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Annual Reports

1998UC IPM Competitive Grants Program

Decision Support

Decision Support includes projects whose aim is the development and promotion of decision systems useful in crop production and pest management. General areas of such work might include but are not limited to development or improvement of sampling or detection methods, quantification of crop loss, development or improvement of damage thresholds or action levels, risk assessment, economic or other evaluation of IPM programs, development or evaluation of expert systems or other computer models.

Development of monitoring programs for pests and biological control agents involving sampling decision rules, control action thresholds, or improved methods for quantifying pest abundance or potential crop loss can significantly reduce the riskiness of pest control, reduce the number of pesticide applications, and improve adoption of new practices. Control action threshold research should be based on the relationship of pest occurrence or abundance to measurable crop loss in order to ensure cost-effective decisions.

High priority is given to proposals that clearly define objectives; will produce programs that can be easily and directly used in the implementation of integrated pest management; will focus on a biological system that has had sufficient preliminary investigation to allow completion and validation of the program in a reasonable amount of time; and will involve principal investigators who have demonstrated experience in systems modeling or systems management.

New Projects Funded for 1998-99
Continuing Projects Funded for 1998-99
Projects that Ended in 1997-98
Final Reports for Projects that Ended in 1997

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New Projects Funded for 1998-99

Use of Cal/EPA’s Pesticide Use Reports to Quantify the Impact of Selected IPM Programs on Pesticide Use.

Principal Investigator: L. Epstein, Plant Pathology, Davis

Objectives: Finish the development of software that converts individual application records in the Department of Pesticide Regulation’s Pesticide Use Reports into an output (a Profile of Pesticide Users) that graphically and quantitatively describes growers’ pesticide applications programs.

Use our Profile of Pesticide Users to compare the reported pesticide use with the recommended use by the model. In selected cases in which there are new environmentally-driven models for pesticide applications.

Document selected cases in which incorporation of an IPM program between 1992 and 1996 has possibly resulted in reduced pesticide use in California using our Profile of Pesticide Users.

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Continuing Projects Funded for 1998-99

Biology and Life Cycle of the Sting Nematode.

Principal Investigators: J. Ole Becker, Nematology, Riverside, UC Cooperative Extension

Objectives: Describe life cycle of the sting nematode as observed in culture of excised corn roots.

Determine influence of various physical and chemical environmental factors on behavior and population development of the sting nematode.

Summary of Progress: Direct observations of ectoparasitic nematodes in their natural habitat are difficult given their microscopic size, near transparency, and almost constantly movement in the darkness of a plant rhizosphere. Therefore, more than 50 years after its first discovery in Florida, much of the life cycle and behavior of the sting nematode Belonolaimus longicaudatus remains obscure. With its recent spread into California and consequent serious threat to the turf and agricultural industry, we have begun to unravel some of the mysteries concerning this pest. The key to observe the complete life cycle was the in vitro culturing the sting nematode on excised roots of corn. At 28C, first stage juveniles molted in the eggs four days after egg deposition. The second stage hatched one day later, moved towards the root tips and started to feed. Two days after the first molt the second occurred. Third-stage juveniles continued feeding for three days until the third molt took place. Fourth stage feeding extended over 4-5 days. At the end of the last, fourth molt, nematode gender became apparent. Male sting nematodes found females without much search which strongly indicated the presence of a sex attractant. Females began to lay eggs two days after mating. The life cycle was completed in 24 days. Vigorous feeding by many nematodes usually caused discoloration of root tips and termination of growth. The basic biological information gathered here will prove invaluable in eventually developing management tactics to minimize the spread of this pest.


Modeling of Pear Scab, Venturia pirina, Epidemiology and Disease Risk Using an Onsite Weather Monitoring System.

Principal Investigators: D. Gubler, Plant Pathology, Davis, UC Cooperative Extension; C. Feliciano, Plant Pathology, Davis; K. Dell, Plant Pathology, Davis; G. McGourty, UC Cooperative Extension, Mendocino County

Objectives: Monitor pear scab pseudothecial development, ascospore maturation, ascospore release, and infection of plant parts from primary inoculum at two pear orchard field sites in Mendocino County, each with an onsite weather station with sensors for rain, leaf wetness, temperature, and relative humidity.

