IV Report 1999-2000

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 commence 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® and Lorsban®) are organo-phosphate type compounds which are currently are under review by US EPA for possible future restriction under the provisions of the Food Quality Protection Act. Currently, there are no well-established alternatives to the use of these materials for sugarbeet seedling protection.

Methods

To evaluate alternative seedling protection strategies and document loss to insects and other causes, a trial was conducted in the Imperial Valley near Brawley in a 45 acre sugarbeet field. Fifteen strips, each with 20 thirty inch rows, 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, 16, 19, and 25 days after irrigation. At the last date, the above-ground portions of 30 seedlings were collected from row 8 of each subplot, dried and weighed for comparison.

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 dessicated, 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 19 using a Monosem air planter. The amount of seed remaining after planting the field was weighed to get an exact weight for the seed planted. In this trial, 144,600 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 high rate compared to the target root population at harvest of approximately 35,000 plants per acre and reflects the common growers’ anticipation of low levels of seedling survival. Irrigation was initiated on September 19^th following planting.

Herbicides are also used in a program of sugarbeet seedling protection. The most common and effective herbicide used is a selective material called Betamix® (phenmedipham+desmedipham). It is a photosynthesis inhibitor, and under conditions of intense light and high temperature, also can harm sugarbeet seedlings. Growers in the Imperial Valley report that if seedlings are damaged by insect chewing, Betamix® causes more damage to seedlings than otherwise, resulting in increased post-emergence seedling mortality. So an additional objective of insect control for the growers is the desire to avoid leaf injury leading to additional herbicide damage. Betamix® was applied to all plots on October 8^th (day 16). Plots were counted immediately prior to application and then three days (day 19) and nine days later (day 25). Higher seedling losses in the shriveled or diseased category in treatments in which insects were not controlled compared to treatments in which they were should reflect insect-herbicide interactions.

Thinning was carried out with a mechanical, wheel-type thinner. The field was managed uniformly following thinning until harvest. At harvest, all the sugarbeet roots in two adjacent 150 foot long rows (rows 7 and 8 above) were counted and then harvested mechanically using a sugarbeet plot harvester. Two subsamples per plot were analyzed for sugar content and impurities. Plant populations at harvest were compared to plant populations prior to thinning the previous fall. Prior to harvest, the distances between the first 50 seedlings in row 8 were measured. These were averaged and compared. Plant spacings were also classified into groups, based on the distance from their nearest neighbors, using a target or theoretical spacing at harvest. The theoretical spacing (TS) was estimated using the initial seeding rate (SR), adjusted for germination percentage (GP) (92 %) and combined thinning and post-thinning losses (TL).

TS = SR / {0.92 * (1-TL)}

This assumes that seed placement was uniform and thinning and post thinning losses were identical in all treatments. This is an approximation only, but allows for comparisons of the evenness of plant spacing among the treatments. When planting to a stand, for example, if a large emergence rate is expected, seeds can be spaced at close to the desired final distance and plant population. Large gaps are most likely to cause yield loss under these conditions. Treatments were compared based on the percentage of plants separated from their neighbors by a distance 1.5 times greater than the theoretical spacing. This method of comparison is based on methods of evaluating planter performance proposed by Kachman and Smith (1995).

Results

Cumulative emergence. Seedling survival was greatest when pre-emergence insecticides were used (Table 2). There was no significant difference between the Grower’s treatment using pre-emergence Lorsban® applied to the soil and seed treated with Gaucho® (Table 3). Emergence was delayed slightly, however, by the Gaucho® treatment, which is known to slow emergence (fig. 1). Substantially fewer seedlings emerged in all the other treatments lacking pre-emergence insecticides (Table 3). Pre-emergence losses are determined by difference (Table 2). Average pre-emergence losses of all seed planted were approximately 20% in treatments receiving insecticides, and 42 % in those not. The germination percentage of this seed lot was 92 %. Accounting for non-viable seed ( minus 8 %), reduces estimated pre-emergence losses to approximately 11 % of the viable seed for the treatments receiving insecticides and 35 % for all the other treatments.

