are slow to emerge, such as carrots and onions, and causes substantial yield losses.

Solanaceous weeds are alternate hosts for many plant pests and viral vectors and may perpetuate insect (Myzus persicae), disease (Rhizoctonia solani, Alternaria solani, Phytophthora infestans), and nematode (Meloidogyne chitwoodi, M. hapala, and M. incognito, Pratylenchus neglectus and P. penetrans) problems in potatoes and other rotation crops (Rogers and Ogg 1981).

No herbicides are available that can totally eliminate volunteer potatoes in rotation crops. Several herbicides suppress volunteer potato growth and/or eliminate exposed shoots, but new sprouts emerge from the tuber. Control tactics are often more effective after initial potato growth (Boydston 2001; Lutman 1977), presumably as some of the tuber reserves are exhausted. However, daughter tubers forming after tuberization and yield loss associated with early season potato competition constrains the extent to which growers can delay control measures. Cultivation is an effective tool for volunteer potato control between crop rows, but three or more cultivations are required to substantially reduce new tuber production (Williams and Boydston, 2002). Herbicides combined with cultivation have reduced new tuber formation in corn and onions more than either strategy alone (Boydston 2001, Boydston unpublished data). The extent to which similar results could be obtained by coupling early shoot removal by herbicides or cultivation with arthropod herbivory is unknown.

Carrots are an example of a potato rotation crop where farm worker exposure to pesticide residues can be significant. Since herbicides currently labeled for use in carrots provides no suppression of volunteer potato, the primary weed, handweeding is a central tool. Handweeding is most effective when the volunteer potato is large enough that, when hand pulled by the shoot, the buried tuber is removed from the ground. Large handweeding crews move from field to field over a six-month period. Carrot fields are treated with a number of pesticides prior to worker entry, including 1,3-dichloropropene (300 lb ai/A; NFPA1 = high), malathion (0.5 lb ai/A; NFPA = moderate), and endosulfan (0.75 lb ai/A; NFPA = moderate). Growers pay up to $200/A for a field to be handweeded and are eager to find ways to cut the cost of production.

A Solanaceous herbivore

The oligophagous Colorado potato beetle (CPB) is native to southwestern North America. The beetle feeds primarily on Solanaceous species and original hosts were buffalobur (Solanum rostratum), silverleaf nightshade (Solanum elaeagnifolium Cavanilles), and Solanum angustifolium Miller. Host range has expanded to include potato in the PNW as well as Solanaceous weeds, including hairy, black, and cutleaf nightshade and volunteer potato (Hsiao and Fraenkel 1968; Weber et al. 1995).

Utilizing CPB as a beneficial insect in rotational crops is unlikely to increase the extent to which the beetle is a pest in potato, and instead, may mitigate buildup of resistance to insecticides. First of all, CPB is ubiquitous within potato growing region of the Columbia Basin and is effectively controlled in potato. Current insecticide use patterns in Washington are geared towards controlling green peach aphid, the primary arthropod pest, since it vectors potato leaf roll virus. Control of the beetle is a secondary result of targeted control efforts of green peach aphid. Secondly, maintaining a reservoir of susceptible individuals in CPB populations is a principle of IPM (Weber and Ferro 1994). The beetle has developed resistance to nearly all major classes of chemical insecticides and lack of non-crop hosts to serve as reservoirs of susceptible beetles is cited as an important factor which favors insecticide resistance (Weber and Ferro 1994). Practices that utilize CPB feeding to assist in Solanaceous weed suppression would reduce selection pressure for insecticide resistance.

Plant stress can attract CPB

Beetle attraction (anemotaxis) is enhanced by exposure of Solanaceous plants to damaging levels of ozone (Schutz et al. 1995), mechanical damage (Bolter et al. 1997), and to feeding by beetle larvae on plant foliage (Bolter et al. 1997). Beetles are also attracted to potato treated with volicitin and methyl jasmonate, chemicals implicated in the induction of defense chemistry of plants (Landolt et al. 1999). The extent to which herbicide-induced plant stress influences beetle attraction is currently being evaluated (Williams and Walsh).

