Objective 3: Improving Sugarbeet Stands and Reducing Pesticide Use in the Imperial Valley⁽¹⁾

Summary

The effects of different methods of protecting emerging sugarbeet seedlings were compared in a field trial in the Imperial Valley. Treatments included the current preferred growers' practice involving the use of an insecticide at planting combined with three or more post-emergence sprays for insect control, seed treatment with a systemic insecticide at two rates (imidicloprid or Gaucho®), and no control of measures. Seedlings were counted five times up until thinning. At harvest, stands were evaluated and yields were compared. Pre-emergence pesticide applications resulted in significantly larger numbers of seedlings than other treatments. Gaucho® was not as effective as the use of insecticides applied to soil and aerially, but still resulted in adequate numbers of seedlings for a successful sugarbeet stand. Root yields and percent sucrose were not affected by treatments, including the control treatment, but gross sugar yields of control treatments were approximately 500 lb per ac lower than other treatments. There were no losses of beets during the late fall to harvest period. Flea beetles were the principal cause of damage at emergence and are well controlled by Gaucho®, but it has no effect on armyworms. Armyworms caused little damage during the trial this year, but some post-emergence insect protection remains important in the Imperial Valley when fields are irrigated early in the fall. The amount may be reduced significantly by using a seed treatment insecticide like imidicloprid.

Introduction

Sugarbeet production in the Imperial Valley is thriving. The reduction of chronic loss from lettuce infectious yellows virus and the improved performance of new sugarbeet varieties have led to world record sugarbeet yields over the last decade. Once established, sugarbeet plants grow well during the winter and spring months in the low desert. Planting takes place, however, during September and early October, when air and soil temperatures are above optimum, and the populations of insects preying on sugarbeet seedlings such as flea beetles and armyworms are large. Growers believe that control of insects on sugarbeet seedlings should begin as soon as seedlings appear and continue until after thinning approximately 40 days later. Otherwise, stand failure is considered certain. Management based on this assumption has been successful for many years, but the most commonly used materials for control (Lannate® (methomyl), Lorsban® (chlorpyrifos), and Diazinon®) are carbamate or organophosphate type compounds which currently are under review by US EPA for possible future restriction under the provisions of the Food Quality Protection Act. Diazinon was recently withdrawn voluntarily from the home garden market because of concerns about public exposure. Currently, there are no well-established alternatives to the use of these materials for sugarbeet seedling protection.

Methods

To demonstrate alternative seedling protection strategies and document loss to insects and other causes, a trial was conducted in the Imperial Valley near Brawley in a 87 acre sugarbeet field in the fall of 2000. Fifteen strips, each with 20 thirty inch rows a half mile long, were planted with Beta 4776R, a commonly planted variety in the area. All of the seed was from the same seed lot. Five different pre- and/or post emergence treatments were applied (Table 1). Each treatment was replicated three times. Emerging seedlings were counted in two twenty foot long subplots in rows 7, 8, and 9 in each plot, at 10, 14, 20, 26, and 46 days after irrigation. At the last date, seedling spacing was determined by measuring the distance between the first 100 seedlings in row 8(W). Also, the above-ground portions of 30 seedlings were collected from row 8 of each subplot, dried and weighed for comparison. Prior to the last count, inadvertent cultivator damage occurred in one of the plots (tmt 2), so that plot has data from only two replications for that date.

Each seedling was labeled with a small wooden stake at emergence. The stake was removed later if the seedling died and the cause of mortality was evaluated visually in the field. If a plant was chewed off or obviously damaged by insects, its loss was attributed to the insect damage category, if it was shriveled or desicated, or a common seedling pathogen could be visually identified, it was classified in the shriveled or diseased category. If there was no seedling next to a stake, it was classified as missing. Using stakes allows for the identification of the majority of seedlings appearing. Those disappearing during the first three or four days from the start of emergence will not have been counted. The sum of the number appearing is cumulative emergence. The last count, just prior to thinning was considered to be the final establishment. Because the amount of seed planted is known, pre-emergence losses can be calculated by difference using observed cumulative emergence. The field was planted on September 12 and 13 using a Milton planter. The amount of seed remaining after planting the field was weighed to get an exact weight for the seed planted. In this trial, 90,000 seeds per acre were planted. This was divided by the known field area to get the seed population. We assume that planting occurred uniformly. The seeding rate used was a reduction from the previous year's trial. Irrigation was initiated on September 14-15^th following planting. An important difference from last year's trial was that this field was not pre-irrigated. The initial irrigation was slowed and required approximately 8 days to completely wet the field. During this period, portions of the field remained saturated for up to eight days.

