1) SUMMARY

Two species of aphids commonly infest sugarbeets in California.  The green peach aphid (GPA), Myzus persicae, has been a recognized pest of sugarbeets for 50 plus years.  The black bean aphid (BBA), Aphis fabae, (also called the bean aphid) is a relatively new pest of sugarbeets in California.  These two aphids are similar in their damage, importance, life history, etc. and will be considered together in this analysis.  The differences will be pointed out when applicable.

The bean aphid is dark olive green to black with a dull, matte-like appearance.  It can be easily confused with the cowpea aphid and the dark morph of the cotton aphid, which do not commonly occur in sugarbeets.  The green peach aphid is pale green and medium-sized.  Both aphid species have been present in California at least since the 1940's.  However, yellow water pan trap samples collected near sugarbeet fields in Yolo and Solano Counties in the late 1960's by Dr. Harry Lange showed that less than one percent of the collected alate aphids were bean aphids; whereas, 80 percent of the aphids were green peach aphids.  Samples collected from 1992-1997 by Dr. Larry Godfrey, in cooperation with sugar company and California Beet Growers Association personnel, showed a dramatic reversal.  Today about 70 percent of the alate aphids collected in the spring in traps adjacent to sugarbeet fields are bean aphids, and green peach aphids comprise the other 30 percent.  The reason for this shift in species composition is unknown.

Populations of GPA and BBA typically build up in the spring (March) and persist through about May.  The exact dates are dependent on the environmental conditions.  GPA cannot tolerate heat; therefore, populations crash as temperatures exceed 85º F; BBA are more tolerant of heat, and populations persist longer, in some cases through the summer.  BBA populations appear again in the fall; whereas, GPA are at very low numbers in the fall and the rest of the year.  The most severe aphid problems in sugarbeets are in the Central Valley, particularly in the lower Sacramento Valley and upper San Joaquin Valley (Glenn County to the north and San Joaquin County to the south).

The primary damage done by these aphids in sugarbeets is their propensity to vector virus diseases including western beet yellows luteovirus and beet yellows closterovirus (BYV).  These diseases, along with others, are commonly called the yellows complex.  Beet yellows closterovirus has been shown to be the most damaging of these virus diseases and is of the greatest concern to the industry.  Some of the key life history traits of GPA and BBA are summarized below.
 

The primary damage and concern from aphids in sugarbeets is the transmission of virus diseases.  Infection with the yellows virus complex early in the growth of the beet can result in a 50 to 75 percent reduction in yield.  This susceptible period is up to six weeks after seedling emergence.  More mature plants are not so severely affected by the yellows complex; however, a 10 percent yield reduction can still result from infections of sugarbeet plants that are nine weeks old, for example.  The yellows virus complex reduces plant leaf area, photosynthesis of the remaining leaves, and can kill plants with early infections.

BBA infestations, besides introducing BYV into the crop, can also reduce sugarbeet yield.  This species, as it feeds, injects a toxin which "stunts" plant growth.  The infested leaves curl and do not develop properly.  Threshold values have been developed for BBA on sugarbeets (Summers, et al, 1996).  GPA feeding alone does minimal obvious damage to the plant.  Both species exude honeydew which coats the leaves and stimulates the growth of sooty mold fungus.  The effects of this coating on sugarbeet growth have not been fully evaluated, but in other systems it is known to be detrimental to plant growth.

Aphid management in sugarbeets involves an integrated program of cultural, biological, and chemical controls.

 2) CURRENT PEST MANAGEMENT PRACTICES

Cultural Controls:  The timing of sugarbeet planting and harvest is highly regulated by the industry.  The characteristics of the plant allow sugarbeets in the Central Valley to, in theory, be planted and harvested during any month.  In addition, it has an indeterminate growth characteristic, especially in the non-bolting varieties.  The industry is organized into a series of districts with specific windows for planting and harvesting.  The primary goal of this program is to assist in management of the yellows complex by temporarily and spatially separating older (overwintered, possibly infected) sugarbeets from new plantings.  This reduces the introduction of BYV into the newly emerging sugarbeets, which is the most critical time for the crop.  This program has been highly successful and is one of the pre-eminent examples of area-wide pest management.

This area-wide strategy is very successful, but BYV is not completely controlled by it.  Why?  The original research in the 1960's suggested that at least a ten-mile buffer (and possibly more) was needed to prevent aphid movement from overwintered beets to newly planted beets.  This is not feasible in many areas, and a five-mile buffer is used.  This still allows for some aphid movement from infected to new beets.  Environmental conditions and winds also contribute to aphid movement between old and new beets.  More importantly, the emergence of the bean aphid as a common aphid pest has greatly threatened this system.  The ability of the bean aphid to withstand hot temperatures has compromised the utility of delaying planting past the timing of the aphid flight peak in March-April.  BBA was uncommon when the planting/harvest system was implemented in the late 1960's.  It is also possible that BBA is able to fly farther than GPA.

