William D. Hutchison
& Christopher D. Campbell
Department of Entomology
University of Minnesota
St. Paul, MN 55108
The sugarbeet root aphid (SRA), Pemphigus betue Doane, continues to be a potential threat to profitable sugarbeet production in southern Minnesota. During the past four years, we have taken abroad-based approach to documenting the economic impact, mechanisms for damage, SRA biology and methods for control. Prior to this project very little was known about the biology or control of SRA. Major accomplishments during the past four years include:
1. Based on a 2-yr study, sugarbeets infested with SRA incur average losses of 31% in sugar content, 32% in yield, and an overall 54% reduction in recoverable sugar/act A root rating system can be used at harvest to provide a more precise estimate of yield loss for a given hot spot. These studies have also shown that, in general, average root ratings must exceed 2.3 before consistent losses in recoverable sugar occur.
2. Although above-average moisture years (> 24 inches) in southern Minnesota tend to minimize damage by SRA, this result is most likely due to a direct beneficial effect on beet growth rather than a direct mortality effect on the aphid (see enclosed manuscript).
3. SRA infestations in southern Minn. appear to develop primarily from wingless adults (apterae)overwintering in the soil.
3a. Year-year SRA infestation dynamics probably results from a combination of the following factors: the abundance of late-season apterous adults that remain in the soil in the fall, the number of severe-weather days during the winter, i.e., the number of days with temperatures < 0°F and snow cover < 5 inches (this study is in progress to evaluate the past 10 years), average to below average precipitation during the current growing season, and a minimum of 1800 degree-days (or heat units > 7.6°C) during the current growing season.
3b. In cooperation with Mark Seeley, Minnesota Extension Service Climatologist, synoptic climate analyses indicate a lack of consistent aphid migration events from the Colorado and Wyoming Rocky Mountain Region (to Minnesota), which is where SRA would have to originate from each spring, if it were coming from galls on narrowleaf cottonwood. These results agree with the absence of winged migrants being trapped in yellow-pan traps, or on beet foliage, in southern Minnesota during late-spring and early-summer, and provide further evidence that most infestations develop from those SRA overwintering locally.
4. Despite the cryptic nature of subterranean life stages that overwinter, most of the infestations developing in the spring can be monitored by sampling lambsquarter - before and during beet emergence.
5. Once SRA infestations are detected in early spring, a forecasting model can be used to predict the timing of the start of reproduction (2 yrs. of data). In addition, the timing of movement from lambsquarter to nearby beet fields has been very consistent the past three years (mid-May to early-June).
A reliable rearing method has been developed for SRA, with colonies having been continuously maintained for over 2 years. The colony has been essential for conducting laboratory studies on SRA biology and insecticide toxicity. The colony has also been useful for artificially infesting field plots to evaluate insecticide efficacy or tolerance by sugarbeet hybrids. Given proper lead time, small quantities of aphids from this colony could be made available to the industry for additional biological or control testing purposes. A paper describing the rearing method will soon be available for other research groups.
Results From 1993 SRA Forecasting & Insecticide Control Studies
SRA Forecasting/Life Table Analysis:
As summarized last year (Hutchison & Campbell 1993), we have developed a forecasting model for spring emergence and reproduction of SRA. To develop the model, we have conducted studies to determine the developmental and fecundity rates of SRA as a function of temperature. This information, along with life table data for spring cohorts, has been used to estimate a lower threshold for development of 7.6°C. During 1993, we extended this work to determine the age-specific survival and fecundity of summer populations of the aphid.
Fig. 1 illustrates the life tables generated for two cohorts of individuals (starting with first instar nymphs of the same age) reared at two constant temperatures. Using this information and the lower threshold for development, the data for each temperature was placed on the same degree-day time scale (Fig. 2). Although these results reflect optimal survival and reproduction under laboratory conditions, they are useful for estimating potential population growth rates in the field.
These data, along with the spring emergence data (Hutchison & Campbell 1993), represent the first such life tables developed for a subterranean aphid. The results, however, are surprisingly similar to those for most foliar feeding aphids. Equally important, these models can be used to forecast the number of SRA generations (e.g., first-instar to adult) that can occur in any given year. To date, simulations for recent cool (1992) and warm (1989) years, indicate differences of about 8 and 11 generations, respectively. This information, along with overwintering survival, helps to explain the presence of damaging SRA populations of 1989.
Field Test & Timing of Insecticides for SRA Control in southern Minnesota:
This study was conducted at the University of Minnesota agricultural experiment station at Rosemount, Minn in sugarbeets planted 14 May 1993. Treatments were replicated 4 times and arranged in randomized complete block design. Each replicate consisted of 3 treated rows on 30 inch centers measuring 70 ft in length. Treatments were separated by a single untreated row. Granular treatments were applied using a single row Noble applicator delivering a 6-inch band to either side of the base of the plant. Liquid treatments were applied using a single nozzle CO2 backpack sprayer with a flat-fan nozzle delivering 27 GPA in a directed band to the base of the plant, with both sides being treated.
Four "pre-infest" treatments were applied 29 Jun. prior to artificial infestation, including Counter 15G, Counter 20CR, AC513858 20G, Knox-Out (2340). The remaining treatments were applied 14 July. Plots were infested 7 Jul. 21 Jul. 10 Aug. and 24 Aug. Plots were infested by placing 8-12 SRA adults and 100-200 nymphs at the base of a single plant in the center row of each treatment, with 1 location for the July infestations and 1 location for the August infestations, separated by 20 ft.