Establishment at thinning. The percentage of seeds resulting in established seedlings immediately prior to thinning (approximately six true leaves) is reported in Table 2. The average number of seedlings counted at each date is also presented in figure 2. There was no significant difference between the Growers and Gaucho® treatments, but all other seed treatments resulted in significantly less (Tables 2 and 3), and approximately similar numbers of seedlings. 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 treatment, there was very little post-emergence seedling loss up to thinning (Table 2, figure 3). Mortality increased with time in all other treatments. There was significantly greater post emergence mortality in Gaucho®-treated plots than in the Growers treatment (Table 3), though the absolute difference was small (Table 2). Insects, plant diseases, and possibly herbicide damage all contributed to seedling loss.

Seedling growth. The dry weight of seedlings at thinning is compared in Table 4 and figure 4. The Growers treatment resulted in the largest seedlings, but seedling DW was not significantly different from Gaucho® treated seeds. All other treatments resulted in significantly smaller seedlings. 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 as well. The Bt treatment seemed to provide marginal protection to the seedlings, and there was a non-significant trend towards larger seedling dry weight, suggesting that some inhibition of armyworm growth may have occurred. Gaucho® treated plants were smaller than the plants that were sprayed frequently. Gaucho® is not effective against armyworms and increasing damage with time occurred as armyworm larva grew and consumed seedlings. This damage may have continued for a period after thinning, because Gaucho® treated plots resulted in fewer plants and larger distances in the row between plants than the Growers treatment at harvest (Table 5).

Plant populations at harvest. Plant populations were largest in the Growers and Gaucho® treated plots, and lower in the others (fig. 5). Plant spacing differed significantly among the treatments and followed the patterns established primarily at emergence the previous autumn (Table 5). The treatments with the greatest number of large gaps were those with the poorest overall seedling establishment levels the previous fall (Table 5).

Yield. Yield was proportional to root weight and inversely proportional to plant population at harvest. Yields were lowest in the Growers treatment (fig. 6, 7). Because of plant over-crowding, the treatments that had the fewest surviving plants at harvest had the largest yields. These were the same treatments that resulted in the largest early season total mortality. There were no significant differences in sugar percent among the treatments (fig. 8) or in impurities or recoverable sugar (not shown).

Discussion.

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 three times greater among treatments that did not include an insecticide with the seed.. 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. If widespread, this is a new observation in California. Such damage has been reported in England and elsewhere in Europe, where Collembola sp. are sometimes implicated in losses (Durrant, et al., 1988) but has not been reported before in California. Alternatively, losses of newly emerged seedlings to flea beetle grazing or other insect predation may have occurred prior to the first observation at day nine after irrigation. However, in many other recent trials elsewhere in California where flea beetles have been present, early post-emergence loss to insects or diseases (1 to 3 days from the onset of emergence) has been rare. We do not consider them likely here. Early seedling damage was due almost entirely to flea beetles. Armyworm larvae had not had time to develop and were not observed.. Gaucho® is very effective against flea beetles, and substituted well for soil applied Lorsban® and the first and possibly the second aerial applications of Lorsban®, as well. This is a significant savings in pesticide use.

In addition to having adequate numbers of seedlings, growers need healthy, vigorous plants. Treatments not receiving a pre-emergence insecticide resulted in severely damaged seedlings by the last counting date. Those seedlings surviving were reduced in size, often having damage to the apical meristem region. Even the Gaucho® treated seedlings were smaller and were beginning to suffer armyworm damage at the last counting date, suggested both by lower seedling weights (Table 3) and increasing rates of mortality (figure 4). These losses resulted in fewer plants at harvest and greater variability in plant spacing compared to fall population estimates and compared to the Growers treatment (table 5). They imply that some post-emergence worm control is necessary in the fall establishment period. Compared to the standard growers treatment, however, the amount of pesticide and the number of treatments needed could be reduced, if these results prove to be characteristic.