Other cultural control measures for sugarbeet aphids include diligence in removing weedy (keeper) beets, minimizing weeds around fields, etc.

Biological Controls:  Several generalist predators and parasites kill aphid pests of sugarbeets.  Lacewings and lady beetles are important predators.  Parasites, such as Lysiphlebus testaceipes, are important.  In spite of these naturally-occurring biological control agents, outbreaks commonly occur.  In addition, both aphid species can transmit BYV in as few as 15 minutes and predators/parasites may not act quickly enough.  Methods to increase and manipulate levels of predators such as releases of lacewings and food sprays to attract native lacewings have been tried with minimal success.  These techniques may offer help in the future following more research.

Host Plant Resistance:  No host plant resistance is available to GPA or BBA.  In addition, resistance to the most detrimental component of the beet yellows complex (beet yellows closterovirus) has been researched for 40 years with no break-throughs to date.  Resistance through transgenic techniques is apparently being examined in Europe.  The exact details of this research are unavailable to us at this time.  The importance of sugarbeets to the economy of many European countries clearly warrants this research.  However, in the U.S., California is the only production area with serious problems with aphids and yellows diseases.  The size of our industry has not yet warranted transgenic efforts in this area.  The direct applicability of the European transgenic sugarbeets to BYV management in California is doubtful.  First, the sugarbeet varieties grown in Europe and California are greatly different.  Secondly, the viruses and/or virus strains involved in the yellows complexes in California and Europe are probably different and may limit direct applicability of the European transgenic efforts to California.

Chemical Controls:  Foliar insecticides are occasionally used to control BBA that have reached levels high enough to cause plant damage; these infestations are often spotty, so spot treatments can be used.  Lorsban®, diazinon, Metasystox-R®, and Thiodan® are used.  However, the primary concern with aphid pests in sugarbeets is the transmission of virus diseases.  The primary use of insecticides for aphid control is to minimize this injury.  Two approaches are used:  1) planting time applications in areas and months when the threat of aphid infestation and virus introduction is severe, and 2) applications of foliar insecticides following seedling emergence and aphid infestation.  The challenge with all these insecticide treatments is that aphids can transmit BYV in as few as 15 minutes.  Few insecticides can kill pests that fast or protect the plant that thoroughly.  Insecticides can, however, reduce aphid densities and subsequent secondary spread of virus diseases within the field.  Planting time treatments include Temik®, Thimet®, and Gaucho® (imidacloprid) seed treatment.  Temik® and Thimet® can also be applied postemergence.  These two products do not seem to appreciably reduce the percentage of plants with BYV; but especially with Temik®, there is a measurable positive yield response.  Gaucho® is a reduced-risk insecticide that was available starting in 1997 for aphid-BYV management.  Results from Europe with the product in sugarbeets have shown a reduction in the incidence of several virus diseases (but not BYV).  We have seen an indication that Gaucho® treatments directly reduce the incidence of BYV, but this needs to be confirmed.  Gaucho® has been shown in California to provide excellent protection of sugarbeet yield.  This product is used at 45 grams active ingredient per 100,000 seeds, or 45 to 60 grams per acre at commond seeding rates.  This rate protects the seedling for three to five weeks.  A higher rate is used in Europe, but this rate is cost-prohibitive in the California system.  Gaucho® also controls other seedling pests such as flea beetles, wireworms, etc.  Foliar insecticides are used during periods of high aphid density to reduce populations and virus spread.  Lorsban®, diazinon, Metasystox-R®, and Thiodan are used (Monitor® was also used before its loss in 1998).

The drawback of the foliar treatments is that they also kill beneficial insects.  These natural enemies assist with aphid control, but more importantly, they impede the buildup of armyworm, leafhopper, and spider mite pests.

Other biorational control techniques have been evaluated for aphid management in sugarbeets.  Stylet oil, insecticidal soaps, and Neem® have been tested in replicated trials with minimal positive results.  No additional synthetic experimental insecticides have been evaluated or are nearing registration for aphid management.

 3) REDUCED-RISK OPTIONS

Adaptation to systemic insecticide applied through low dosage seed treatments.

