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Annual Reports

1995UC IPM Competitive Grants Program

Applied Field Ecology

Research in the area of applied field ecology focuses on the interactions among pests, their hosts, their biocontrol agents, the biotic micro-flora, and environmental factors that affect pest population dynamics, survival, and crop damage. The emphasis is on applied ecology with attention given to the understanding of how pest-host interactions and biocontrol agents are affected by both abiotic and biotic factors. Studies might determine the environmental factors that affect the ability of the biocontrol agent to effectively suppress pest populations or develop a better understanding of the mechanisms by which the biocontrol agent suppresses pests. Laboratory studies are expected to be closely related to field experimentation.

Because of the general nature of field ecology, it is expected that projects here would include components found in other research categories. For example, studies on the interactions among organisms would involve the development and use of monitoring techniques. Possible research areas might include studying dynamics of pest populations or natural enemy and antagonist populations, development or improvement of optimal cropping system design, host-pest-environment interaction studies, or research on the mechanisms affecting interactions between organisms. Highest priority will be given to field-oriented research that demonstrates a high potential to lead to the control of pests or a reduction in pesticide use.


New Projects Funded for 1995-96
Continuing Projects Funded for 1995-96
Projects that Ended in 1994-95
Final Reports for Projects that Ended in 1994

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New Projects Funded for 1995-96


Monitoring Mites on Hens

Off-host ecology and sampling of northern fowl mites in poultry systems. (Year 1 of 2; $17,025)

Principal Investigators: B. A. Mullens, Entomology, Riverside; N. C. Hinkle, Entomology, Riverside

Objectives:
Document the practicality of a previously developed sequential direct bird examination sampling plan for northern fowl mites on caged laying hens.
Correlate estimated mite numbers with actual mite numbers on hens.

Determine the relationship between estimated on-host numbers of mites and numbers of mites appearing on the eggs.

Examine off-host distribution (eggs, cages, manure surface) of northern fowl mites relative to time of year, time of day, temperature, and estimated level of infestation of hens.


Why Don't Insecticides Always Work?

Agronomic and environmental factors influencing control of cotton aphids with insecticides. (Year 1 of 3; $16,054)

Principal Investigator: L. D. Godfrey, Entomology, Davis

Objectives:
Examine effect of cotton aphid morph on the susceptibility to commonly-used insecticides in cotton IPM.
Examine effects of several agronomic factors in cotton production, such as plant nitrogen level, plant water status, plant age and boll load, on cotton aphid susceptibility to commonly-used insecticides in cotton IPM.
Study susceptibility/response of cotton aphids reared on various host plants to commonly-used insecticides.

Habitat Management for Pierce's Disease

Management of riparian woodlands for control of Pierce's disease in coastal California. (Year 1 of 2; $20,284)

Principal Investigators: A. H. Purcell, Environmental Science, Policy and Management, Berkeley; J. R. McBride, Environmental Science, Policy and Management, Berkeley

Objectives:
Test the effectiveness of manipulating the structure and composition of riparian woodlands to reduce populations of Pierce's disease vectors.
Manipulate the structure and composition of riparian woodlands bordering typical intermittent-flow coastal streams near commercial vineyards, by selective plant removal and control and by planting of tree and shrub species, to reduce breeding of the blue-green sharpshooter, its infectivity with the bacterium Xylella fastidiosa, and its dispersal to adjacent vineyards.
Assess through monitoring the effects of riparian vegetation management on population density, dispersal and infectivity with Xylella fastidiosa of the blue-green sharpshooter vector of Pierce's disease.


Predicting Weed Problems

Phenology predictions of common annual weeds in California. (Year 1 of 2; $38,448)

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

Objectives:
Quantify phenology, including emergence, flowering, and seed production of common annual weeds planted in two different field locations.
Determine the effect of planting date on phenology of these weeds.
Correlate phenological data from two locations and four planting dates with air and soil temperature to evaluate the utility of degree-days to predict phenology of these weeds.


