1) SUMMARY

A major problem for California sugarbeet producers is the beet armyworm, Spodoptera exigua, and the western yellow-striped armyworm, Spodoptera praefica.  Both pests can occur at the same time in sugarbeet fields and can be hard to distinguish.  Damage and control measures are similar for both.

Beet armyworm larvae are generally light colored and have a small black dot on the middle thoracic segment.  The western yellow-striped armyworm is darker and has a black spot on the first abdominal segment.  The adults are mottled gray, non-distinct moths that fly at night. Moths lay eggs in large clusters on the underside of leaves and cover them with abdominal hairs.  There can be three to four generations in California, and a broad host range ensures that generations will quickly overlap.

Armyworms are a problem in sugarbeets in the summer months.  Seedling loss can be significant in the interior valley when planting occurs in June.  Damage from armyworms at emergence is loss of seedlings and lower yield.  In the Imperial Valley and other fall plant regions, seedlings are sprayed routinely.  Seedling loss is more critical as growers continue to eliminate labor for thinning by space planting and planting to a final stand.

Much of the discussion of armyworm and cutworm damage revolves around establishing and maintaining high populations of sugarbeet plants.  It has been shown repeatedly, in California and elsewhere, that plant population and yield are highly correlated.  Growers have been made aware of this and attempt to manage pests that can cause seedling loss accordingly.

Growers and pest control advisors in the Imperial Valley have noticed greater damage from postemergence herbicides (Betamix® and Betamix Progress®) following minor armyworm feeding on emerged sugarbeets.  It has been suggested that minor feeding damage stresses the young beets enough to cause increased susceptibility to herbicides.  This minor feeding damage may also physically compromise the plants' ability to tolerate the herbicide.  In order to maintain herbicide control without sugarbeet injury, growers spray insecticide on armyworm populations that, by themselves, would not cause economic injury.  The detrimental effect of these herbicides is increased by high temperatures, which typically occur at time of sugarbeet emergence.

Sugarbeet armyworm becomes a more geographically distributed pest later in the season.  Older sugarbeets can suffer yield loss from reduction in leaf area by armyworm feeding.  This damage occurs in July and August and can continue into September and October.  In general, yield loss from armyworm feeding is in direct proportion to the amount of leaf area lost, but this has not been established quantitatively. Increased losses from root rot disease are associated with armyworm defoliation late in the summer.  Armyworm feeding provides an entry for fungal pathogens, such as Rhizopus.  The loss of leaf area increases plant stress and susceptibility to soil pathogens, such as Phytopthora and Pythium.

The worst armyworm problems develop in the San Joaquin Valley in July and August but can continue through October. Sugarbeets are planted in the fall and harvested in July and August on the west side of Fresno County.  Sugarbeets have a dense canopy at this time that is very difficult to penetrate with contact insecticides.  Growers have treated five or more times, trying to stop complete defoliation of the crop and subsequent loss of sugar and yield.  In many years, spider mites and Empoasca leafhoppers reach damaging populations, possibly the result of armyworm control efforts that eliminate all beneficial insects.

Field evaluations over several years indicate that the behavior of the armyworm may be changing.  Armyworms are found nearer the crown of the plants as opposed to the outside leaves.  More often, crown feeding is occurring with associated complications with root rotting organisms.  This may be a result of insecticide selection pressure producing armyworms that are hiding in the dense foliage nearer the base of sugarbeet plants.  Older studies on damage thresholds reported by Suh (1980) may no longer apply.

 2) CURRENT PEST MANAGEMENT PRACTICES
 
Chemical Control:  The use of insecticides currently under review by FQPA figures prominently in chemical control of sugarbeet armyworm.  Lorsban® and Lannate® are interchangeably applied by growers up to five times per year.  If a leafhopper problem is increasing, growers will mix Lannate® and Dibrom® to control both pests.

Lorsban® and Lannate® are the most frequently used insecticides in sugarbeet production. Nearly 35 percent of the base acres were treated with Lorsban® and 22 percent with Lannate® in 1995, more than any other pesticide.  Growers report wide variability in the use of armyworm insecticide from year to year, so the information from a single year is only marginally useful.  Growers with a large infestation, any year, have a critical need for insecticides.

While some Lorsban® may have been used for black bean aphid control, the majority of use has been targeted toward armyworm control.  Control has been variable most years and is perceived by most growers to be declining.  Whether this is due to loss of efficacy, a change in the habits of the larvae, or built-up resistance is unknown.  Cost of armyworm control can be as high as ten percent of the overall sugarbeet production costs.  Production costs vary by growing area but, in general, range between $900 and $1000 per acre.

The use of biological insecticides has not become established in sugarbeet production.  Bacillus thuringiensis (Bt) has not been effective late in the season, when armyworm pressure is at its worst.  As sugarbeets develop a large canopy, microbials are less effective.  Microbial insecticides are only moderately effective early in the season when worms are small.  Many growers have tried the many brands of Bt and have decided they are not effective.  The biggest advantage is that Bt's do not destroy beneficial insects, a minor consideration for growers when sugarbeets are being defoliated.
 