Refine the Spotts-Cervantes model for predicting pear scab ascospore release as well as conditions favorable for disease development, and incorporate it into an electronic format.

Test a treatment regime at one field site based on applying fungicides only when needed, i.e., when inoculum is viable and present in sufficient numbers and conditions favorable for infection.

Summary of Progress: Leaves overwintered on the orchard floor in Ukiah and Potter Valley were examined for presence of various maturity classes of asci within pseudothecia. Data revealed that asci matured over a longer time than predicted by the Spotts-Cervantes model at both Ukiah and Potter Valley during 1997. Viable mature ascospores were observed in leaves up to and including the final sample from Ukiah, on 5 May, indicating the maturation curve may be even more drawn out than we have indicated. By contrast, the Spotts-Cervantes model predicted 100% of ascospores would mature by 31 March. Spore releases, observed until 30 May at Ukiah, support the maturation curve found in leaves. Just over half of all pseudothecia at both sites were found to contain no identifiable fungi, or fungi other than Venturia pirina. Asci of class one, two, and three (immature, mature, and empty) were often found co-mingled in the same pseudothecia, and appeared throughout the season. Asci initials were found only in the earliest samples, and aborted asci were only found from mid- to late-season samples.

Control of the disease will be more difficult due to the extended duration of mature ascospore availability, and the need for a model of inoculum potential is greater under these circumstances. A fungicide trial demonstrated that post infection applications with a combination of Procure 50W and Dithane can be highly effective in controlling pear scab.


Precision Agriculture: Comparison of Weed Seed and Previous Weed Populations for Prediction of Subsequent Weed Populations.

Principal Investigator: W. Thomas Lanini, Vegetable Crops, Davis

Objectives: Evaluate two sampling methods, weed seedbank (Elutriation-Germination method) and weed population sampling the previous growing season, alone or in combination, for predicting summer weed populations and their distribution within a field.

Analyze weed populations at crop harvest and weed seedbanks during winter, in terms of the species composition and spatial distribution, and to determine the most "reasonable" sample size to describe such populations at a farmer’s level of resolution.

Summary of Progress: Most herbicides are applied uniformly throughout a field, prior to weed emergence, yet it is known that weed distribution is not homogeneous. If weed species and location were known, appropriate treatments could be applied at each location. Some portions of the field could receive lower rates (soil texture change or more sensitive weed species) or no herbicide (few weeds or species not controlled by labeled herbicides). This study is examining methods for predicting weed species, density and distribution within a field, in order to apply pre-emergence herbicides precisely where they are needed. In year one, it was determined that weed seedbanks were better at predicting weed species and location than previous years escaping weeds. Correlation coefficients between seeds and seedlings, although significant for many species, were low. Evaluations of ways to improve sampling efficiency found that germinating weeds in the greenhouse for one flush was as reliable as three or four flushes, which could reduce the time needed for this step. Seedling emergence in the field was less than 10% of the estimated seedbank. Weeds were found to be clumped in the field, rather than evenly distributed. Distance between samples partially explained nightshade spatial structure, but did not explain barnyardgrass structure. This would indicate a need for more sampling to explain barnyardgrass structure within a field. Weed populations at tomato harvest in 1996 were less reliable in explaining nightshade seedlings in 1997, but better at predicting barnyardgrass. These variables will be evaluated in 1998 when tomatoes are once again grown.


Using Phenology Prediction to Improve Weed Management.

Principal Investigators: J. S. Holt, Botany and Plant Sciences, Riverside; T. S. Prather, UC IPM Project, Kearney Agricultural Center, Parlier

Objectives: Quantify phenology of common annual weeds in two different locations to identify functional weed groups defined by similar threshold temperatures and degree-day requirements. Determine the effect of crop presence on weed phenology and phenology predictions.