Sheep for Weed Control

The manipulation of sheep grazing pressure for weed control in seedling alfalfa. (Year 1 of 2; $15,720)

Principal Investigators: C. E. Bell, UC Cooperative Extension, Imperial County; J. N. Guerrero, UC Cooperative Extension, Imperial County

Objectives:
To compare different sheep grazing pressures in seedling alfalfa for their effect on weed control, crop yield, crop cover, lamb weight gain, and soil compaction.


Ground Covers: Help or Hindrance for True Bugs?

Manipulation of orchard-floor vegetation for control of hemipteran pests of pistachio. (Year 1 of 2; $23,619)

Principal Investigators: W. J. Bentley, UC IPM Project, Kearney Agricultural Center; P. G. da Silva, Laboratory of Biological Control, Berkeley; K. M. Daane, Laboratory of Biological Control, Berkeley

Objectives:
Determine whether species composition and population densities of the major hemipteran pests are influenced by the presence or absence of vegetative ground cover in San Joaquin Valley pistachio orchards.
Determine whether hemipteran-caused crop damage is influenced by the presence or absence of vegetative ground cover in San Joaquin Valley pistachio orchards.


Ant Repellents

Developing an IPM-compatible technology for using semiochemicals and related chemicals to disrupt foraging of ants. (Year 1 of 2; $29,544)

Principal Investigator: H. S. Shorey, Entomology, Davis

Objectives:
Using two quantitative laboratory bioassays that discriminate between chemoreception by olfaction alone vs. olfaction and gustation operating together, determine the relative biological activity of candidate repellents to three ant species as it relates to chemical structure, duration of activity, and mode of chemoreception.
Evaluate a number of different schemes for formulating the most effective ant repellents, so as to arrive at one or more long-duration delivery systems that are appropriate for banding perennial crops.
Determine for a variety of perennial crops the long-term phytotoxic effect of candidate ant repellents and formulating materials.

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Continuing Projects Funded for 1995-96

Controlling Crabgrass with Less Herbicide

Crabgrass management in turfgrass: Understanding invasion and improving control with reduced herbicide use. (Year 3 of 3; $23,632)

Principal Investigator: C. L. Elmore, Vegetable Crops, Davis; A. Harivandi, UC Cooperative Extension, Alameda County; M. Rejmanek, Evolution and Ecology, Davis

Objectives:
Evaluate the utility of using tall fescue cultivars and timing and rate of overseeding of tall fescue for crabgrass control.
Determine the relationship of gap size and the ability of smooth and hairy crabgrass to invade tall fescue turfgrass.
Use selected tall fescue cultivars to reduce preemergence herbicides.
Develop overseeding of cultivar methods for crabgrass control in tall fescue turfgrass.

Summary of Progress: Smooth and large crabgrass are the most prevalent summer annual grass weeds found in turfgrass in California. Commonly preemergence herbicides are used annually in the late winter or early spring for control. Fall planted turf-type tall fescue cultivars have inhibited smooth and large crabgrass from establishing the following spring and summer. In spring plantings of tall fescue, smooth crabgrass is more competitive than large crabgrass and will establish, irrespective of the tall fescue cultivar. Preliminary data from this year have shown that an opening (gap) in turf is needed for crabgrass to establish. When 4 to 8 inch openings are made in the turf leaving bare soil, smooth or large crabgrass can germinate and establish if seed is present. When the turf is scalped severely once, or clipped repeatedly to 3/4 inch, the regrowth of tall fescue is rapid enough that little or no crabgrass will establish. A few smooth crabgrass will establish in large closely clipped areas. Large crabgrass did not establish until there was 4 inches or more of bare soil. By understanding the gap or hole size for necessary the two crabgrass species to establish, it is possible to gauge when bare spots should be overseeded. Fall seeding of tall fescue is preferable to spring seeding to give the greatest competitive edge to the turf over crabgrass.