Xentari® (Bt) was not very effective as a post-emergence worm control practice. It has no effectiveness against flea beetles, the principal pest during the earliest stages of growth. Other, newer botanicals or low impact insecticides may be combined with Gaucho® in the future, however, to form a complete alternative to currently used organophosphate and carbamate insecticides.

Costs of establishment. The percentage of seed resulting in established seedlings was high in this trial when pre-emergence insecticides were used. Generally, when 70 % or more of the seed planted results in viable plants, sugarbeets can be planted to a final stand density, and hand thinning is no longer needed. Hand thinning costs in the Imperial Valley typically average between $50 and $100 per acre. In addition, seed is over-planted by approximately three times the needed amount, if the emergence rates observed in the best treatments in this trial can be repeated in most locations. With seedling protection in this trial, money could have been saved and several pesticide applications spared.

Yield. Mechanical thinning, applied uniformly across all treatments, left too many plants un-thinned. A large population of roots survived until harvest (fig. 5). Average losses from thinning until harvest (including the thinning process itself) equaled 41% of the seedlings in all treatments. Because of very large populations, many small roots were present at harvest in treatments 1 and 2, but these could not be gathered by the harvester and where lost, skewing the yield comparison.

Of particular interest, however, was the observation that treatment 3, which received no insect control of any kind, resulted in a good commercial yield. There were no seedlings in the untreated test plots that escaped damage by insects, but even in uncontrolled plots sufficient seedlings survived to develop into healthy sugarbeet plants and provide good yields. This result contradicts conventional wisdom that there would be no useful plant stand without insect control, and occurred despite the observation that the autumn of 1999 was notable for severe armyworm pressure. It is also a testimony to the inherent toughness of sugarbeet seedlings once emerged.

Limiting the results of this experiment, however, was the irrigation date. This field was initially irrigated during the middle of the sugarbeet planting period in the Imperial Valley. Growers begin irrigating the earliest sugarbeet stands at the end of the first week of September and it is a common observation that the earliest stands are the most severely damaged by insects. Our results do not reflect the consequences of applying these seedling protection treatments at the earliest planting date possible. Additional tests started earlier in the year are needed. Our results do suggest, however, that fields initially irrigated even later in the season than this field may need less pest protection than previously thought necessary, and that later planting is itself an alternative management technique.

Plant spacing at harvest. Yield was not a useful way to compare the different treatments applied in this trial. Thinning was inadequate to allow roots to develop to full size in some of the treatments. Yields suffered as a consequence. For a grower concerned about establishing a uniformly spaced stand, the number of large gaps at harvest compared to the target population is the most important consideration. This can be determined by evaluating the number of plants in the population with spacings greater than 1.5 times the target population. The proportion of plants at harvest greater than 1.5 times the target spacing lowers average yields in a field because of lost light capture by the plant canopy. In this trial, treatments 1 and 2 had fewer large gaps (Table 5). If an Imperial Valley grower were interested in attempting to plant to a stand, soil or seed applied insecticides are essential.

Conclusions

1. Pre-emergence pesticide applications resulted in significantly larger numbers of seedlings than other treatments.

2. Gaucho® applied to seeds worked as well as Lorsban® applied to soil and aerially immediately after emergence. Flea beetles were the principal cause of damage at emergence and are well controlled by Gaucho®. Approximately 7 to 10 days after emergence, armyworm control became important. At this point, an effective post-emergence insect control measure was probably required in Gaucho®-treated plots.

3. Establishing a large percentage of seeds as seedlings both 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®.

5. Despite the survival of unprotected seedlings in this trial, it would be incorrect to conclude that no seedling protection is necessary. Growers cannot risk a large financial investment in crop production when they have the opportunity to protect that investment. Reduced spraying based on economic thresholds or knowledge of probable damage is far different from leaving the fate of seedlings to chance, and depends upon the use of remedial measures like insecticides as necessary.