Test phenology predictions for weed management in field experiments and growers' fields.

Summary of Progress: Seeds of annual weeds were collected at UC Riverside and Kearney Agricultural Center for weed phenology experiments. Weeds studied at UCR include Amaranthus albus, A. blitoides, A. palmeri, Chenopodium album, Conyza canadensis, Digitaria sanguinalis, Echinochloa crus-galli, Hirschfeldia incana, Portulaca oleracea, Setaria glauca, Solanum nigrum, and Sonchus oleraceus. Weeds studied at KAC include many of the above, plus Brassica geniculata, Conyza bonariensis, Euphorbia maculata, Malva neglecta, Physalis wrightii, Raphinus sativus, and Salsola kali. Field experiments were planted in summer and fall 1997 in both locations to quantify timing of weed emergence and development. Experiments were planted in randomized complete block designs with six blocks; weed species constitute the treatments. Parallel experiments were established in fall 1997 in fields just planted to a crop to observe weed phenology in the presence of a crop canopy. Data are being collected at two to three day intervals and include times to emergence, one to ten leaves, one to ten branches or tillers, flowering, and seed set. Using previously determined emergence and development thresholds, these data will be converted to cumulative degree-days to each event for each species. In winter 1998, further experiments will test phenology predictions in experimental crop fields and in growers’ fields. Weed phenology predictions can be used to choose crop planting dates when weed germination is least likely; to improve timing of seedbed preparation, seedling management, and cultivation for weed control; and to plan post-emergence herbicide applications to control the greatest number of weeds with the least amount of herbicide.


Pheromone-Based Monitoring Systems for Major Stink Bug Pests of Agriculture in California.

Primary Investigator: J. G. Millar, Entomology, Riverside

Objectives: Identify attractant pheromones for major stink bug pests in California, particularly species which infest a number of crops.

Develop, optimize, and evaluate pheromone-based monitoring systems for pest stink bugs for field use by growers and PCA.

Transfer monitoring systems technology to the agricultural industry of California as rapidly as possible.

Summary of Progress: Stink bugs cause damage and/or vector diseases across the entire spectrum of crops, from forage (e.g., alfalfa), through vegetables (e.g., beans and tomatoes), to fruits and nuts (e.g., pears, pistachios). Identification of attractant pheromones for these bug species for use as trap baits would provide rapid, simple, sensitive, and selective methods for sampling pest stink bugs.

In the first six months, we have made excellent progress towards our objectives. We have established lab colonies of five stink bugs (Thyanta pallidovirens, Acrosternum hilare, Chlorochroa sayi, C. uhleri, C. ligata) to provide material to work with year round. We have identified the sex pheromones of Thyanta pallidovirens, and worked out syntheses of the components. Lab bioassays have shown that the attractant pheromone of Acrosternum hilare consists of a single component, the gross structure of which has been identified. However, this compound exists as one of two mirror-image forms (enantiomers), and both forms must be synthesized to determine which one the bug produces. Compounds produced specifically by sexually mature males of three Chlorochroa species, i.e., likely pheromone components, have been identified and are awaiting bioassay. Finally, minor male-produced components of the stink bug Euschistus conspersus, for which the major component of the pheromone is known, have been identified. Baits for most or all species will be available to begin field testing and optimization of blends, doses, and effective field lifetimes of baits in 1998.

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Projects that Ended in 1997-98

Development of Sampling Procedures for Western Flower Thrips, Frakeliniella occidentalis, in Greenhouse Flower Production.

Principal Investigators: M. P. Parrella, Entomology, Davis; B. C. Murphy, Entomology, Davis

Objectives: Determine the within- and between-plant distribution of western flower thrips on roses and chrysanthemums for common cultivars during the growing season.

Determine the optimal sample unit for estimating WFT abundance.

Develop and evaluate a practical, grower-oriented sampling procedure using sticky cards, plant samples, or a combination of the two for monitoring WFT for determining the need for control measures. Estimate the relationship between WFT density on plants and the degree of feeding injury on foliage and flowers.