Spiders and Grape IPM

Effects of spiders on pest populations in vineyards: Determining their role in grape IPM. (Year 2 of 2; $23,381)

Principal Investigator: K. M. Daane, Laboratory of Biological Control, Berkeley/Kearney Agricultural Center, Parlier; M. J. Costello, Kearney Agricultural Center, Parlier

Objectives:
Determine spider species composition and relative abundance, and evaluate and refine a spider sampling procedure.
Evaluate the role of the more common vineyard spider species on leafhopper densities through laboratory and field feeding studies.
Determine the effect of cover crops on the spider species composition and relative abundance in the vines.

Summary of Progress: Many grape growers attributed reduced pest numbers in grape vineyards to high spider densities; however, there was little hard evidence on the spiders' population levels.

To better understand the role of common spiders, we have refined a sampling method which has enabled us to better estimate spider species composition and density in the grape vineyard. Three spider sampling methods were evaluated: 1) beat and shake onto a drop cloth, 2) beat and shake into a cloth funnel suspended from the trellis wire and, 3) D-vac (insect vacuum). All methods were compared to a control in which we stripped off and searched the foliage of entire vines. The D-vac altered the proportion of spider species sampled, collecting a significantly higher percentage of Sassacus vitis (family Salticidae), Oxyopes spp. (family Oxyopidae), and Linyphiidae, than the control, indicating a sample bias by this method towards day-active spiders, especially those which are hunters. Spider species composition was not altered by either the beat or funnel methods. The funnel method has the advantage of speed without a significant decrease in accuracy.

Both laboratory and field studies on spider feeding behaviors have begun to identify spider species which show promise as effective biological control agents. The laboratory studies are developing life history parameters on three common spider species: Trachelas pacificus, Cheiracanthium inclusum, and Theridion dilutum. For example, this past year we found that Theridion dilutum ate, on average, ate fewer leafhopper nymphs per day than did either T. pacificus or C. inclusum, indicating that spider species composition may be important for leafhopper control.

We are also using serological analysis of spider gut contents to determine what spiders are feeding on in the field. Great progress was made in finding a serum of antibodies which will react with leafhoppers but will not cross react with other prey which are commonly found in the grapevines. For example, we developed a polyclonal serum which reacted with leafhopper extract, but not with aphid or fly extract. After further tests for cross reactivity, we hope that these antisera can be used to detect the presence of leafhoppers in spider stomachs.

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Projects that Ended in 1994-95


Economic Thresholds for Barnyardgrass

The influence of barnyardgrass interference and spatial distribution on direct-seeded tomatoes: An economic threshold.

Principal Investigators: W. Akey, Botany, Davis; C. Elmore, Vegetable Crops, Davis; R. Norris, Vegetable Crops, Davis; M. Rejmanek, Evolution and Ecology, Davis

Objectives:
Determine the influence of spatial distribution of barnyardgrass on its interference with growth, development and yield of direct-seeded tomatoes.
Determine the impact of direct-seeded tomatoes on barnyardgrass growth and reproduction.
Establish a single-season economic threshold level for barnyardgrass in direct-seeded tomatoes.

Summary of Progress: Most predictions of crop yield loss due to weeds are based on a regular or uniform spacing of weeds. When weed distribution is patchy, as in most crop fields, overestimation of yield loss may occur and predicted economic weed threshold densities may be too low. We examined the influence of regular, random and clumped distributions of barnyardgrass on its interference with direct-seeded tomatoes. No comparable studies with these or other species have been reported in the literature to date.

At a commercial planting density, tomato yield losses in 1994 ranged from 8 to 50% for the clumped arrangement and from 11% to 75% for the regular and random arrangements, respectively, for equivalent barnyardgrass densities. Yield loss differences were related to changes in the proportion of weed-weed and weed-crop competition associated with the different spatial distributions of barnyardgrass. Competition of barnyardgrass with itself increased 14 and 49%, respectively, as the spatial arrangement was changed from regular to random or from regular to clumped. In contrast, the intensity of competition experienced by tomato from barnyardgrass decreased by 8 and 78%, respectively, for the same changes in barnyardgrass aggregation. Changes in the amount of shading of the tomato canopy by barnyardgrass contributed to yield loss differences for the various spatial arrangements. Clumped barnyardgrass caused significantly less shading than barnyardgrass in regular or random arrangements.

