OVERWINTERING BIOLOGY OF THE SUGARBEET ROOT APHID: DEVELOPMENT & VALIDATION OF A FORECASTING MODEL

William D. Hutchison & Chris D. Campbell
Assistant Professor & Assistant Scientist

Department of Entomology
University of Minnesota
St. Paul, MN 55108

Introduction

Pemphigus betae Doane, occurs throughout the major sugarbeet growing areas of North America, including infestations in Texas (Winter 1980), Alberta, Canada (Harper 1963), California (Grigarick & Lange 1962) and Michigan (Landis 1990, personal communication). SRA infestations have nearly always been more severe under dry soil moisture conditions, due either to dry years under dry land conditions in the upper midwest, or to using less water in the irrigated areas of the west and southwestern U.S.

In Minnesota, the drought of the late 1980's, especially during 1988 and 1989, was probably most responsible for the buildup of economically damaging SRA infestations in the southwestern part of the state. In contrast, 1990 and 1991 were very wet years with soil moisture returning to normal levels in southern Minnesota. Although the 1992 season was dryer, it was one of the coolest seasons on record. Not too surprisingly, SRA infestations were less abundant during the past three seasons and infestation "hot spots" developed much later compared to the previous drought years (about mid- to late-Aug. vs. July 1st).

Of the studies that have been done to determine the impact of SRA on yield, there have been conflicting results with regard to actual sugar losses. For example, Maxson (1916) observed a 4.55 T/A (25% less than uninfested fields) loss in beet yield with sugar reductions of <1%. Harper (1961) reported yield losses of 1.3 to 3.1 T/A but did not record sugar losses. Most recently, Summers and Newton (1989) found that SRA in California reduced yield by 36-60%, but in only one of four commercial fields studied, did they observe the sugar percentage to be significantly lower in infested areas. However, because of the drastic reduction in beet yield, total sugar/A was significantly reduced in three of four fields. Our results from the 1990-1991 seasons in Minnesota clearly indicate that if the infestation approaches a beet rating of > 2.75, beet yields are typically reduced by 50% and sugar percentage by 40-50% within aphid hot-spots. While gross yields were variable, sugar content and actual recoverable sugar per acre in SRA infested areas were significantly (P<0.05) reduced in 73% (8/11) of the fields examined (Hutchison & Campbell 1992,1993).

Because of the sporadic and cryptic nature of subterranean SRA, very little work has been done to understand the biology, ecology or management of the pest on sugarbeet. In addition, damaging infestations continue to show up locally each year. If unusually dry conditions occur in subsequent years, more widespread damaging infestations are likely as the commercial hybrids grown in Minnesota are highly susceptible to SRA. Without a basic knowledge of the life cycle of SRA and its mechanisms of infestation, high risk fields cannot be identified nor can management strategies be effectively implemented. The focus of this article is to provide a summary of the work we have done on overwintering and spring phenology of SRA, validation of a model to forecast spring phenology, and summer population dynamics. This information will be useful for the development of a risk-rating system for estimating which fields may be likely to become infested.

OVERWINTERING BIOLOGY:

In many locations, SRA's primary host is narrow-leaf cottonwood, which probably does not occur in Minnesota. Narrow-leaf cottonwood usually occurs only at elevations >5,000 ft. In addition, the aphids we have collected from galls on our native "eastern" cottonwood, were not identified as P. betae, but as other related Pemphigus species. Despite this finding, we have observed significant numbers of a late (winged) aphids being produced in the fall. After emerging from sugarbeets, aphids have been trapped on yellow sticky cards placed on nearby cottonwood. However, trees in the area were monitored closely during the spring of 1991 for galls; no SRA developed on these trees. We also exposed SRA alates emerging from potted sugarbeet plants in the greenhouse to cottonwood bark to determine whether or not SRA will lay eggs for overwintering. Alates collected from field samples and potted beets were dissected and found to contain an average of 6.5 sexuales/female. However, these sexuales did not lay the eggs that overwinter on cottonwood trees.