Coupling arthropod herbivory and herbicide-induced stress

Integrated pest management of weeds aims to manage populations of weeds through a series of mortality- and fitness-reducing events. Decreased seedling emergence and decreased early vigor often increases susceptibility of weeds to arthropod attack, as well as many tactics used to manage weeds. There is limited documentation of how vigor reduction due to minimal herbicide use can be coupled with arthropod herbivory.

Preliminary studies provide the basis for quantifying Solanaceous weed fitness as a function of sub-lethal herbicide dose and CPB herbivory. In the absence of CPB larvae on young plants, 91.9 g ae/ha (2X the recommended dose) of fluroxypyr killed volunteer potato (Williams et al, unpublished data). On the contrary, 5.7 g ae/ha (0.1X the recommended dose) effectively killed the weed when a modest level of CPB herbivory was introduced.

Methods

Objective 1 – Identify alternatives

If CPB is to serve as a biocontrol agent of Solanaceous weeds in non-potato crops, then selective, targeted IPM should supplant broad-spectrum insecticide use that currently reduces or eliminates CPB populations. Current arthropod pest management recommendations will be reviewed for rotation crops, including sweet corn, field corn, onions, carrots, and wheat. Specifically, insecticides recommended for use in each crop will be reviewed in various sources (e.g. Hirnyck, 2002) and ranked in terms of their CPB safety. Ratings will include ‘safe’ (less than 1/3 mortality expected when field rate is used), ‘moderately harmful’ (>1/3 to 2/3 mortality expected when field rate is used), and ‘harmful’ (>2/3 mortality expected when field rate is used). This information will be made available for inclusion in the online and hardcopy version of PNW Weed Management Handbook, for a special section on weed IPM. The purpose is to make information available to growers, regarding potential implications of insecticide use on IPM of Solanaceous weeds. Those chemical and non-chemical methods of arthropod pest management that are expected to result in minimal reductions in beetle populations will be highlighted.

Objective 2 - Functional relationships

No-choice tests replicated in time will be conducted in the weed ecology greenhouse at WSU-Prosser. Experimental design will be a randomized complete block design with 5 replications. An experimental unit will be a single pot with an emerged potato (cv. Russet Burbank). The treatment design will be a 4x8 factorial arrangement of treatments (beetle density x dose). The herbicide dose factor (8 levels of fluroxypyr ranging from 0 to 2X the label recommendation) will be applied when shoots are 15 cm tall using a chambered spray system. Two days after herbicide treatment, the herbivore factor (4 levels: 0, 5, 10, 40 larvae per plant) will be imposed by placement of first instar CPB larvae on plant leaves. After 14 days, larvae will be removed and plants will be clipped at the soil surface and analyzed for leaf area, vegetative biomass, and tuber number and biomass. For each beetle density, the logistic model will be used to regress plant response to herbicide dose (Seefeldt et al., 1995). Logistic model parameters will be used to calculate the herbicide dose required for 95% control (I95 – a biologically effective dose). A linear model will then be used to regress biologically effective dose to beetle density. Based on preliminary data (Williams, unpublished), Figure 1 illustrates generalized hypotheses for this objective.

Figure 1 - Hypothetical Relationship between herbicide dose and beetle density. (Unable to include because of incompatibility of graphic provided.)