Verify effectiveness of the sampling procedures under commercial greenhouse conditions.

Summary of Progress: During 1997 we completed all major objectives for WFT in fresh cut roses. Spatial distribution data for WTF were obtained from five commercial rose greenhouses in three regions (San Mateo, Monterey, and Santa Barbara counties) over the past two years. Data were used to quantify and assess the within-plant and between-plant distribution of WFT. We also determined a high degree of correlation existed between WFT captured on blue and yellow sticky cards and WFT density in roses demonstrating the potential for a sticky card monitoring procedure. Preliminary sample size estimates for estimating WFT abundance were generated for both a plant and a sticky card monitoring program. Sample size estimates showed development of a plant and/or sticky card monitoring program is feasible and likely cost effective. We also successfully estimated the relationship between WFT density in roses and the degree of feeding damage to foliage and flowers and developed preliminary estimates between WFT density and the proportion of the rose crop showing feeding injury. These results indicate development of a quantitative and meaningful economic and action thresholds for roses is feasible. Work is progressing to develop practical, grower-oriented sampling procedures and action guidelines in roses.

Completion of the objectives for chrysanthemums has progressed more slowly. Preliminary data on the within- and between-plant distribution indicate WFT abundance using plant samples can be estimated cost effectively in mums. Data are still being analyzed for some of this work. The degree of variation found in chrysanthemum cultivars and growing practices, in addition to interference from pesticides sprays, has required additional time and effort to obtain a sufficient database.


Review and Refinement of Decision Thresholds for Lygus Bugs on California Blackeye Beans.

Principal Investigator: P. B. Goodell, UC IPM Program, Kearney Agricultural Center, Parlier

Objectives: Evaluate the effect of a range of Lygus bug populations on yield and quality of CB46 blackeye beans.

Evaluate current decision thresholds relative to timing of infestation.

Summary of Progress: Lygus bug is a key pest in cowpeas or blackeye beans. This insect can cause damage in two ways, by feeding on flower buds it can cause a loss of potential bean pods or by feeding after the pods have formed it can reduce the quality of the beans. This project used 6’x6’x6’ cages to cover two rows of beans (CB46) before the plants flowered. These cages provided protection from Lygus bugs during the experiment while allowing a known number of insects to be introduced on plants. Just before flowering, immature Lygus bugs were collected from the beans or alfalfa. Into half the cages, 0, 20, 40, or 60 insects were placed. Just after flowering, the other half of the cages received 0, 20, 40, or 60. The insects were allowed to remain in the cages for the duration of the experiment and reproduction did occur. These insect densities represent field estimated populations of 0, 0.5, 1, and 2.5 bugs per sweep. We did find the amount of damage did increase as length of exposure to Lygus increased and as the number of Lygus in a cage increased. These differences did not translate into measurable yield differences, at least as determined by quality grade of number one beans.


Using Detection of Latent Infections of Monilinia fructicola in Stone Fruits in California to Predict Brown Rot at Harvest and Postharvest and Reduce Fungicides.

Principal Investigator: T. J. Michailides, Plant Pathology, Davis/Kearney Agricultural Center, Parlier

Objectives: Determine if incidence of latent infections of peaches and nectarines by Monilinia fructicola can be a predictor of brown rot of fruit at harvest and postharvest.

Use the predictor (findings in objective one above) to minimize the use of preharvest fungicide sprays of stone fruit for the control of brown rot at harvest and postharvest.

Use the predictor (findings in objective one above) to minimize the use of postharvest fungicide treatment of stone fruit for the control of postharvest brown rot.

