Montana Crop Health Report – Cutworm Survey Special Issue

December 28, 2001 Vol. 14, No. 8

IPM Western Region Cutworm Survey and Risk Warning

Principle Investigators:

Dr. Sue L Blodgett

Mr. William T. Lanier

Dr. Gregory D. Johnson

Contributors:

Montana Reports

Chouteau County

Pale western cutworm larvae were detected in early May, ranging between ¼ and ¾ inches. No army cutworm larvae were detected. (Judee Wargo)

Custer County

No army or pale western cutworm larvae were detected, Spring 2001. (Kent Williams)

Fergus County

No army or pale western cutworm larvae were detected at two sites in late April. Chemical treatments were applied to approximately 25,000 acres. (Dave Phillips)

Judith Basin County

Pale western cutworm larvae were detected the first and 8th of May. No army cutworms were detected. (Jim Moore)

Liberty County

Pale western cutworm larvae were detected ranging in size from ¼ to ½ inch. No army cutworms were detected. Approximately 780 acres were treated in 2001. (John Maatta)

Madison County

No Pale western or Army cutworm larvae were detected. (Maggie Scott)

Other Montana Cooperators:

Kirk Barnett, Big Horn

John Maki, Beaverhead

Mike Schuldt, Blaine

Wayne Fuller, Broadwater

Bobbie Roos, Daniels

Bruce Smith, Dawson

Larry Brence, Fallon Carter

Ron Carlstrom, Gallatin

Darren Crawford, Glacier

Paul McCarthy, Hill

John Pfister, Musselshell

Marko Manoukian, Phillips

Ardis Oelkers, Roosevelt

Jamie Vowell, Toole

Dan Clark, Teton/Pondera

Kevin Johnson, Teton

Jeff Farkel, Pondera

Other State Contributors:

Nebraska

Dr. Gary Hein, University of Nebraska.

South Dakota

Dr. Louis Hesler, USDA-ARS, Brookings South Dakota

Dr. Michael A. Catangui, South Dakota State University

Colorado

Bob Hammon, Fruita Research Center CSU

Tom Hooten, Colorado State University

Idaho

Juan Manuel Alvarez, Aberdeen Res. & Ext. Center, UI

Wyoming

Randy Violette, Northwest College, Powell Wyo.

Materials

Phero Tech

British Columbia, Canada

Pheromone testing

Dr. David Weaver, Montana State University

Web Application and Modeling

Michael Grinder, Montana State University

Degree Day Modeling and Prisim Mapping

Dr. Leonard Coop, Oregon State University, Integrated Plant Protection Center.

IPM Western Region Cutworm Survey and Risk Warning

Two major cutworm species, the pale western and the army cutworm, are serious but sporadic pests for Montana producers. Outbreaks can occur when cutworm populations are high and weather conditions are favorable for survival. However, outbreaks do not necessarily occur in successive years. It is difficult to predict in which years damaging larval cutworm populations may occur. Monitoring small grain for the larval stage of the two species, army and pale western cutworm, is difficult and time consuming. Forecasting the potential for damaging cutworm outbreaks allows producers, consultants and Extension personnel to improve the timing of costly and time consuming monitoring practices for the damaging larval stage.

Distribution: Both cutworm species are native to North America. Pale western and army cutworms have distributions through the western Great Plains (Fig 1,2).

Fig 1. Pale Western cutworm distribution

Fig 2. Army cutworm distribution.

Pheromones, identified for both army and pale western cutworms are used to monitor the incidence and relative abundance of both moth species. A project funded by the Western Regional Integrated Pest Management program will allow us to incorporate key environmental data into a risk model with adult moth pheromone trap catches. This project will result in improved predictions of army and pale western cutworm activity and risk assessments.

Components of the Army and Pale Western Cutworm Risk Warning are (1) total number of moths collected during adult monitoring, (2) degree day accumulation from Jan. 1 for occurrence of larval stage, (3) precipitation data from May through June.