The most obvious SRA overwintering strategy appears to be adult (non-winged) survival in the soil. Fig. 1 summarizes our current model of SRA overwintering in Minnesota. Examples of SRA movement from overwintering sites to beets are shown in Fig. 2, where the SRA dispersed from culled beets in 1991, or fallow soil in 1992, to lambsquarter. Similar results from both years' research clearly demonstrates the role of lambsquarter in facilitating the spring buildup and movement of aphids into beet fields. The orientation of the 1992 infestations, in relation to the adjacent 1991 hot spots, are illustrated in Fig. 3. These results also suggest that good weed management can help to suppress SRA infestations. The decline of SRA on lambsquarter in late June also corresponds to the time at which we can begin to see aphids building up in newly planted beet fields. An additional piece to the puzzle is that first-instar nymphs (immature stage), rather than adults, could very well be the primary migrating stage. We have observed them moving in wind currents in large incubators in the laboratory.

Fig. 4, Fig. 5, and Fig. 6 summarize the life table (death and birth rates) studies generated for overwintering SRA adults collected in March-April, 1992. This is the first publication of age-specific life tables for the a non-feeding adult stage in aphids. A lower threshold for development was estimated from the developmental time (collection date to first reproduction) data to analyze all temperature data on a degree-day (DD) time scale. As with crop development, degree-days are a useful tool for quantifying the impact of temperature on insect reproduction, development, etc. Using the 6.6C threshold, time to first reproduction averaged from 90-100 DDs. This DD estimate incorporated fluctuating temperature accumulations in the field up to the collection date, as well as constant temperatures in the laboratory. As illustrated, despite the different collection dates, and rearing temperatures, reproduction and survival patterns are quite similar for each of the three constant temperatures, with most of the reproduction occurring between 100-200 DDs. On a day (calendar) time scale, however, SRA at 12C took nearly 3X longer to complete reproduction (mx) than at 24C.

The degree-day (DD) model is also in agreement with an independent field observation of the progression of aphids that are reproductive (i.e., containing live embryos, but not yet larvipositing). Fig. 7 summarizes this trend for 1992 and indicates that the 50% point occurs just at 95-100 DDs, which is when larviposition is expected to occur.

These studies are on-going through the 1992-1993 winter season and will continue into the spring of 1993 and winter of 1993-1994. Based on more conclusive evidence from the 1991-1992 winter and 1992 fall, we are still processing field samples that should enable us to document the proportion of aphids that overwinter in the soil and those that produce winged forms in the fall, that is an apparent latent, suicidal behavior in Minnesota. To do these studies, we are again monitoring a small plot (100 rows X 250 ft) of sugarbeets planted in 1992 where we had confirmed SRA "hot-spots" during the season (Clara City location). In 1993 we will again be monitoring any possible gall formation on cottonwood trees adjacent to the sugarbeet plot to refute the possibility of gall forming aphids infesting beet fields.

In addition to documenting field colonization in the spring, a primary goal of the 1993 season will be the validation of the degree-day model at a minimum of four field sites. At selected sites, we will set up ambient air and soil temperature recording probes for site specific temperature data. In addition each site will be selected in close proximity to weather monitoring sites maintained by SMSC and for the University of Minnesota, to evaluate regional differences in temperature accumulation and the effect on forecasting precision. These data will provide a good foundation for testing the model in 1993. We see this step as an integral component of our overall goal to develop the early-warning system.

SPRING & SUMMER PHENOLOGY:

As part of the beet sampling project, an "end-of-season" area-wide survey has been conducted during each of the last three seasons. Of the 51 fields sampled in 1990, 11 had at least some SRA infestations (22 %), while only 5 were heavily infested (10 %). In 1991, the region-wide survey indicated that 9 of 39 fields (23 %) were infested with SRA. In 1992, 9 of 42 fields (21 %) were found to have one or more SRA infestations. These data will provide a good base-line for comparison with future years.

WITHIN-FIELD DISPERSAL: As illustrated in Fig. 8 and Fig. 9, during the cool 1992 season,SRA progressed slowly from field edges toward the center of the field. The two figures summarize data from two different field edges in the same field (see also Fig. 3). The primary difference was that the low site was adjacent to a 1991 beet field that was heavily infested, separated only by about 3' of grassy border. The Northeast corner infestation began with only a few infested beets along the field edge. The limited infestation along the edge of the northeast spot apparently limited the magnitude of the subsequent spread, relative to that of the low spot.