At the final count in the fall and again at harvest, the distances between one hundred beets in one row per plot were measured. The same rows were used in both fall and spring. Results were compared to determine losses over the winter, prior to harvest. The field was not thinned.

Results

The results reported here are from the fall stand establishment period only (September through November, 2000). A final report with this season's results and comparisons between the two year's trials will be submitted in June(2001) when yields and final plant populations are measured.

Cumulative emergence. On average, a far smaller percentage of seeds resulted in sugarbeet seedlings in 2000 than in 1999. In 1999, emergence reached 80% of seeds planted while in 2000, the best treatment resulted in approximately 50% emergence. Seedling survival was greatest when pre-emergence insecticides were used (Table 2). There was a significant difference between the Grower's treatment using pre-emergence Lorsban® applied to the soil and seed treated with Gaucho® (Tables 2 and 3). Emergence was delayed slightly as well by the Gaucho® treatment, which is known to slow emergence (fig. 1). Substantially fewer seedlings emerged in the control treatment, lacking pre-emergence seedling protection (Tables 2 and 3).

Pre-emergence losses are determined by difference (Table 2). Average pre-emergence losses for all treatments were high and ranged from 50% to approximately 70%.

Establishment at thinning. The percentage of seeds resulting in established seedlings immediately prior to thinning (six to eight true leaves) is reported in Table 2. The average number of seedlings counted at each date is also presented in figure 2. There were significant differences between the Growers and Gaucho® treatments, and the Gaucho treatments and Control treatments (Tables 2 and 3). The Growers treatment remained largely constant. In the Gaucho® treatment , they increased and then decreased slightly, while in the untreated plots, seedling numbers declined steadily with time (fig. 2).

Cumulative mortality. In the Growers and Gaucho treatments there was very little post-emergence seedling loss up to thinning (Table 2, fig. 3). Mortality increased with time in the control treatment.

Seedling growth. The dry weight of seedlings at thinning is compared in Table 4. The Growers treatment resulted in the largest seedlings, but seedling DW was not significantly different among the three treatments. From initial emergence onwards, flea beetles were present in the plots and damaged seedlings, even at the cotyledon stage. Later, armyworm larvae appeared, and began to damage seedlings.

Overwinter losses. Table 5 provides a comparison of changes in the plant populations in the two sample rows counted in each plot. Essentially, there was no loss of roots in any treatment during the November to late April period.

Yields. Root yield, percent sucrose, and gross sugar yields are presented in Fig. 5. There were no significant differences in roots yields among the treatments or in percent sucrose. Gross sugar yields for the control treatment were approximately 500 lbs lower that the other treatments. Root weights were smallest in the grower's treatment because of greater rates of emergence and establishment in fall (Fig. 6). But since the overall emergence rate was still poor, spacing between roos was uneven and bunched, resulting in a larger number of very small beets in this treatment and a lower average root weight.

Discussion.

Cumulative emergence and seedling establishment. Successful stand establishment is the result of many interacting factors, as is stand failure. Only 30 % to 50 % of the seed planted emerged in this trial, compared to 50 % to 80% in the previous year. The most significant management difference between the two trials was grower irrigation practice. Not pre-irrigating in 2000 resulted in an extended and probably non-uniform initial irrigation. The portion of the field used for the trial (near the head end) was observed to be saturated for the entire eight day irrigation period. Seed likely experienced anoxic conditions during some or all of this time, a circumstance known to be damaging to the seeds of many crop species, including sugarbeet. Not only was emergence reduced, but it was delayed, a classic response to anoxia (Fig. 4).

Plant protection. In the Imperial Valley, and other locations where pre-emergence losses are high, an insecticide applied with or to the seed appears necessary. Pre-emergence losses were 20% to 40 % in the control treatment without an insecticide than in the other treatments. The significantly larger number of seedlings emerging and becoming established in treatments including a pre-emergence insecticide in this trial leads to the inference that insect damage is occurring to seeds and emerging seedlings before they appear above ground. This is the second year in a row in which such an observation has been made in the Imperial Valley. Early seedling damage was due almost entirely to flea beetles. Armyworm larvae had not had time to develop and were not observed. Very few armyworm larvae were active in the plots during this trial and in the Imperial Valley generally this last autumn. Only three aerial applications were used by the growers to control armyworm and flea beetle predation on seedlings in 2000, compared to four in 1999. The lack of armyworm damage affected seedling dry weight as well. In contrast to 1999, there were no significant differences among treatments.

Gaucho® was very effective against flea beetles, and substituted well for soil applied Lorsban® and the first and possibly the second aerial applications of insecticides as well. The lower rate imidicloprid treatment (20 g per 100,000 seeds) appeared to perform as well as the higher rate treatment. This treatment represents a significant savings in pesticide use compared to current practices.