Compared to the regular and random arrangements, barnyardgrass growth was substantially reduced in the clumped spatial arrangement, primarily due to increased competition of barnyardgrass with itself. Competition from tomato suppressed seed production of barnyardgrass in 1993. Averaged over all spatial arrangements, seed production was reduced about 60, 70 and 80% by tomato densities of 5, 10 and 20 plants/m, respectively. With less competition from tomato in 1994, suppression of barnyardgrass seed production is expected be similarly reduced.

According to yield loss/weed density relationships developed in this study, predicted economic threshold densities of barnyardgrass for a commercial planting density of tomato (10 plants/m) would then be 104, 130, and 186 plants/acre, respectively, for regular, random and clumped spatial distributions of barnyardgrass. Based on these results, a producer might accept a somewhat larger average density of barnyardgrass before deciding to implement weed control if the barnyardgrass were growing in patches as opposed to being more evenly distributed over the field. However, seed production from even the lowest density of barnyardgrass, which exceeded 100 million per acre and at which spatial arrangement was not critical, necessitates that long-term thresholds for this weed be less than one plant per acre.


New Approaches to Olive Knot Control

Integrated control of the olive knot disease.

Principal Investigator: M. N. Schroth, Environmental Science, Policy and Management, Berkeley

Objectives:
Determine if propagation wood carries olive knot bacteria and is the source of the inoculum that causes widespread disease in young olive orchards.
Confirm the newly found systemic nature of olive knot bacteria and monitor the "population swings" throughout the year to determine when pruning can be done safely without leading to new infections.
Enhance the efficacy of copper compounds to control the olive knot disease by using an iron supplement which alters the physiology of the bacterium and special adjuvants which affect the permeability of bacterial membranes and enhance chemical penetration of sites harboring the infective propagules.

Summary of Progress: The hypothesis that olive wood used for propagation purposes can internally carry olive knot bacteria was proven true. This has to be a principal reason why young, previously healthy olive orchards rapidly become severely diseased, especially following a hail storm or other environmental condition that favors the disease. Similarly, the hypothesis that olive knot bacteria is systemic and that the population size fluctuates throughout the year was found to be correct. In December, 1994, the population sizes of knot bacteria in healthy limbs were estimated to be well over one million per 0.5 gram of tissue. The bacteria usually cannot be detected in healthy limbs of olive trees between the months of May and November. However, during these warm months, it also can be difficult to isolate the bacteria even from diseased knots. In December, a relatively low population of knot bacteria was found on leaves, suggesting a distribution from diseased knots. However, the orchards were treated with a copper compound. A new method of extracting bacteria from internal limb tissues (vacuum infiltration) was superior to other methods for detecting bacteria in these tissues. No additional work was done to enhance the efficacy of copper compounds in controlling olive knot by adding ferric chloride with special adjuvants, because the new material was tested in two different areas the previous year and a 100 % improvement was obtained. Pruning must be done during the hot summer months to avoid infection of wounds.


Controlling Ants with Semiochemicals

Developing an IPM-compatible technology for using semiochemicals and related chemicals to disrupt foraging of ants.

Principal Investigator: H. H. Shorey, Entomology, Davis

Objectives:
Develop laboratory bioassay procedures for repellents against three ant species, enabling quantitative, comparative indexing of the strength of repellency (or inhibition of forward movement), as it is mediated through either the olfactory or the gustatory sense.
Using the laboratory bioassays, determine the relative biological activity of candidate repellents, as it relates to chemical structure, duration of activity, and mode of chemoreception.