The detailed data collected in 1992 on field colonization will be very useful in validating the degree-day model of aphid spread during the season. Additional field sites, under different temperature/precip. conditions, will be essential to complete model validation. Fields found to be SRA-infested in 1992 will be scouted in early 1993 to determine the extent to which aphids have successfully overwintered. New beet fields adjacent to or in close proximity of the 1992 SRA infestations will be sampled in 1993 to determine if and when migration and colonization has occurred. Early season scouting involves digging soil samples in areas where culled beets remain as well as examining roots of weed species known to host SRA (i.e. Chenopodium spp.). Early sampling is usually done on field margins and near field entrances. Any new infestations found will be monitored throughout the 1993 season to document the extent and rate of spread within the field. This was done in 1992 by weekly sampling of beets, row by row, within a 100 ft radius of the initial infestation. Soil samples taken from beets found to be infested provide estimates of aphid density and rate of increase.

INSECTICIDES:

During 1992, an insecticide plot was set up within each of the two dispersal sites at the Clara City location. Plots were arranged in a latin square design, with 6 treatments (20' x 3 rows) replicated 6 times. An untreated row separated each treatment. Treatments consisted of two rates of Knox Out 2FM as a directed spray and Counter 15G applied to the base of plants. Sprays were either incorporated manually or left as a lay-by. Granular treatments were incorporated manually. Treatments were applied when sampling indicated dispersal had occurred to at least 10 rows from the field edge.

Infestations did not spread throughout either of the plots. Therefore, only treatments within the first 12 rows were analyzed, and the two plots were combined to keep the number of replications at 6. Analyses based on percent infested, mean root rating, and yield showed no statistical significance (Table 1; P=0.05, ANOVA).

These results are consistent with past insecticide trials involving SRA (Campbell & Hutchison 1991, 1992). However, aphid density samples for the 1992 test have yet to be counted, and may provide additional information. The data taken at the time of evaluation is based on the presence of wax produced by the aphids. This wax remains even after aphids have died or left the beet. Therefore, percent infestation and root rating give no indication of whether viable colonies remained in treated areas. The information here indicates that infestations at the time of treatment were not at economically damaging levels, based on final root ratings (Hutchison & Campbell 1992). Levels of infestation in the untreated check remained below levels known to be economically damaging up to the harvest. This was most likely due to cool summer temperatures which hindered colony development and dispersal.

We will continue to conduct insecticide trials, using different materials and application timing strategies. As in 1992, we will attempt to conduct trials in at least three locations. We will also continue to focus on Counter 15G and Diazinon formulations (14G or Knox-Out), as these materials are currently labeled for sugarbeet. In a field situation, a decision to treat a large area with insecticides, would have to be made before the canopy closes. Early evaluations of SRA dispersal into the field, as well as knowing the likelihood of hot, dry conditions will be important in the decision making process. Economically damaging infestations may have to be present early in July before insecticides can be recommended. All data gathered to this point indicates that insecticides are unpredictable in light pressure situations. Additional studies are needed to determine if early treatments can suppress dispersal. Given the fact that insecticidal control currently represents the only realistic option for SRA control, these studies are necessary and complementary to the degree-day forecasting research.

SRA DENSITY - DAMAGE - WELD RELATIONSHIPS:

Examples of previous SRA-yield loss relationship were presented at previous research reporting sessions (Hutchison & Campbell 1991, 1992). As in 1990-1991 seasons, work was done to quantify SRA impact on sugar yields in commercial fields that were infested with SRA. A good cross-section of fields were sampled during the 1990-1991 field seasons. During the 1992 season, however, infested "hot spots" that were sampled (9/46 fields surveyed were infested), root ratings were < 2.5, indicating significant yield losses were unlikely.

These studies will be repeated in 1993 to provide data under different agronomic and environmental conditions (e.g., a warmer, dryer year). The same approach will be used. Five sets of 10-row-feet samples will be taken within SRA-infested "hot-spots" and compared to adjacent uninfested areas within the same field. As in 1990 and 1991, beet and sugar yield, as well as impurity analyses will be determined at the SMSC research laboratory. Beet root ratings and percentage of beets infested will also be determined for each yield sample. Where possible, we will also attempt to identify potential "hot-spots" relatively early (July-August) so that within-season estimates of SRA infestations can also be correlated with yield effects.