Costs of establishment. In 2000, the percentage of seed resulting in established seedlings was closer to the average reported anecdotally for many years in the Imperial Valley, compared to high levels of seedling establishment in 1999. Nonetheless, populations at thinning were adequate in Gaucho treated plots (approximately 30,000 plants per acre) for successful beet production, good in the Growers plots (approximately 40,000 seedlings per acre), but too few in the control plots (approximately 20,000 plants per acre). Thinning would not be required at any of these levels except to even out plant spacing. Nonetheless, there were no root yield differences among the treatments, and only the control plots resulted in significantly reduced gross sugar yields, though the absolute amount of gross sugar differences were small. One reason why low root populations in control plots resulted in similar root yields is the early harvest date. The effects of gaps on yield reduction would have increased with a longer growing season. Also, uneven plant spacing in the higher population rows, resulted in a large number of beets that were too small for the harvester to collect, particularly in the growers treatment. More uniform spacing, associated with greater emergence percentages would have increased the yield differences among the treaments.

Conclusions

1. Pre-emergence pesticide applications resulted in significantly larger numbers of seedlings than the control treatment without them.

2. Gaucho® applied to seeds was a satisfactory method of controlling per-emergence seedling losses and resulted in adequate numbers of sugarbeet seedlings for a successful commercial crop. Flea beetles were the principal cause of damage at emergence and are well controlled by Gaucho®.

3. Establishing a large percentage of seeds as seedlings saves growers money on seed costs and reduces the amount of pesticides applied, with imputed environmental benefits.

4. Some post-emergence insect protection remains important in the Imperial Valley when fields are irrigated early in the fall, but the amount may be reduced by using a seed treatment insecticide like Gaucho®.

References

Durrant, M.J., Dunning, R.A., Jaggard, K.W., Bugg, R.B., and Scott, R.K. (1988). A census of seedling establishment in sugar-beet crops. Ann. Appl. Biol. 113:327-345.

List of tables

Table 1. Treatments and associated costs.

Table 2. Seedling emergence and establishment (percent)

Table 3. Treatment contrasts.

Table 4. Seedling dry weights at thinning.

Table 5. Growing season losses: comparisons of root populations in fall and at harvest.

List of figures

Fig. 1. Imperial Valley, fall_2000. Cumulative emergence (seedlings per 20 feet of row). Error bars are standard errors.

Fig. 2. Imperial Valley, fall_2000. Number of plants established (seedlings per 20 feet of row). Error bars are standard errors.

Fig. 3. Imperial Valley, fall_2000. Cumulative mortality from all causes (seedlings per 20 feet of row). Error bars are standard errors.

Fig. 4. Comparison of relative emergence patterns in 1999 and 2000.

Fig. 5. Root yields, percent sucrose and gross sugar yields.

Fig. 6. Average root weight at harvest.

Table 1

Treatments (2000)

* Seed planted at the rate of 90,000 per acre.

Table 2

Seedling emergence and establishment at thinning

Imperial Valley, (fall 2000). Data collected at 47 days after initial irrigation. Includes 5.7% non viable seed.

Table 3

Treatment contrasts (Days since initial irrigation = 46, final count)

*See Table 1 for treatment descriptions

Table 4

Seedling dry weights at thinning

(g DW per seedling at 47 days, average of 30

seedlings)

Table 5

Growing season losses: comparisons between fall and spring root populations

Fig. 1. Imperial Valley, fall_2000. Cumulative emergence (seedlings per 20 feet of row).

Error bars are standard errors.

Fig. 2.ImperialValley, fall_2000. Number of plants established (seedlings per 20 feet of row).

Error bars are standard errors.

Error bars are standard errors.

Fig. 4. Comparison of relative emergence patterns in 1999 and 2000.

Fig. 5. Root and gross sugar yields and percent sucrose.

Fig. 6. Average root weight at harvest

1. Supported in part by a grant from the California Department of Pesticide Regulation, Betaseed, Inc, Spreckels Sugar, Inc., Gustafson, Inc., and the California Sugarbeet Industry Research Committee. Acknowledgments are extended to Tom and Kurt Rutherford, Larry Godfrey, Tom Terini, and Gary Peterson for their cooperation and help with the field experiment.

2. Extension agronomist, Department of Agronomy and Range Science, University of California, Davis; formerly, agronomist, Spreckels Sugar, Inc., Woodland, California, now, California Department of Pesticide Regulation, extension specialist, Department of Entomology, University of California, Davis; and UCCE, Imperial. County.