In recent years, the Beet Armyworm, Spodoptera exigua, has surpassed aphids as the most economically important insect pest of California sugarbeets. Armyworms are recognized for their ability to reduce seedling stand densities, defoliate leaves, and feed on the beet roots at the soil level which predisposes plants to opportunistic root rot pathogens. Armyworm control centers around the use of Lorsban® and Lannate®, both of which are susceptible to Food Quality Protection Act (FQPA) legislation. Additionally, beet growers and PCAs are reporting unsatisfactory control from these materials on armyworms and flaring effects which result in secondary pest outbreaks of mites and leafhoppers.

This study was conducted to establish economic injury levels for the beet armyworm. Adoptable thresholds would allow growers to lower insecticide use by maximizing the natural ability of sugarbeet plants to compensate for defoliation. Decreased insecticide use would in turn reduce the incidence of secondary pest outbreaks by not disrupting naturally occurring biological control organisms.

One and a half acres of sugarbeets were planted on May 12 and grown according to standard grower convention at the Armstrong Plant Pathology Field Station on the UC Davis Campus. The experimental plots were 6 rows wide, with two rows representing fall 2000 harvest, two rows for spring 2001 harvest, and the other two rows acting as borders. Plots were 17 feet long (artificial inoculation experiment) and 30 feet long (natural population experiment). Plots were organized in a randomized complete block design with 4 repetitions of each treatment.

Plant damage was evaluated through weekly leaf area measurements as well as harvest evaluations of tonnage, sucrose content, and root rot incidence. Weekly leaf samples consisted of 10 of the first fully expanded leaves taken from random plants in each plot. Area measurements were recorded for each individual leaf using a Li-Cor LI-3100 leaf area meter. Harvest evaluations were completed from October 25^th to 27^th. Plots were mechanically topped and lifted and then manually counted and weighed. Beet samples were sent to the Spreckels Sugar Company tare lab for sucrose analysis and clean beet percentage.

Armyworm densities were established through the artificial inoculation of armyworm eggs suspended in corn cob grit. Plants were inoculated I, 2, or 3 months before fall harvest with 0, 20, 40, 80, or 120 eggs per plant. Malathion applications were used one week before each inoculation date to minimize predation on eggs. Two additional treatments were manually defoliated 1 and 2 months before harvest to serve as checks.

No significant differences were found in leaf area, sucrose percent, tons/acre, or sucrose/acre among the different treatment levels at any given date or among inoculation dates. Significant differences were found when comparing inoculated plots with plots artificially defoliated one month or two months before fall harvest. This suggests that even 1 complete defoliation event can be sufficient to significantly reduce all yield components, and that inoculated armyworm densities were insufficient to produce the damage equivalent to one such defoliation event. The trial was unable to determine if the observed damage resulted in sufficient economic losses to justify the application of chemical insecticides.

Field observations attribute the low damage to poor survivorship of armyworm eggs. Eclosion (in the absence of predators and parasites) averaged 18.5% (July), 34.0% (August), and 17.8% (September). This is the equivalent of 22.2, 40.8, and 21.4 first instar larvae per plant in the 120 egg treatment. Yet, despite these high rates of inoculation, field data from three days after each inoculation date reported nearly 100% mortality of armyworm larvae. This included over 600,000 eggs over three inoculation dates at rates up to over 4 million eggs per acre.

Natural armyworm populations were allowed to establish in the plots from planting on May 12 until July 27. A single Lorsban application was sprayed on June 19^th to aid in sugarbeet seedling establishment. Treatments of different armyworm densities were established through the use of the insecticide Success®. During the three months preceding fall harvest, each plot received from 0 to 4 applications of Success® at 6oz./acre. Single application treatments were established to represent early (July), middle (August), and late-season (September) control. Additional treatments represented season-long control at a low rate (2 applications) and a high rate (3-4 applications) as well as unsprayed control plots.

No significant differences were found in leaf area, sucrose percent, tons/acre, or sucrose/acre between the single and multiple application treatments. Also, no significant differences were found among early, middle, and late season control treatments.

Natural armyworm densities were insufficient, particularly mid to late season, to cause detectable losses in yield data. Pheromone trap catches showed three armyworm flights, each of decreasing magnitude throughout the season. During most years, each successive peak is greater in magnitude with highest worm pressures resulting in late summer. Highest trap catches this year were observed between late-June and mid-July, with peak populations of 140 adults during the week of July 10-17. The first Success application was sprayed on July 27. Later flights in August and October reached peaks of 69 and 40 adults per weeks ending August 29 and October 4 respectively. As a comparison, some armyworm traps in sugarbeet fields of the central valley of California during the same time periods consistently caught over 1000 adults per week.

High levels of biological control within the field probably also contributed to low pest densities. By using a selective armyworm insecticide, pest outbreaks common with organophosphate and carbamate use were avoided. This was confirmed in both experiments by mid-season crashes of mite populations. Mite populations on August 10 were high enough to debate the need of a miticide application. By September 5^th, both mite and armyworm populations had disappeared completely without the application of a single miticide.

Weekly sweep net samples in individual plots were used to monitor within plot differences of armyworm densities. Averages across all treatments of armyworm densities after Success applications showed a direct relationship between worm density and period of time after insecticide application.

Density of armyworms collected in sweep net samples after applications of Success.

Season-long treatments of 2, 3, and 4 applications provided the best overall control. Highest worm densities varied according to date and application timing. Even the highest infestation level, though, proved insufficient to reach economic injury levels since no significant yield differences were observed. Data from this experiment does suggest that no significant losses occur for one time density counts of 7 larvae per 40 sweeps in August, 14 larvae in September, and 9 larvae in October. Also, no significant differences were found for cumulative damages of 3.3 larvae per 40 sweeps over the 11 weeks before fall harvest.

We were not able to establish economic injury levels for beet armyworm during this research year. This was due in part to high levels of biological control as well as low general insect pressure. We did, though, make valuable observations regarding the interactions among armyworms, mites, and beneficial generalist predators. We also determined that at the levels of armyworm infestations present in these experimental plots this year, all uses of chemical insecticides for armyworm within three months of harvest were not economically justifiable.