Summary of Progress: California’s 100,000 hectare stone fruit industry suffers tremendous losses due to the brown rot disease in some years. If weather conditions are wet and cool in the spring this disease can attain epidemic levels killing blossoms and rotting green and mature fruit as well as fruit in storage. In previous years’ research, we obtained information that supports the hypothesis that we could be able to develop a simple method to predict fruit brown rot. Because of the expected very low levels of brown rot in 1997, fruit from 8 nectarine, 11 plum, and 9 prune orchards were sampled and processed using the freezing-incubation method that we developed in our laboratory to detect latent infections caused by Monilinia fructicola. Because of the very unfavorable disease conditions in 1997, the levels of latent infections were in most cases zero or when latent infections were detected were below 3%. As expected the levels of disease recorded in the field just before commercial harvest did not exceed 0.65% for nectarines, 0.3% for Casselman and Friar plums, or 2% for prunes, although postharvest brown rot ranged from 0.4 to 23.0% in nectarines, 0.9 to 22.7% in Casselman plums, 0.4 to 9.9% in Friar plums, and 0.1 to 9.0% in prunes. Because of the extremely low and sporadic incidence of latent infections in 1997, no significant relationships could be determined between the incidences of latent infections of green fruit and brown rot of mature fruit at harvest and/or postharvest. However, our method could, at the least, show that in years with very unfavorable conditions for disease development, stone fruit growers should not worry about brown rot and reduce preharvest fungicide sprays accordingly.

In contrast to the 1996 results, the incidence of latent infections in prunes was also very low in 1997, and we could not obtain significant relationships between latent infections and disease in the field and postharvest. Based on previous years’ history, however, we identified three orchards with expected different levels of brown rot potential and applied fungicide sprays as proposed in this proposal. The two day preharvest fungicide treatment was not applied as proposed in the original proposal because of the voluntary increase by the registrant of the fungicide iprodione (Rovral) of the preharvest interval (PHI) from zero to seven days. Results from the preharvest fungicide trials showed that indeed none of the two sprays were needed in the Fresno County orchard predicted to have low risk of brown rot (0% disease in the field) or the orchard in Glenn County expected to have medium potential for brown rot (0.2% brown rot in the field). However, in these two orchards one late spray or two sprays were necessary to reduce the incidence of postharvest brown rot. In the orchard in Butte County with the high brown rot potential, only one spray was needed to reduce significantly brown rot in the field. A second spray did not add to even more reduction of brown rot (no significant differences. In addition, with the exception of the spray applied on 3 July, one or two preharvest sprays were needed to reduce the incidence of the postharvest brown rot.

After fruit was surface sterilized with 0.01% chlorine and treated with the postharvest treatments (iprodione for brown rot or Alisan for Rhizopus rot) as proposed in the original proposal, no significant differences were detected in brown rot, and therefore for such low levels of disease none of the postharvest treatments were needed for the prunes harvested from the three studied orchards. Therefore, in 1997 the method of freezing-incubating stone fruit showed 1) that the expected potential for disease was extremely low, 2) that depending on the orchard, all the preharvest sprays were not needed, and 3) that none of the postharvest treatments were necessary.

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Final Reports for Projects that Ended in 1997

A Monitoring System for Predicting Tospovirus Epidemics by Sampling for Infective Thrips.

Principal Investigator: D.Ullman, Entomology, Davis

Objective: Develop a monitoring system that will allow prediction of tospovirus epidemics by sampling for infective thrips.

Summary of Accomplishments: Tomato spotted wilt virus (TSWV) is transmitted by thrips and causes severe crop losses in California coastal regions and worldwide. Thrips cause important direct damage and some action thresholds have been established, but the relationship between thrips numbers and TSWV incidence is unknown. This information is needed to determine action thresholds for thrips as vectors. A valid monitoring system that differentiates between non-infectious and infectious thrips, and identifies epidemiological "hot spots" within the crop, would assist growers in directing control actions and containing virus spread. Through collaborative efforts with Karen Robb, UC Cooperative Extension at San Diego County, we were able to develop a trapping system that monitored for the direction of thrips movement and location of infectious thrips. We tested this prototype at the Flower Fields of Carlsbad, approximately 40 acres of Ranunculus sold as both cut flowers and in the tuber market. This site has a history of tomato spotted wilt virus outbreaks with an incidence of over 70% in the year before our experiment. Our trapping system, which included a directional sticky trap and a petunia indicator plant at each monitoring station showed that thrips movement was from the north of the field, and that infectious thrips were also most abundant in one northerly location. This guided us to a key source of TSWV adjacent to the production area. The grower eliminated the identified source and the result was a reduction in virus incidence to record low levels (below 1%) at this site. Additional information gleaned from our sticky trap data showed that western flower thrips was not always the dominant species of thrips in the field. This demonstrated that basing pesticide decisions on total thrips/card could result in unnecessary spray treatments against nonpest thrips species. The pest control advisor received weekly reports from our sticky trap counts, and made his recommendations for pesticide spray treatments based on the numbers of vector thrips. We conclude that our monitoring system can locate the source of infectious thrips and guide management strategies to reduce virus spread within a crop and reduce unnecessary use of pesticides. This results in increased productivity and economic benefits for the grower.