Cultural/Biological Control:  Researchers at the University of California have done work to establish a damage threshold based on age of plants, percent defoliation, and number of defoliations (Suh, 1980).  A four-year study in the late 1970's tested the yield effect of natural and artificial defoliation of sugarbeets.  The clover cutworm was used in these studies to do the defoliation.  This species does not do any crown feeding but feeds only on the leaves.  Sugarbeets six weeks after emergence could tolerate three larvae per plant, at 12 weeks five to ten larvae, and after 12 weeks as many as 15 larvae per plant, as long as damage only occurred once.  Significant yield reductions were reported with increasing number of defoliation events.  Results from the study were not always consistent and have never been field tested.  Field experience has demonstrated that root rot increases with uncontrolled armyworm damage at any level.  This threshold has never been used by growers, and the UC Integrated Pest Management guideline for sugarbeets does not mention it.

Rachael Long, a farm advisor with the University of California, has cooperated with Spreckels Sugar Company research over the past several years to find better ways to manage sugarbeet armyworm.  Long has evaluated the use of an armyworm phenology model to predict the need for sprays and to better time sprays.  Fourteen sugarbeet fields in three counties were monitored with beet armyworm pheromone traps.  Eight of these fields were also monitored weekly for larvae in the fields.

In all cases, the number of moths caught in traps seemed to have little connection with the number of larvae found in fields.  This may have been due to biocontrol early in the year by Hyposoter wasps, which can be effective parasites of beet armyworms.  There may be other predators and parasites that decimate larvae before they become a problem.  It may also be explained by moths flying into sugarbeet fields from other hosts.  Information is needed about how many moths enter sugarbeet fields from other crops in relation to how many are generated from within fields.  A whole farm and regionalized approach to armyworm management will rely on this type of information.

Long reported that when moth activity was low (less than three moths per trap per day throughout the season), growers did not need to control worms.  If confirmed, this could be useful information for growers.

New information was gathered on the dynamics of beneficial insects in sugarbeet fields. Hyposoter wasps also were abundant early in the season and in some cases were 100 percent effective in parasitizing sugarbeet armyworm.  Long reported that early treatment with insecticides often removed these beneficial parasites from fields, and armyworms would cause damage and growers would have to spray.  Growers who sprayed early had to spray often, sometimes as much as five times.

It was also found that a hyperparasite became abundant in August and decimated populations of Hyposoter.  As Hyposoter declined, sugarbeet armyworm populations increased and growers were forced to spray.  Conservation of Hyposoter early in the season may be critical to sugarbeet armyworm management.

Microbial insecticides (Bt's) are reported to be moderately effective early in the season when worms are small.  Many growers have tried the many brands of Bt and have decided they are not effective.  The biggest advantage is that Bt's do not destroy beneficial insects, a secondary consideration for growers when sugarbeets are being defoliated.

An inherent problem with biological control is lag time.  Rapid defoliation by large populations of armyworms preclude any reliance on biological controls. Growers base treatment decisions primarily on damage.  Some of the growers in Long's study treated "damage," even as the armyworm population was declining from parasitism.

A phenology model has been developed for sugarbeet armyworm on cotton.  Insect development is measured in terms of degree-days.  A degree-day is defined as 1º F above an organism's lower developmental threshold for 24 hours.  The lower developmental threshold for sugarbeet armyworm is 54º F (cotton).  Laboratory studies have determined how many degree-days are required for each stage of armyworm development.

The sugarbeet armyworm phenology model (cotton) predicts that an armyworm egg laid in Woodland, California, on May 1 would become a first instar larvae in seven days, continue through the next four instars in 32 days, pupate and emerge as a moth in 22 days, for a total generation time of 61 days.  Based on average temperature over the past three years, the three generations of armyworms would complete their life cycle (egg to adult) on July 3, August 18, and October 7.  A functioning degree-day model allows a grower to predict when the most susceptible stage of an insect will be present in a crop, increasing the effectiveness of any control measure.

Pheromone traps can be an effective and easy way to monitor sugarbeet armyworm adults. Further research is needed to validate the sugarbeet armyworm phenology model and determine its usefulness to the sugarbeet industry.

 3) REDUCED-RISK OPTIONS
 
Pest management options are presently limited to insecticide applications.  Growers cannot allow the crop to be defoliated by armyworms, nor can they afford to spray needlessly.  Information is not available on effective biological controls that are reliable from a grower's perspective.

Late season armyworm control is hampered by the dense sugarbeet canopy. Control measures targeted at the moths to prevent egg laying (mating disruption) have more potential in this situation.

 4) CHALLENGES

Pest management challenges are many.  The greatest challenge appears to be regulatory.  The loss of Lorsban® and Lannate® will cause severe hardship on the industry until alternatives are researched and subsequently proven effective to growers.