Summary of Progress: Two different laboratory bioassays were developed to quantitatively determine the repellent activity of semiochemicals, such as farnesol and methyl eugenol, against foraging workers of the Argentine ant, the native fire ant, and the native gray ant. One assay was based on the ability of ants to cross barriers of beeswax containing differing concentrations of test chemicals; this assay was not designed to discriminate between olfactory and gustatory modes of chemical detection. The other assay was based on the ability of the ants to detect airborne chemicals and, therefore, only measured the olfactory responses of the ants. Both assays permitted construction of dosage/response curves and determination of a BI-50 (that amount of chemical providing 50% inhibition of foraging). The utility of the assays in discriminating between inhibitory activities of a number of candidate repellent chemicals was demonstrated. Of particular interest is the use of the olfactory assay to discriminate between the ant-repellent activities of extracts of plant parts taken from a number of commonly found plant species. These assays may provide a rapid means to find more effective, naturally occurring ant repellents for banding the stems of perennial plants.


Cover Crops & Biological Control

Improving biological control of San Jose scale using flowering cover crops.

Principal Investigators: G. E. Heimpel, Entomology, Davis; J. A. Rosenheim, Entomology, Davis

Objectives:
Determine whether the planting of various flowering cover crops can improve naturally-occurring biological control of San Jose scale by the parasitoid Aphytis aonidiae.
Determine whether A. aonidiae feeds upon the nectar of dill.
Quantify the contribution of various nectars to the longevity of A. aonidiae in the laboratory.

Summary of Progress: We set out to test a number of hypotheses that are crucial to determining whether or not flowering cover crops can lead to improved biological control of pests. In particular, we were interested whether plantings of dill within an almond orchard could improve biological control of San Jose scale, Quadraspidiotus perniciosus by its dominant natural enemy in northern California, the parasitoid Aphytis aonidiae. We hypothesized Aphytis may increase their lifespan by feeding on dill nectar, resulting in higher parasitism rates and lower San Jose scale densities. We established dill plantings in an organically-managed almond orchard in northern California, and compared adult parasitoid abundance, parasitism rates, and scale densities in these plots with plots left to resident vegetation. No differences were found in any of these measures of parasitoid activity between the dill and control plots. Thus, dill plantings did not improve biological control of San Jose scale in our experiment. This occurred despite the fact that dill nectar can increase the lifespan of Aphytis.

The failure of dill to improve biological control of San Jose scale can probably be attributed to a lack of movement of wasps from the almond trees to the flowers. Although a number of other parasitoid species were observed feeding on dill nectar in the field, including wasps as tiny as Aphytis (ca. 1 mm long), no Aphytis were seen on dill plantings. Also, only a single Aphytis was recovered by shake sampling into alcohol-coated pans, and no differences in sticky-trap catches of Aphytis were detected between dill and control plots. It seems therefore that Aphytis did not forage for dill nectar.

We conclude that, for some parasitoid species that are potentially food-limited, ways may need to be developed to deliver food into the tree canopy itself.

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Final Reports for Projects that Ended in 1994

Impact of Lacewing Releases Disappointing

Evaluation of augmentative release of Chrysoperla carnea (Stephens) for control of variegated grape leafhopper Erythroneura variabilis Beamer.

Principal Investigators: K. M. Daane, Laboratory of Biological Control, Berkeley; K. S. Hagen, Laboratory of Biological Control, Berkeley

Summary of Accomplishments: The mass release of commercially reared green lacewings has been used by grape growers to improve biological control of the variegated grape leafhopper. Our laboratory studies found that lacewing larvae are good leafhopper predators, with a maximum consumption rate of over 180 fifth stage leafhoppers. However, results from field studies suggest that lacewing predation in the vineyard is much reduced.