Biology of the Army Cutworm: The army cutworm begins laying eggs in late August and can continue through October. Eggs hatch from 9-21 days during the fall and winter, following rain or snow. The army cutworm over winters in the larval stage. Larvae begin feeding in late winter or early spring, feeding above ground at night and staying below ground during the day except on very overcast days. This nocturnal behavior plus the small approximate 1/2-inch (1.27 cm) size of the early instar larvae makes them difficult to detect even though the feeding damage to foliage is obvious. Larvae vary in color, ranging from greenish-brown to greenish-gray with a narrow, pale mid-dorsal stripe. The head is pale brown with brown to dark brown freckles. When abundant and in short supply of food, the larvae will move en masse to adjacent fields, thus the common name.

Adult moths, also known as millers, emerge from pupal cells in May and early June. An interesting phenomenon of this cutworm species is its seasonal migration to high elevations in the Rocky Mountains to avoid high summer temperatures. During the day they form dense aggregations under stumps, logs, and other structures that offer them protection from direct sunlight. It has been reported that these dense aggregations of cutworm moths are an important food source for grizzly bears in the absence of high-quality prey alternatives in July and August. Adult moths return to the plains in late summer and begin laying eggs. Dry periods during August - October are detrimental to egg hatch and larval survival of army cutworms.

Biology of the Pale Western Cutworm: The pale western cutworm has a life cycle similar to the army cutworm. Moths deposit up to 300 eggs per female in the upper 2 inches (5 cm) of loose soil in cultivated ground in early fall. Most eggs hatch the following spring. The newly hatched larvae are small, approximately 1/2 inch (1.27 cm), and difficult to detect. Larvae are subterranean and feed on plant stems below the soil surface. The general body color of the larva is pale yellowish-gray with a distinct white mid-dorsal line. The head is yellow-brown with two distinct vertical black dashes that form an inverted ‘V’. Mature larvae measure approximately 1 1/2 inches (3.81 mm) gradually becoming less active, pupating within an earthen cell. Adult moths emerge from pupation during late summer, and begin the cycle over again. The prepupae and pupal periods are longer in the southern part of the range so that a single generation occupies a full year throughout its distribution.

Dry weather favors a pale western increase. Potential for damaging pale western cutworms have been mapped (Fig. 3). Excessive precipitation in the spring can drown larvae in low-lying areas, force larvae to the surface of the soil where they are exposed to attack by predators and parasites, and produce conditions favorable for the increase of pathogens. A method of predicting outbreaks is based on the number of wet days (i.e., days on which >0.25" (0.63 cm) precipitation falls) in May and June. More than 10 wet days during this period result in decreased numbers of cutworms; however less than 10 wet days were followed by an increase. Pale western cutworm also has one generation per year. (Sue Blodgett, IPM Coordinator 994-2402)

Fig 3. Average interval between local outbreaks of pale

western cutworm (Montana Exp Station Bulletin No. 225)

IPM Program:

A pilot cutworm monitoring effort across Montana was initiated in 1992 with sites, in Gallatin, Hill and Pondera counties. The pilot effort expanded to include sites in Big Horn, Chouteau, Daniels, Fallon/Carter, Fergus, Hill, Liberty, Phillips, Pondera, Prairie, Richland, Rooosevelt, Teton and Toole by 1998. With funding from the USDA Western Region IPM program in 1999 the project expanded to sites in Montana (23), Nebraska (4), South Dakota (5), and Wyoming (1). In 2001 sites were added in Colorado, Idaho and Utah. The information collected from this trapping has made it possible to identify the peak Army and Pale Western cutworm moth flight period for Montana. Monitoring adults through the peak flight provides the most reliable information about adult abundance and flight duration.

Fig. 1 Cumulative Army Cutworm Moth Flight, 1992 - 2001.	Fig. 2 Cumulative Pale Western Cutworm Moth Flight, 1992 - 2001

Adult Monitoring: County Agents, 4-H participants and producer - cooperators place and maintain pheromone traps at each monitoring site. Traps are monitored for eight weeks from August - early October. An internet-based Cutworm Survey report was developed for web entry of cutworm moth flight data and is available on the web at <Cutworm.org>, in March 1998. Cooperators are encouraged to submit a record of the number of moths trapped for each date and site each week via the web form at <Cutworm.org>. Web browser screens enabled the cooperators to enter moth trap catch data and graph and print entries. Number of PW or AC moths are summarized in a "moth count data" form, which reports the total numbers of moths collected.