No proven, low impact control strategy is currently available.  We have little information about how biological insecticides might reduce FQPA chemical use or how they may reduce pesticide applications by maintaining beneficial predators and parasites.  Armyworm control with insecticides is implicated in secondary outbreaks of spider mites in the San Joaquin Valley and leafhoppers in the Sacramento area.

A big challenge, certainly with current pest management practices, is controlling armyworm populations late in the season within dense sugarbeet canopies.

Growers in California have many cropping options.  The industry must find new and better ways of managing pests in order to compete for acreage with other commodities.  If pest control options become too costly or complicated, growers will likely move to crops that are "easier" or less costly.

 5) INNOVATIVE FEATURES IN REDUCED-RISK PROGRAM

The use of pheromone confusion on a farm scale basis needs to be assessed for its ability to keep armyworm populations below economic levels.  Disruption of mating and egg laying is the end result of this approach.  Unlike chemical application, mating disruption should work equally well at all crop stages, though protection at the stand establishment stage requires further evaluation.  Since the target is now egg laying, rather than "worms," it will be effective even late in the year when the crop canopy is dense.

The reduction in worker exposure is obvious as are other environmental impacts.
 
Pheromone confusion/mating disruption would be applicable to all areas of the state and to several crops.  Thus, it will have far reaching impacts on pesticide use on a whole farm and regional scale.  No effect on beneficial insects is anticipated, so a reduction in overall pesticide use can be expected.  Reduction in egg laying by 75 percent reported in previous work indicates the potential for this approach.  Programs to manage aphid transmitted virus on a regional scale have given the sugarbeet industry with this approach and an organizational structure accustomed to managing pests on a regional basis.

Whether 75 percent reduction in egg laying is adequate for sugarbeet growers needs to be determined.  Economic threshold information in sugarbeets is incomplete, old, and little publicized.  A new, thorough, economic analysis detailing the level of defoliation that can be tolerated by various stages of sugarbeets is needed as a part of the project.

This approach to armyworm control has been used on a small scale for several years.  It has progressed to the point where "early adopters" could begin to use it.  The costs are lower than current control practices, and the potential benefits are great.  The application/delivery system for dispensing the pheromone has been commercialized.  The pheromone is readily produced and obtained.  A large-scale test/demonstration is underway with a large grower in Fresno County.

Another approach is augmentation of naturally occurring beneficial insects.  In 1996 Ehler, et al. (1997) investigated a novel approach to controlling armyworm through enhancing predators of beet armyworms and black bean aphids in sugarbeets.  His small plot study used food sprays to encourage the development of populations of predators.  Results from his work showed some success but also encouraged the development of hyperparasites.  If successful on large-scale production areas, this technique would be cost effective and certainly less risk than current practices.

 6) BARRIERS TO ADOPTION OF REDUCED-RISK METHODS

There are no major perceived barriers to adoption.  The industry is organized in a way that enhances acceptance of new practices.  The curly top control program and the beet free program for virus yellows control are examples of cooperative pest control programs sugarbeet growers are involved in.  Information is extended to growers through University publications and the Extension Service, the California Beet Grower Association's Bulletins, Spreckels' bulletins, frequent and timely meetings, field discussions, the Internet, and personal contact by company field personnel.
 

REFERENCES

Enler, L. E., R. F. Long, M. G. Kinsey, and S. K. Kelley. 1997. Potential for augmentative biological control of black bean aphid in California sugarbeet. Emtomophaga. 42(Omega), 241-256.

Long, R. and C. Lamb. 1995. Area-wide pheromone trapping and field monitoring of beet armyworm in sugarbeet in the Sacramento Valley. University of California Sugarbeet Research Progress Report. California Beet Growers Association, Two West Swain Road, Stockton, CA 95207-4395.

Maxson, Asa C. 1948. Insects and Diseases of the Sugar Beet. The Beet Sugar Development Foundation, Fort Collins, CO., pps. 131-134.

Shorey, H.H. 1997. Development of a farm-wide system for control of the beet armyworm in sugarbeets based on disruption of premating pheromone communication between female and male moths. Final Report to the Pesticide Environmental Stewardship Program, U.S. Environmental Protection Agency.

Shorey, H.H. and R. G. Gerber. 1996. Disruption of pheromone communication through the use of puffers for control of beet armyworm (Lepidoptera: Noctuidae) in tomatoes. Environmental Entomology. 25: 1401-1405.

Suh, J.B. 1980. Effects of simulated and natural defoliation on yield and sugar content of sugarbeets in California. M.S thesis, University of California, Davis.

Summers, C.G., L.D. Godfrey, and R. Long. 1996. Sugarbeet: Sugarbeet Armyworm. UC IPM Pest Management Guidelines. UC IPM Guideline Series, UCDANR Publication 3339.