Between 1991-1993, lacewings were released into small plots and large on-farm trials and the number of leafhoppers monitored. Release rates used in small plot trials were between 2,500 to 100,000 lacewings per acre per brood (commercial releases are typically between 2,000 - 10,000 per acre at an average cost of $3 per 1,000 lacewing eggs). In plots that received lacewings at rates equivalent to >12,000 per acre per brood, leafhopper densities were reduced by up to 40%. At lower release rates there were no consistent differences between release and control plots. In on-farm studies, 8 of 18 trials had significantly fewer leafhoppers in release plots; however, the average leafhopper reduction was never greater than 50% and was typically less than 20%, as compared to control plots. More importantly, the reduction in the number of leafhoppers plateaus at only 7-8 leafhoppers per leaf. Apparently, leafhopper density influences the percent reduction. At low leafhopper densities (<5 per leaf) there is very little effective control, at moderate leafhopper densities (10-20 per leaf) the percent reduction is greatest, and as leafhopper densities increase to economically damaging levels (>20 per leaf) the percent reduction becomes progressively lower.

One reason for reduced field performance of released lacewings was the release process used, which led to egg mortality and poor dispersal of eggs. In one study, there was a significant 35.5% mortality of eggs delivered to the vines. Another study using fresh lacewing eggs found only 20% egg hatch over a 4 day development period, indicating that field egg hatch may be even lower because of desiccation. Release timing is also important to maximize effectiveness. Currently, improved methods of lacewing egg delivery are being developed by university and insectary personnel.


Bloom Fungicides Not Effective for Some Bunch Rots

Role of bloom time sprays for control of bunch rot of grapes.

Principal Investigators: J. J. Marois, Plant Pathology, Davis; J. J. Stapleton, UC IPM Project, Kearney Agricultural Center

Summary of Accomplishments: Fungicide sprays at bloom time have been recommended for bunch rot prevention in grape growing areas of California for many years. This project confirmed the biological benefit of bloom sprays in reducing bunch rot at harvest in cooler and more humid areas where Botrytis rot is prevalent, but not in warmer and more arid climates of the central and southern San Joaquin Valley where rots caused by other fungi prevail.

Although only one bloom spray is normally applied, thousands of acres are treated annually. The most common fungicides used have been iprodione, captan, and benlate. Five field experiments were conducted on rot-susceptible 'Zinfandel' in commercial vineyards in the central and northern San Joaquin Valley in 1993-94. Applications of iprodione at 10%, 50%, and 100% bloom and preclose stages of grape development were evaluated for control of components of the summer bunch rot complex. Botrytis bunch rot predominated in the cooler, more humid delta climate of Sacramento County, while rot caused by Aspergillus niger and sour rot were more prevalent in the experiments further south in Stanislaus and Madera Counties. The iprodione bloom sprays were effective in reducing the incidence of Botrytis bunch rot in all experiments. However, they did not have a consistent beneficial effect on sour rot, rots caused by A. niger or Penicillium glabrum, or on total rot. In some experiments, rot caused by A. niger increased after fungicide application.

The data obtained in 1993-94 indicate that bloom sprays can be helpful in cooler grape growing areas of the central valley where Botrytis bunch rot is prevalent, but are of less value in warmer, more arid areas where rots caused by other organisms predominate.


Mulch vs. Herbicide for Weed Suppression

Cultural management of vine row weeds in North Coast vineyards.

Principal Investigators: L. Varela, UC IPM Project, Santa Rosa; C. Elmore, Vegetable Crops, UC Davis; K. Klonsky, Agricultural Economics, UC Davis; W. A. Williams, Agronomy, UC Davis

Summary of Accomplishments: Conventional methods of managing weeds in the vine row rely on herbicide applications and mechanical cultivation. A project was initiated in the fall of 1991 at two Sonoma County vineyards to compare these conventional systems with an alternative which utilizes the mulch from mowed covercrops (oats, vetch or an oat/vetch mix) in the middles. A reseeding subclover was established in the vine row as a subplot in 1991.