Objective 3 – On-farm testing

Field experiments will be conducted in carrots with Todd Crosby, Agronomist, at Mercer Ranch, Alderdale, WA. Mercer Ranch annually grows some 4,000 A of carrots, representing more than 40% of Washington production, which is the state that leads in US processing carrot production. A commercial carrot field will be selected that was in potato production the previous year. Mercer Ranch will raise the carrots under conventional practices, however, care will be taken to avoid the use of insecticides efficacious to CPB. Experiment design will be a split-split plot design with 4 replications. The treatment design will be a 2x4x2 factorial arrangement of treatments (crop x herbicide x beetle). Main plots will be assigned presence or absence of the crop (carrots), subplots will be assigned one of four herbicide treatments, and sub-subplots will be assigned beetle presence (naturally occurring) or absence (excluded from plots). After crop emergence, no-crop main plots will be removed of carrots by hand. To minimize beetle feeding in no-beetle plots, imidacloprid will be soil-applied to appropriate plots as volunteer potatoes emerge. Retreatment of no-beetle plots will occur as needed. When potatoes are 15 cm tall, two timings each of fluroxypyr (biologically effective dose based on results of objective 2) prometryn (biologically effective dose based on current research) will be applied with a backpack sprayer. Crop, where appropriate, and weed will be monitored on a bi-weekly basis for leaf area, vegetative biomass, and reproductive biomass. After senescence, plants will be harvested and evaluated for yield and grade.

Impact Assessment

The extent to which plant stress alters the effects of arthropod herbivory on weed fitness has received very little study. Despite volumes of research on individual weed management components, no one has demonstrated the significance of coupling reduced herbicide use with arthropod herbivory for weed IPM. The scientific community will benefit as the publication of results from this research will provide insight into increasing the efficiency of IPM with a novel approach. Measurement of the impact to the scientific community will be broad because the general concept of this work has many specific applications. Growers in the PNW potato rotation systems will be closer to having a new strategy for IPM of Solanaceous weeds, namely volunteer potato, that could supplant use of large handweeding crews that are exposed to pesticide residues. Results will have limited application to other Solanaceous weeds and other non-potato crops. Because handweeding is expensive (Mercer Ranch currently spends some $300,000 in carrots alone), effective alternatives will be seriously considered. At the end of the study, the Mercer Ranch will provide feedback on the extent to which they think their pesticide use patterns could change, as a result of the project.

Objective 1 will identify those insecticides that are potential limitations to utilizing a Solanaceous herbivore as a biocontrol agent in rotation crops. As a grower, the question is - What can I use to economically control my arthropod pests without killing the biocontrol agent? Under controlled conditions, objective 2 will provide the level of beetle density and herbicide dose required to achieve a weed management goal (e.g. 95% control). From this, one could infer the level of herbivory required to limit herbicide use to a given amount. Through the series of no-choice tests, the functional relationships developed from this objective will provide an empirical dataset, which can also be used to infer treatment recommendations based on site-specific field conditions. The approach will serve as a model for others and could easily be modified for different circumstances (biocontrol agent, weed, herbicide, etc.).

Extending research findings to field conditions is critical in defining practical obstacles to any IPM approach. This work will provide critical data on the dynamics between the weed, the crop, the beetle, and imposed management of the weed. It will assist in addressing questions such as: To what extent does application of sub-lethal herbicide dose predispose the weed to a) reduced fitness, b) beetle herbivory, and c) reduced fitness as a result of beetle defoliation and crop competition? The third objective will provide an opportunity to analyze the significance of the IPM approach in production fields. We anticipate finding opportunities to increase suppression of Solanaceous weeds through IPM, shift from broad-spectrum to selective insecticides for arthropod pests, and mitigate exposure of handweeding crews to pesticide residues by expanding the concept of IPM.

Literature Cited

Anonymous. 2000. Washington Agricultural Statistics. Washington Agricultural Statistics Service.

Bolter, C.J., M. Dicke, J.J.A. van Loon, J.H. Visser, and M.A. Posthumus. 1997. Attraction of Colorado potato beetle to herbivore-damaged plants during herbivory and after its termination. J. of Chemical Ecology. 23:1003-1022.

Bond, W. 1993. Evaluation of ioxynil, fluroxypyr and clopyralid for the control of volunteer potato in vegetable crops. Asp. App. Biol. 35:123-130.