 4) CHALLENGES

The Food Quality Protection Act could have significant impact on management of aphids and yellows virus diseases.  The strengths of the present system are the variety of strategies used, including cultural, biological, and chemical.  Within the chemical control strategy is the availability of products possessing a range of modes of action and with several use patterns.  The range of modes of action inhibits the build-up of insecticide resistance in the aphids.  Aphids, especially green peach aphids, have the propensity to quickly attain resistance.  The ability of materials used as seed treatments, as granular soil-applied treatments, and as foliar applications provides growers with considerable flexibility and options for aphid management.

The implementation of FPQA could result in the loss of registrations of organophosphates, carbamates, and organochlorines; this would leave only Gaucho® (imidacloprid) for aphid management.  The availability and use of of only one material would be the worst-case situation for IPM.  The use of Gaucho® would likely increase as growers would be left with no foliar treatment options (foliar treatments are not the best method to manage yellows diseases but do provide options for growers in emergency situations).  Therefore, the implementation of the FQPA would greatly hinder aphid/disease management and IPM in sugarbeets.

Several other challenges are present to threaten the current system of managing aphids and virus diseases in sugarbeets.
 

• GPA and BBA have a history of the development of insecticide resistance.  This has been shown in several systems and countries.
• Temik®, a key product used for aphid control, is under constant regulatory scrutiny because of its environmental and human health concerns.
• The availability of Gaucho®, a reduced-risk material, is an asset for the sugarbeet industry.  However, aphids, especially BYV, tend to cycle in importance; and during years with outbreaks, insecticidal control has not been sufficient to prevent substantial crop losses.
• The planting date restrictions are useful for minimizing aphid and BYV incidence, but these restrictions compromise the crop potential and increase the damage from other "pests."  Delaying the planting until May/June (after the aphid flight) increases the stress from rhizomania and decreases seedling establishment due to hot weather.  Managing aphids and virus diseases through host-plant resistance, biological control, etc. would greatly benefit sugarbeet production.

 5) INNOVATIVE FEATURES IN REDUCED-RISK PROGRAM

Innovative features include the substitution of seed-applied systemic insecticide to reduce quantity and loss to environment compared to foliar and soil-applied material.  The beet free program is a model for regionalized pest management.

 6) BARRIERS TO ADOPTION OF REDUCED-RISK METHODS

  Barriers are adequate demonstrations and extension of the effectiveness of reduced-risk seed treatments.
 

 REFERENCES

Bennett, C. W. 1960. Sugar beet yellows disease in the United States. USDA-ARS. Tech. Bull. No. 1218.

Godfrey, L. D. & P. A. Mauk. 1993. Interactive effects of aphid injury and beet yellows virus on sugar beet photosynthesis and yield. J. Sugar Beet Res. 30: 95.

Godfrey, L. D. and P. A. Mauk. 1998. Sugar beet plant growth response to bean aphid (Homoptera: Aphididae) infestation and beet yellows closterovirus infection. J. Econ. Entomol. in review.

Godfrey, L. D. and P. A. Mauk. 1998. Influence of bean aphid (Homoptera: Aphididae) infestation and beet yellows closterovirus infection on sugar beet yield parameters. J. Econ. Entomol. in review.

Hills, F. J., W. H. Lange, R. J. Shephard, & J. S. Mcfarlane. 1982. Sugarbeet pest management: aphid-borne viruses. Div. Agric. Sci., Univ. of California, Spec. Publ. 3277.

Kaffka, S. & P. G. Lemaux. 1996. The use of molecular breeding methods to advance the sugarbeet industry in California. Div. Agric. and Nat. Res., Univ. of California Spec. Publ. No. 21544.

Lange, W. H. 1987. Insect pests of sugar beets. Ann. Rev. Entomol. 32: 341-360.

Limburg, D. D., P. A. Mauk, & L. D. Godfrey. 1997. Characteristics of beet yellows closterovirus transmission to sugar beets by Aphis fabae. Phytopathology 87: 766-771.

Summers, C. G., L. D. Godfrey, & R. Long. 1996. Sugarbeet: Bean Aphid. UC IPM Pest Management Guidelines. UC IPM Guideline Series #24, pp. A4-A6.

Summers, C. G., L. D. Godfrey, & R. Long. 1996. Sugarbeet: Green Peach Aphid. UC IPM Pest Management Guidelines. UC IPM Guideline Series #24, pp. A1-A4.

Weber, C. A., L. D. Godfrey, & P. A. Mauk. 1996. Effects of parasitism by Lysiphlebus testaceipes (Hymenoptera: Aphididae) on transmission of beet yellows closterovirus by bean aphid (Homoptera: Aphidae). J. Econ. Entomol. 89: 1431-1437.

Aphids contributed by Larry Godfrey, PhD., Entomology, University of California, Davis.