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.

Kachman, S.D., and Smith, J.A. (1995). Alternative measures of accuracy om plant spacing for planters using single seed metering. Trans. ASAE 38(2):379-387.

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. Plant population comparisons.

List of figures

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

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

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

Fig. 4. Seedling above ground dry weights (g per 30 seedlings). Error bars are standard errors.

Fig. 5. Plant populations at harvest by treatment. Error bars are standard errors.

Fig. 6. Root yields by treatment. Error bars are standard errors.

Fig. 7. Plant population, yield and root weights.

Fig. 8. Sugar percentage by treatment. Error bars are standard errors.

Table 1

Treatments

Number

Description

Pesticides used

Timing (Days since first irrigation)

Rates

Type of application

Cost

($/ac)

Standard practice in the region (Growers’)

Lorsban 15G

Lorsban 4E

Lorsban 4E +

Diazanon4E

-2

4.6 lb/ac

1.15 pt/ac

1.18 pt/ac + 0.59 pt/ac

1.06 pt/ac + 0.88 pt/ac

Soil applied with seed

Aerial

8.92

15.84

16.65

16.87

73.49 (total)

Seed applied systemic insecticide (Gaucho)

Imidicloprid

(Gaucho)

Applied to seed prior to planting

45 g per 100,00 seeds; 67.5 g per acre.

With seed

72.34 (total)

No pre- or post-emergence treatments (Control)

none

Bacillus thuringiensis application post-emergence (Bt)

Xentari

1.25 lb/ac

Aerial

22.82

91.28 (total)

One application of standard pesticide (1X)

Lorsban 4E

1.15 pt/ac

Aerial

15.84 (total)

Table 2

Seedling emergence and establishment

Treatment

Cumula-

tive

emer-

gence

Cumul-

ative. mortality

(% of seed sown)

Cumul-

ative mortality

(% of plants emerged)

estab-

lished

Diseased

(% of seed sown)

Diseased

(% of plants emerged)

Insect loss (% of seed sown)

Insect loss (% of plants emerged)

Pre-emergence loss

(%)*

Grower’s

82.2

2.7

3.5

79.3

1.2

1.6

0.1

0.2

17.8 (9.8)

Imidicloprid

79.4

5.1

6.9

74.1

3.0

4.3

0.8

1.1

20.6 (12.6)

Control

56.3

8.1

15.6

47.5

4.4

10.0

1.7

3.2

43.7 (35.7)

55.6

5.5

7.1

49.7

3.0

6.4

0.7

1.5

44.4 (36.4)

Control + Lorsban (1x)

58.2

6.2

11.5

51.6

3.8

7.8

0.7

1.3

41.2 (32.2)

* Includes 8 % non-viable seed. Values adjusted for non-viable seed are in ( ). Additional viable seed may have remained un-emerged in the soil, but this seed is of no agronomic significance and is regarded as lost.

Table 3

Treatment contrasts (Days since initial irrigation = 25, final count)

Treatments*

Variables

p =

Growers vs Gaucho

Cumulative emergence

160.4

0.56

0.4556

Number established

584.4

1.86

0.1756

Cumulative mortality

148.1

6.84

0.0106

Control vs Bt and 1X

Cumulative emergence

10.7

0.04

0.8469

Number established

277.1

0.88

0.3498

Cumulative mortality

176.3

8.15

0.0054

Pre-emergence insecticide vs control

(1+2 vs 3)

Cumulative emergence

17226.8

60.3

0.0001

Number established

24390.0

77.82

0.0001

Cumulative mortality

602.1

27.83

0.0001

*See Table 1 for treatment descriptions

Table 4

Seedling dry weights at thinning

(g DW per 30 seedlings)

Treatment

Dry weight (g)

Grower’s

5.75

Imidicloprid