Biorational Management of Beet Armyworms in Sugarbeets in the Central Valley

Introduction. Beet armyworm (Spodoptera exigua) larvae remain a significant insect pest of sugarbeets in the Central Valley. This species as a wide host range and is significant pest (in addition to sugarbeets) on tomatoes, cotton, cucurbits, alfalfa, lettuce, and other crops (Fig. 1). Beet armyworm eggs are deposited in clusters of ~100 on the leaf surface. Egg masses are covered with hairlike scales. Newly-emerged larvae feed in a cluster initially and than move apart over the plant. The larvae skeletonize plant leaves leaving the veins. On sugarbeets, this defoliation can caused significant yield losses (Fig. 2). In addition, in recent years the larvae appear to feed in more protected areas as opposed to populations in the 1970's and 80's, for instance. This has resulted in the larvae often feeding on the beet roots near the soil surface or slightly below the soil surface (larvae crawl into soil cracks caused by the roots). This root feeding provide entry ports for root rotting organisms into the beet roots. This root rot diseases can quickly decimate a sugarbeet stand or nearly mature crop. Finally, beet armyworm larvae also inhibit sugarbeet seedling establishment by clipping emerging seedlings.

Control of beet armyworm infestations during the growing season is largely accomplished with applications of organophosphate insecticides (primarily Lorsban® and Lannate®). In recent years in the Central Valley, repeat applications are often needed and control has still been inadequate. These applications have eroded the profitability of sugarbeets and the lack of control has reduced the sucrose yields. In addition, the multiple applications have flared populations of secondary pests such as spider mites, leafhoppers, etc (Fig. 3, 4). In many areas, the beets are nearly completely defoliated by about 1 month before harvest. The plants regrow at this time, which utilizes stored energy that could go into sucrose at harvest.

Parasitoids, Hyposoter exigua, predators, and virus diseases potentially inflict a high degree of natural control on beet armyworm populations (Fig. 5). However, given the high populations and the need for quick control, these have not been important factors in the Central Valley. The efficacy of the organophosphate insecticides appears to be waning probably because of the development of resistance. Resistance to these materials has been verified in vegetable systems. In addition, the regulatory actions of FQPA may limit use of these products. Therefore, there is a need to design alternative, improved IPM programs for beet armyworms on sugarbeets in the mid and southern San Joaquin Valley.

Methods. A demonstration project was conducted in Fresno County to attempt to manage beet armyworms using biorational means in comparison with the standard grower practice (PMA project ‘Reduced Risk Management of Insect Pests in Sugarbeets’). Two late fall/winter planted fields were utilized in which the biorational practices were used on 30 acres compared with the standard practices on the remaining ~130 acres (Fig. 6). The PCA was involved in making decisions on the grower-practice side and we (Babb and Godfrey), in concert with the PCA, made management decisions on the biorational side. The concept for the biorational management was to use pheromone traps to monitor the beet armyworm moth flights and to make visual inspections of foliage for egg masses. The control tactic was to use B.t. sprays (Lepinox or XenTari) at the onset of egg hatch. This would concentrate the activity of B.t. onto the early instars, where it is most effective. The grower practice was to use repeated applications of Lannate, Lorsban, or other organophosphate insecticides.

The following samples were collected on a weekly interval (irrigation and/or chemical treatments prevented sampling on a few dates).

1.) wing/sticky pheromone traps baited with BAW pheromone were placed in each field on 6 June.

2.) bucket pheromone traps baited with BAW pheromone were placed in each field on 14 June.

3.) sweep net samples were taken in each field (grower and biorational portions as soon as this segregation occurred), samples were taken to the laboratory and the numbers of beet armyworm larvae, Empoasca leafhoppers, and beneficials (lygus bugs, stink bugs, minute pirate bugs, big-eyed bugs, assassin bugs, damsel bugs, lacewings, lady beetles, collops beetle, parasitic wasps, and spiders) were counted.

4.) visual inspections were done on 20 leaf samples in each field to assess the numbers of beet armyworm egg masses and larvae.

5.) leaf samples for spider mites were collected on 9 Aug.; samples were processed in the laboratory with a washing technique.