Risk Assessment for Fruit Scarring by Citrus Thrips, Scirtothrips citri (Moulton), from Soil and Air Temperatures before Petal-Fall.

Principal Investigators: H. Schweizer, Entomology, Riverside; J. G. Morse, Entomology, Riverside

Objective: Develop a model for estimating the amount of fruit economically scarred by citrus thrips from temperature information accumulating prior to petal-fall which can be used for decision support regarding whether or not to apply control measures (e.g., insecticides) against citrus thrips (CT) (Scirtothrips citri (Moulton)).

Summary of Accomplishments: Using 25 years of historical fruit scarring and air temperature data from a field at the University of California, Lindcove Citrus Research Extension Center we developed regression models to estimate the amount of fruit scarring in a given year from temperature patterns before the end of petal-fall.

We evaluated the economic consequences for a grower if she/he were to use the models for predicting the level of fruit scarring in order to decide about the application of an insecticide treatment against citrus thrips. We compared this strategy to a standard of applying prophylactic treatments every season. Following model predictions resulted in fewer insecticide applications and increased the average financial return per acre. Indirect benefits from reducing insecticide applications, such as postponing insecticide resistance in citrus thrips, and reduced interference with biological control of other pests (mainly scales) are difficult to quantify. However, even without taking such effects into account, using model predictions appeared financially advantageous in the long run. The risk of losing money due to a false decision seems acceptable. In our worst case scenario the grower would have lost less than $15 per acre in one out of 13 seasons.

Our "best" model suggests that chill degree-days accumulating during early March and heat degree-days accumulating during bloom are associated with increased citrus thrips fruit scarring. We emphasize that for an optimal decision about citrus thrips treatments the above relationships (model predictions) must be supplemented with the specific experience and field sampling from the orchard in question.

We have a reasonable explanation for the association between warm weather during bloom and high fruit scarring. Warm weather stimulates the activity of adult citrus thrips resulting in the deposition of more eggs into small fruit as soon the flowers have lost their petals. The reasons for the association between chill units in early March and high citrus thrips fruit scarring are less obvious. Our favored hypothesis is that cold weather stimulates more leafless inflorescences. This will result in more small fruit right after bloom. Thus, there are more suitable fruit available for citrus thrips' egg deposition. This in turn will result in a higher total number of thrips larvae per tree which will cause accordingly more damage.


Phenology, Seasonal Host Selection, and Biology of the Silverleaf Whitefly (Bemisia argentifolii Bellows & Perring) in the San Joaquin Valley, California.

Principal Investigators: C. G. Summers, Entomology, Davis; R. L. Coviello, UC Cooperative Extension, Fresno County; M. J. Jimenez, UC Cooperative Extension, Tulare County

Objectives: Determine the seasonal phenology and population dynamics, the seasonal host plant acceptability, and the crop host selection sequence of the silverleaf whitefly on selected hosts in the San Joaquin Valley.

Determine the over wintering biology of silverleaf whitefly and the role of selected crop and noncrop hosts in over wintering survival.

Establish thresholds values for nymphal populations causing broccoli white stalk and irregular ripening of tomatoes in the San Joaquin Valley.