Boydston, R. A. 2001. Volunteer potato (Solanum tuberosum) control with herbicides and cultivation in field corn (Zea mays). Weed Tech. 15:461-466.

Callihan, R. H., J. C. Ojala, L. C. Haderlie, and D. W. Kidder. 1990. Nightshade biology and control in cropland of the Pacific Northwest. Pacific Northwest West Extension Pub. # 352, 6 pp.

Hsiao, T.H. and G. Fraenkel. 1968. Selection and specificity of the Colorado potato beetle for Solanaceous and nonsolanaceous plants. Annals of the Entomol. Soc. of Am. 61:493-503.

Landolt, P.J., J.H. Tumlinson, and D.H. Alborn. 1999. Attraction of Colorado potato beetle (Coleoptera: Chrysomelidae) to damaged and chemically induced potato plants. Environmental Entomology. 28:973-978.

Lutman, P.J.W. 1977. The effect of tuber size on the susceptibility of potatoes to metoxuron. Potato Res. 20:331-335.

Ogg, A. G., Jr. and J. H. Dawson. 1984. Time of emergence of eight weed species. Weed Sci. 32:327-335.

Ogg, A. G., Jr., B. S. Rogers, and E. E. Schilling. 1981. Characterization of black nightshade (Solanum nigrum) and related species in the United States. Weed Sci. 29:27-32.

Rogers, B. S. and A. G. Ogg, Jr. 1981. Biology of weeds of the Solanum nigrum complex (Solanum Section Solanum) in North America. U.S.D.A. Publication ARM-W-23, 30 pp.

Schutz, S., B. Weissbecker, and H.E. Hummel. 1995. Impact of elevated atmospheric ozone on host plant finding of the Colorado potato beetle (Coleoptera: Chrysomelidae). Med. Fac. Landbouw. Univ. Gent. 60:819-824.

Seefeldt, S.S., J.E. Jensen, and E.P. Fuerst. 1995. Log-logistic analysis of herbicide dose-response relationships. Weed Technol. 9:218-227.

Weber, D.C. and D.N. Ferro. 1994. Colorado potato beetle: diverse life history poses challenge to management. Pages 54-70. in Zehnder, Powelson, Jansson, and Raman, eds. Advances in Potato Pest Biology and Management, St. Paul: APS Press.

Weber, D.C., F.A. Drummond, and D.N. Ferro. 1995. Recruitment of Colorado potato beetles (Coleoptera: Chrysomelidae) to Solanaceous hosts in the field. Environ. Entomol. 24:608-622.

Williams, M. and R. A. Boydston. 2002. Effect of shoot removal during tuberization on volunteer potato (Solanum tuberosum) tuber yield. Weed Technol. (In press).

Timetable

Project Timetable
Objective	Timing
1 - Identify alternatives	Fall 2002 - Spring 2003
2 - Functional relationships	Spring 2003 - Spring 2004
3 - On-farm testing	Spring 2003 - Spring 2004

Major Participants

Martin M. Williams II, Ph.D, Research Weed Scientist for Washington State University is the project coordinator. Dr. Williams will coordinate the project, administer funds, and provide guidance to personnel hired under this project.

Douglas B. Walsh, Ph.D., Integrated Pest Management Coordinator for Washington State University is a collaborator. Dr. Walsh will assist in the review of arthropod pest management recommendations.

Eric Sorensen, Ph.D., Extension Educator for Washington State University is an educator. Dr. Sorensen will disseminate research results to vegetable growers during annual field days and grower meetings.

Todd Crosby, Agronomist, Mercer Ranch, is a grower-collaborator. Mr. Crosby will provide the on-farm research site, raise the crop, and evaluate usefulness of integrating favorable research results into Mercer Ranch farming operations.

Project Budget

Project Period: 9/1/02 - 8/31/04

Funding Request
Funding Requested	Other Funding	Total Funding
$40,000	$15,828	$55,828