Environmental monitoring: Winter/spring temperatures are the second factor incorporated into development of the Risk warning. Degree-days are accumulated from January 1 and compared among years. A simple rating method based on accumulated degree-days (high, medium, low relative to average accumulation) is used to indicate the relative timing of larval cutworm occurrence. Degree-days are a component calculated by the model. Each day weather databases across the region are accessed and maximum and minimum temperatures for each site are recorded daily. Cutworm larva development responds to temperature and their activity and occurence can be forecasted by monitoring accumulated degree-days.

To increase the accuracy of the degree-day based warning, precipitation data is collected and included when appropriate in the risk warning for each site. The weather component of the model also calculates accumulated moisture. Rain fills the soil pores and forces the larval to the surface where they are more apt to be affected by predators, disease and adverse weather. For Army cutworm the impact of precipitation varies with time of year. For example, total precipitation for August through October of 4.5 inches (11.4 mm) favors egg hatch and larval survival. Pale Western cutworm research has shown that the number of Wet days (March - June) affect larval development. A wet day is a day when > .25 inches (.63 mm) of rain occur. Very Favorable conditions for pale Western are < 6 wet days. Favorable conditions are < 11 wet days.

Scouting Larvae and Larval Monitoring: Detecting potentially damaging populations of pale western or army cutworm moths does not always provide accurate forecasting of damaging larval populations. To improve our ability to forecast damaging larval populations, a larval scouting program was initiated in 2000. Cooperators will provide counts of larval numbers, which will be used to correlate larval populations with fall adult moth counts.

See Table 1. for current data. Additional information will need to be collected to determine a relationship.

Table 1. Army and Pale Western Cutworm numbers / foot and size in inches for site in

Montana, 2000.

Rating Cutworm Damage

Cutworm larvae are difficult to monitor in cropland situations. Small cutworms are well camouflaged in soil. Searching for the larvae within a square foot or row-foot soil sample can be extremely time consuming. However, populations may be assessed by how frequently larvae are found and how widespread their occurrence. For example, Alfalfa and cereal grains are less susceptible to cutworm damage than sugarbeets. Rating cutworms on a wider scale e.g. countywide basis can help to determine relative population and risk to each crop. (Sue Blodgett and Will Lanier, IPM 994-5690)

Rating Scale of Army and Pale Western Cutworm Larval Infestations in the Spring

1 - Not present or very difficult to find

2 - Present but not found frequently

3 - Commonly seen but no local economic infestations in alfalfa (ACW) or wheat (ACW, PWC); Economic infestations may be present in sugarbeets.

4 - Commonly found but limited economic infestations / treatments in alfalfa or wheat (less than 1% of fields require treatment). Economic infestations likely in sugarbeets.

5 - Commonly found with significant economic infestations in alfalfa or wheat (approximately 10% of fields require treatment).

6 - Widespread economic infestations (more than 20% of fields require treatment).

(Contributed by Gary Hein, University of Nebraska).

Tillage Practices Survey

Cutworms were a serious problem in many parts of Montana in 2001. Although the cutworm monitoring program provides potential for cutworm risk on an area-wide basis, we do not fully understand why some fields are at higher risk of cutworm infestations than other fields. We are interested in being able to identify specific field characteristics and farming practices that might put a field in the category of high risk relative to pale western and army cutworms. To assist us in determining characteristics of high-risk fields, we will be mailing a Pale Western and Army Cutworm Infestation and Cropping Practices survey to producers in areas where cutworms were a problem. Questions on the survey deal with crop damage, soil type, tillage practices, weed management, etc. We assume many of these factors influence cutworm presence. If you receive a copy of this survey, please fill it out and return it in the addressed envelope. Your observations and experience with cutworms are valuable in adding to our knowledge base and understanding of this pest. (Greg Johnson, Extension Entomologist 994-3861)

If you would like to participate in this survey contact:

Dr. Gregory D. Johnson

Department of Entomology

Montana State University

Bozeman, MT 59717

Development of a Simple Cutworm Forecast:

We are currently developing a simple forecast for pale western and army cutworms that is based on (1) adult moth flight (2) critical environmental data. The goal is to present an ‘Unfavorable’, ‘Neutral’ or ‘Favorable’ rating to cutworm risk warning to producers. Similar levels of risk warning have been employed successfully across the Canadian prairies to forecast Bertha Armyworm and Orange Wheat Blossom midge (O. Olfert, B. Eloit, P. Mason, Saskatoon Research Center, 107 Science Place, Saskatoon S7N 0X2 and L. Harris, Saskatchewan Agriculture and Food, Walter Scott Building, Regina).