SUMMARY

Our overall goal for the 1993 season is to develop and validate a comprehensive risk rating model to determine whether or not a given field may have a high probability of SRA infestation, and therefore requires treatment. Specific objectives related to this project include:

1) confirmation of SRA overwintering sites underground within beet fields and field borders, AND exactly when overwintered SRA adults begin to reproduce to initiate the spring infestations (SPRING EMERGENCE AND REPRODUCTION DEGREE-DAY MODEL),

2) document the timing of SRA populations colonizing sugarbeet fields and their movement from old sugarbeet fields to lambsquarters, and back to sugarbeets (SUMMER PHENOLOGY MODEL; SUMMER LIFE TABLE STUDIES),

3) maintain laboratory and greenhouse colonies of SRA to facilitate more in-depth studies of temperature-dependent developmental rates (e.g., time from first-instar nymph to adult), survival and birth rates on sugarbeet,

4) obtain data on the efficacy of granular and liquid insecticide applications on SRA, and

5) use of "at-harvest" beet root rating system and assessment of the impact of SRA on beet yield, sugar yield and impurity relationships.

Acknowledgements

Appreciation is extended to the Sugarbeet Research & Education Board of Minnesota and North Dakota for generous funding of this project. We also thank Mark Bredehoeft and the Quality Laboratory of the Southern Minnesota Sugar Cooperative, Renville, MN for sugar quality analyses, and Atochem North America for the Knox-out material and grant-in-aid.

REFERENCES CITED:

Campbell, C.D. & W.D. Hutchison. 1991. Sugarbeet root aphid control in southern Minnesota, 1990. Insect. & Acaracide Tests. 16: 231-232.

Campbell, C.D. & W.D. Hutchison. 1992. Sugarbeet root aphid control in southern Minnesota, 1991. Insect. & Acaracide Tests. 17: 284.

Grigarick, A.A. & W.H. Lange. 1962. Host relationships of the sugar-beet root aphid in California. J. Econ. Entomol. 55: 760-764.

Harper, A.M. 1961. Effect of insecticides on the sugar-beet root aphid, Pemphigus betae. J. Econ. Entomol. 54: 1151-1153.

Harper, A.M. 1963. Sugar-beet root aphid, Pemphigus betae Doane (Homoptera:Aphididae), in Southern Alberta. Can. Entomol. 95: 863-873.

Hutchison, W.D. & C.D. Campbell. 1992. Proposed new name for the sugarbeet root aphid, "Cryptic root aphid": second-year results. 1991 Sugarbeet Research & Extension Reports. 22: 146-160.

Hutchison, W.D. & C.D. Campbell. 1991. Biology, yield impact and management of the sugarbeet root aphid in southern Minnesota: First-year results. In 1990 Sugarbeet Research & Extension Reports; vol 21: 151-159.

Hutchison, W.D. & C.D. Campbell. 1993. Economic impact of Sugarbeet Root Aphid (Homoptera: Aphididae) on yield and quality of sugarbeets in southern Minnesota. J. Econ. Entomol. (In Prep.)

Landis, D. 1990. Michigan State University, E. Lansing, MI; personal communication, April, 1990.

Little, E.R., Jr. 1968. Southwestern Trees: a guide to the native species of New Mexico and Arizona. USDA, Forest Service, Agric. Handbook No. 9. 109 pp.

Maxson, A.C. 1916. Some unpublished notes on Pemphigus betae Doane. J. Econ. Entomol. 9: 500-505.

Parker, J.R. 1914. The life history of the sugarbeet root-louse (Pemphigus betae Doane). J. Econ. Entomol. 7: 136-141.

Summers, C.G. & A.S. Newton. 1989. Economic significance of surgarbeet root aphid, Pemphigus poculivenae Fitch (Homoptera: Aphididae) in California. Appl. Agric. Res.4: 162-167.

Winter, S. 1980. Research News Report 80-19. Texas Agric. Expt. Stn., Bushland,TX; personal communication, Oct. 1990.


1992 Sugarbeet Research and Extension Reports. Volume 23, pages 129-144.


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