6.) harvest samples (from a commercial harvest) were collected in October from both fields and from the biorational side and the grower standard side)

7.) sucrose content was determined at the Spreckels tare lab. and sucrose yields were calculated

Pheromone traps: Beet armyworm flights occurred about 3 weeks earlier compared with normal. That was seen in research plots near Davis and noted by PCAs in the San Joaquin Valley. Moths were captured during the first sample period (6-14 June) and this appeared to the declining side of the first BAW flight (Fig. 8). The peak of the second and third flights occurred in mid-July and mid-August, respectively. The bucket traps captured many more moths than the sticky trap from July to Sept. For the first few sample periods, the bucket trap was not effective. Overall, the bucket trap withstood field conditions better than the wing trap, i.e., was not hindered by dust, and collected more moths. The high moth captures could however be a downside as counting the 1000+ moths captured in one week was not quick or easy.

Research in cotton has shown that ~930 degree-days (882 for females and 977.9 for males) (54⁰F lower threshold) are needed for development of BAW from egg to adult. The developmental rate on sugarbeets is unknown (developmental rates can vary significantly among hosts). Using 1 June as the estimated initial date of oviposition, the new adults should appear about 14 July and the next generation adults should appear on 23 Aug. These approximate our trap captures well.

Field Treatments: The following treatments were applied to the biorationally managed and grower managed areas.

The first application was made primarily for sugarbeet webworms (Fig. 9). Lepinox was also applied to the grower managed side on this date. The 13 July and 21 July applications were made for beet armyworms, as well as the 28 August application made only in the grower-managed side. The M-Pede was included to reduce spider mite numbers.

Sweep Net Samples: Sweep net samples from the biorationally-treated area and the grower-treated area (28 June and once treatments had been applied) showed initially a high number of beneficials followed by a gradual decline in both treatments to no beneficials on 9 August (Fig. 10). This corresponds to the time when the sugarbeet tops were decimated in both treatments. This made collecting sweep net samples difficult and also made the field not conducive to beneficials. Leafhopper populations built-up to significant levels in both treatments (Fig. 11). Populations started at ~50 per 50 sweeps on 14 June and peaked at ~300 per 50 sweeps in mid-July. Levels declined thereafter to less than 20 per 50 sweeps. There were no consistent trends between the two treatment regimes. The leafhopper threshold is based on leaf turn samples (threshold being ~15 leafhoppers per leaf). Preliminary research has shown that 1 leafhopper per leaf turn . 50 per sweep. Beet armyworm larval data from the sweep net samples showed very low populations until 5 July (Fig. 12). These were likely larvae that arose from the moths captured by pheromone traps in mid-late June. The rationale being that for the moths in pheromone traps it would take a few days (3-5) for them to mate, develop and deposit eggs, ~4-5 days for the eggs to hatch, and 7-10 days for the larvae to develop to the third instar. The early instars feed more commonly in a webbing in a mass and may not be adequately sampled with a sweep net. Larval populations remained at 5 or less per 50 sweeps in the grower treated area but peaked at 26 per 50 sweeps in the biorationally-managed area.

Visual Inspections: Inspections of sugarbeet leaves revealed very few larvae and/or egg masses (Fig. 13). The highest count was 3.25 larvae per 20 leaves on 24 July in the biorational treatment. The larvae are reclusive during the heat of the day.

Damage Observations: Defoliation damage was minimal during June and early July. Sugarbeet webworm, Loxostege sticticalis, populations were present during this time and damaged the plant terminals. In mid-July, some moderate defoliation damage was noted and the leaf health started to decline noticeably. Leafhopper feeding and spider mite damage contributed to this decline. The heat was also a factor as daily high temperatures averaged over 100⁰F from July 28 to August 6. By ~9 August, the entire leaf canopy in both fields and both treatments was photosynthetically nonfunctional (Fig. 14, 15). The remaining leaves were chlorotic or dead. Some leaf regrowth occurred from 25 Aug. and by 8 Sept. the plants had a small cluster of leaves (6-8).

Mite Counts: Spider mites can be an important pest of sugarbeets. The exact effects of spider mites on beet yield have not been quantified, but based on observational data the effects can be significant. In addition, we have seen cases where spider mite populations develop following application of broad-spectrum insecticides. Leaf collections (20 leaf sample) from 9 August showed that there were 4700 mites per sample from the grower standard field portion and 3590 mites per sample from the biorationally-treated field section.

Yields: Sugarbeet yields and sucrose contents were variable across treatments and across fields. In both fields, the percentage sucrose was slightly greater in the biorationally-managed plot compared with the grower standard. However, beet yields were variable, i.e., each treatment had the advantage in terms of beet yield in one of the two fields. Sucrose yield was higher for the biorational treatment than the grower standard in field 2 and the inverse was true in field 1 (Fig. 16).