The user chooses a site for estimation of cutworm risk for which data are available. The resulting table lists the thresholds and the data relative for that site. Each component allows the user to enter "what if" data. For example if the user suspects that moth flights were higher than the trap estimate provided they can enter their prediction and view the "what if" risk assigned by the model.

The model assigns a ranking of 0 to 3 (Low is < 1, medium is = 1 - 2, high is > 2) to each component ie. Moth count, degree-days, moisture. Each component is also given a weight by the program. The weights then determine how much each component influences the overall risk. The default is that each component has the same weight as the others. For example a user might give a greater weight to moth counts or weather data that is collected on farm and may discount or reduce the weight of those variables that are borrowed from a distant source. Each factor's risk value is multiplied by its % of the total weight and summed.

For example in the case where all the weights are five, let's say that the moth count has a risk value of 0.5, degree days has a risk value of 1.5, and moisture has a risk value of 2.3. The sum of the three weights is 15, so each factor has approximately a 33% share. (0.5 x 0.33) + (1.5 x 0.33) + (2.3 x 0.33 = 1.419, so the over all risk would be medium. If just degree days and moisture have been chosen so far, then it would be 1.5 x 0.5 + 2.3 x 0.33 = 1.9 which would be medium as well. Adjusting the weights so that one factor has a larger weight that the other factors would give it more influence in the overall score. If the forecast is chosen then the forecast degree-days and moisture are used instead of the observed degree-days and moisture. (Will Lanier and Michael Grinder, 994-5690)

Future Survey:

Darksided/Dingy Cutworm

This species occurs rarely in Montana and has a broad range of host plants including lentils and chickpeas. The darksided/dingy cutworm is one of the latest occurring species, found in May and June. Although it is well known as a climbing species eating leaves that it cuts, it also has been known to cut stems off near the ground surface, as it did in northeastern Montana in lentils and chickpeas in 2001. During the day the larvae rest below ground, usually within ½ inch of the soil surface, feeding at night or on overcast days.

We have been testing two pheromone lure formulations for monitoring darksided cutworm moths. This past year we were able to capture moths with both formulations. Further work will be conducted on darksided cutworm trapping. (David Weaver, Stored Grain Entomologist)

Deliver the pale western and army cutworm risk forecast, regionally.

Delivery of forecast and risk warning are facilitated by the use of geographic information systems (GIS). GIS systems are used as a tool to model or depict data points (insect numbers, weather data) and to display the results spatially in a map format. GIS systems employ different algorithms to average point data, such as insect population numbers.

During the third and final year of the Western Region IPM Cutworm Risk Warning Project the above warning modules (Fall adult trap counts, accumulated weather, forecasted weather, larval counts) will be used to form a graphical risk warning. The graphical risk warning will present the above warning modules in a format resembling a weather map.

Using the interconnectivity of the Web a cooperative arrangement is being explored with the Integrated Plant Protection Center at Oregon State University. Dr. Leonard Coop has developed an extensive IPM Weather Data and Degree-Day Web site for pest management decision-making in the Pacific Northwest. The cooperation will expand the scope of this site and use it ability to graph GIS information and interpolate between data points to represent cutworm infestations.

A visual display, such as a map with contour lines, enhances the interpretation of the point data. Because of the variability of insect sampling methods, the dynamic nature of agriculture systems and the assumptions made when models were developed, it is important that explanatory notes accompany maps used for risk warnings. It is important that researchers and extension agrologists not only highlight the potential risk of a pest species but also make producers aware of the limitations of the forecast. (Sue Blodgett and Will Lanier, 994-5690)

Fig. 4. Example of proposed Cutworm Risk Warning image.