Treatments were harvested 21 Sep and evaluated for percent of beets infested and level of infestation using the following SRA Root Rating Index (0-5): 0 - no aphids nor wax present;1 - 1 colony, wax (or both) < 2.5 cm. diem.; 2 - 2 or more colonies, wax (or both) <2.5 cm diem.each, covering less than 50% of root surface; 3 - 1 or more colonies, wax (or both) >2.5 cm diem. each, covering <50% of root surface; 4 - multiple colonies, wax (or both) covering 50 to 95% of root surface; 5 - multiple colonies, wax (or both) covering >95% of root surface. Samples were evaluated for percent sugar and impurities at the Southern Minnesota Sugar Cooperative tare laboratory in Renville, MN.
July infestations (Table 1) resulted in more substantial at-harvest SRA infestations than the August infestations (Table 2). These results again indicate the need for the aphid to become established early in the season, especially during a cool year, to cause yield loss. Although some treatments resulted in root ratings just above the 2.0 level, no significant differences were observed in percent sugar or recoverable sugar.
The August infestations (Table 2) were quite low with no more than 20% of beets infested and root ratings between 0 and 1. Although none of the infestations could be considered damaging (yield was not significantly lowered), some differences were observed among treatments. Lorsban 15G exhibited significantly higher percent SRA infestation and root rating as in past studies, presumably due to predator mortality. Counter 15G (pre-infest) resulted in lower percent infestation and root rating, but no treatment was significantly different from the untreated check. No phytotoxicity was observed. Also, none of the treatments demonstrated any negative effects on sucrose or total recoverable sugar.
Development of Laboratory Bioassay for Testing Insecticide Toxicity & PreliminaryData for Encapsulated Diazinon:
Because of year-year variability in conducting insecticide field trials with SRA, along with the lack of toxicity data on SRA, we developed a laboratory bioassay method to quantify the dose-mortality response of several insecticides on SRA. These studies are useful for comparing many materials under the same controlled conditions, and thereby provide estimates of the inherent toxicity of different compounds to the test insect species. This information can also be useful for modifying field rates for improved performance. To date, we have done all testing with a new encapsulated formulation of Diazinon from Elf-Atochem, N. Am.
Preliminary bioassays were conducted to determine the toxicity of microencapsulated Diazinon 2FE (formulated product: 2340-1) to first-instar sugarbeet root aphid (SRA). First instar nymphs were collected by isolating adult SRA and allowing larviposition to occur for24 hours. The resulting cohort of nymphs was assumed to have an average age of 12 hours.
A series of 6 dilutions was prepared from a stock solution of formulated Diazinon anddistilled water. Filter paper discs (5.5 cm) were soaked to saturation in each dilution (+distilled water check) and placed in a 6.0 cm petri dish to dry. Approximately 60 nymphs were placed in each dish when the filter paper appeared dry, but was still moist enough to stick to the dish (1 hour). Dishes were sealed with parafilm and held at 20°C for 24 hours and then checked for mortality. Dead individuals and those exhibiting functional mortality (inability to walk, uncoordinated movement, etc.) were recorded as dead.
In Table 3, we summarize results from 3 separate dilution series (3 replications of each dose). Mortality data were subjected to probit analysis to determine LC60 and LC',6 values of 24.8and 71.1 ug(AI)/ml, respectively. Using the same bioassay, plans are underway to determinethe LC60 for SRA adults, and toxicity to Counter, Thimet and Lorsban.
In conclusion, we believe we have developed a good understanding of the overwintering and spring emergence biology of SRA, and have a reasonable management plan for predicting and monitoring the spring emergence of SRA in southern Minnesota. During 1994-1995, we hope to continue the SRA project with the following projects:
1. Provide early-season monitoring service to SMSC, to conduct annual area-wide survey to assess high-risk fields for SRA establishment. These results would be used to make treat/notreat or "spot-treatment" decisions for specific fields.
2. Complete lab. toxicity studies for Counter 15G, Diazinon and the new nicotine-systemic based insecticide, imidacloprid (Gaucho seed treatment).
3. Continue insecticide field trials.
4. Complete overwintering/survival analysis using a winter severity index for the past 10 years to determine if there is a significant relationship between winter weather and the outbreak years (1984 and 1989).
5. Collect additional yield loss data in SRA-infested fields during a dry year, to build on the original 2-yr study, and to add additional data from wet years.
This research was supported in large part by the Sugarbeet Research and Education Board. Additional funding was provided by Am. Cyanamid and Elf-Atochem, N.A. As in previous years, we extend our appreciation to the Southern Minnesota Sugarbeet Cooperative and the southern Minn. growers for working with us to provide fields for study, and yield analysis of samples.
Hutchison, W.D. & C.D. Campbell. 1994. Economic impact of the Sugarbeet root aphid (Homoptera: Aphididae) on Sugarbeet in southern Minnesota. J. Econ. Entomol. 87: (in press).
Hutchison, W.D. & C.D. Campbell. 1993. Overwintering biology of the Sugarbeet root aphid: development and validation of a spring phonology forecasting model. 1992 Sugarbeet Research & Extension Reports, N. Dakota State University Extension Service, N.Dakota State University, Fargo, ND. 23: 129-144.
1993 Sugarbeet Research and Extension Reports. Volume 24, pages 242-251.