Summary of Accomplishments: In the San Joaquin Valley, silverleaf whitefly is distributed mainly along the east side from Kern to Merced County. Early establishment (March), from overwintering individuals, occurs on spring-planted vegetables including cucumber, squash, eggplant, and tomatoes. Melons, cotton, and late tomatoes are infested beginning in June.

Maximum populations in all crops are reached in September and October. Host crops planted for fall production are at significant risk. The host sequence study showed all susceptible crops will be colonized immediately after emergence. There was a significant difference in host acceptability: melons = cucumber > squash > pima cotton > Acala cotton > tomatoes > black-eye beans > broccoli = cauliflower > alfalfa. Growers are cautioned to avoid planting susceptible fall crops in the vicinity of any severely infested crops. Several hundred new hosts for silverleaf whitefly were discovered in the San Joaquin Valley.

Large numbers of silverleaf whitefly adults migrate to citrus during the fall and deposited eggs on citrus leaves and on numerous weeds in the understory. Citrus, and its associated weed flora, is the principal overwintering site of silverleaf whitefly in the San Joaquin Valley. Ornamental plants are also important. Silverleaf whitefly over winters primarily in the nymphal stage. Silverleaf whitefly crawlers are positively photostactic, but do not respond to gravity. Crawlers can move considerable distances, both on and between plants, in order to find a suitable leaf on which to settle.

There is a direct relationship between silverleaf whitefly nymphal density and the percentage of tomato irregular ripening in both fresh market and processing tomatoes. As few as 25 nymphs per leaf, up until four weeks before harvest, can cause 10 to 12% irregular ripening in fresh market and processing tomatoes. In both fresh market and processing tomatoes, densities of 150 to 200 nymphs per leaf, can result in irregular ripening exceeding 50% of the fruit. Soluble solids and pH are not affected by silverleaf whitefly feeding. Crop maturity, in both fresh market and processing tomatoes, as well as broccoli, is delayed by two to four weeks by whitefly densities exceeding 100 to 200 per leaf.


Integrated Management of Aphids and Aphid-Vectored Virus Diseases in Vegetable Crops.

Principal Investigators: J. J. Stapleton, UC IPM Project, Kearney Agricultural Center, Parlier; C. G. Summers, Entomology, Davis/Kearney Agricultural Center, Parlier,

Objectives: Determine and compare the effectiveness of combinations of soil mulches, reflectorized plant coatings, and the aphid alarm pheromone in delaying and/or reducing the development of aphid-borne virus diseases and subsequent crop loss in susceptible vegetable varieties.

Determine the extent to which these various methods and combinations repel aphids from plants thus delaying and/or reducing colonization and virus transmission.

Determine the effects of these treatments on earliness, health, yield, and economics of susceptible vegetable crops.

Summary of Accomplishments: A complex of aphid-vectored virus diseases has been causing increasingly heavy losses in many vegetable crops grown in the inland valleys of California. Normally, insecticidal sprays used by many growers to combat the aphids are not effective in controlling the virus diseases, since the aphids can transmit the viruses to numerous plants before succumbing to the poison. This project was conducted under the hypothesis that aphids are repelled by highly reflective, silver or gray colored soil mulches, and therefore less likely to land, feed, or transmit virus on plants growing over the mulches. More than a dozen field experiments at UC facilities and commercial farms were conducted in during 1994-1996 in crops including cantaloupe melon, cucumber, pumpkin, fresh market tomato, and zucchini squash to test the effectiveness of reflectorized, spray and polyethylene soil mulches for management of aphids and the virus diseases they transmit in the San Joaquin Valley. Effects of treatments on seasonal dynamics and damage caused by the silverleaf whitefly (Bemisia argentifolii) also were determined. The very consistent results obtained across a broad range of experimental conditions and differing crops were that aphid numbers and virus disease incidence on mulched plants were lower than on those which were nonmulched, which was critical for facilitating normal flowering, fruiting, and yield. An added benefit not anticipated at the beginning of the project was the repellency of the reflective mulches to silverleaf whitefly, giving good control of silverleaf symptoms in cucurbits.

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