Weed suppression under each system depended on the year and location. Among the covercrops at each location, suppression was reflected in the amount of mulch biomass produced. At the high soil fertility site, the oat covercrops produced the highest biomass (8000 lbs./ A) and weed control of the covercrops and was not different from the herbicide plots. The vetch and the control (resident vegetation) consistently produced the least mulch (about 3500 lbs./A) and poorest control. At the low fertility site, all annual vegetation, whether sowed or resident, did poorly (typically <1500 lbs./A) and the level of weed suppression depended more on the presence of a living competitor, the subclover, than a smothering mulch. Weed cover was consistently 60% higher when subclover was absent.

Species numbers, indicative of diversity, increased in all covercrops and decreased in the herbicide plots. Though the increases were not considered threatening, western flower thrips were significantly higher in vetch just before and after mulching at both locations. There were no significant differences between the treatments for any of the other key pests assessed.

Grape yields increased through the years at the high fertility site and were highest in the oat plots and lowest in the herbicide plots, which reflects the mulching rate and suggests that the mulch is providing more to the vine than just weed suppression. Yields at the low fertility site tended to be less in the presence of subclover, suggesting that in a nutrient-poor site, anything growing in the vine row will detract from the vines.

The differences in profit between the covercrop systems and the conventional systems depended on site and year.


When is Cotton Aphid a Pest?

Developing an integrated management program for the cotton aphid, Aphis gossypii.

Principal Investigators: J. A. Rosenheim, Entomology, Davis; E. E. Grafton-Cardwell, Entomology, Riverside; T. F. Leigh, Entomology, Davis

Summary of Accomplishments: In the last decade, the cotton aphid has emerged from minor pest status to assume major importance as a pest of California cotton. Our research was aimed at developing a basic understanding of the ecology of this aphid upon which a management program could be built. Our primary results are as follows. (1) Early-season aphid populations which develop on cotton seedlings pre-squaring are generally not damaging. In most areas of the San Joaquin Valley, early-season aphid populations are under strong natural biological control, and outbreaks are uncommon and transient. The key natural enemies (a parasitoid and two predatory beetles) act in a complementary fashion. Furthermore, cotton can compensate fully for early-season damage; there is no effect of early-season aphid feeding on the timing of crop maturation, yield, or fiber quality. Thus, early-season insecticide use directed against the cotton aphid is discouraged, as it may disrupt the natural suppression of other potential pests and accelerate resistance evolution. (2) Mid- and late-season aphid populations are real pests. Even low populations of aphids (5-15 aphids per leaf) present during the late-season (when bolls are open and lint is exposed to honeydew excretions of aphids) can cause "sticky cotton." (3) The cotton aphid is highly variable in appearance. Larger, darker forms (which are produced when environmental conditions are more nearly optimal for aphids -- at lower temperatures, spring and fall day-lengths, and on highly-fertilized plants) are highly reproductive, and are especially capable of producing rapid population increases. (4) Natural biological control during the mid- and late-season is often ineffective, in large part due to one group of predators (generalist hemipteran bugs) consuming another group of predators (lacewings) that would otherwise appear to be effective aphid control agents. Lacewing populations are naturally abundant in San Joaquin Valley cotton fields, making meaningful augmentative releas es of commercially-reared lacewings prohibitively expensive. Even lacewing releases conducted early during the growing season are unlikely to be effective, because predation on lacewings in cotton is too intense to allow lacewing populations to grow.


Predicting Black Root Rot

The development of black root rot on cotton as influenced by inoculum density of Thielaviopsis basicola and soil chemistry.

Principal Investigators: J. G. Hancock, Plant Pathology, Berkeley; B. A. Holtz, Cooperative Extension, Madera; B. A. Roberts, Kings County; R. M. Davis, Plant Pathology, Davis

Summary of Accomplishments: The primary goal of this project was to more accurately predict the disease potential of black root rot in any given field. Growers could use this information to decide whether to implement control practices such as summer flooding or crop rotation. This project was a continuation of a previous IPM project which developed a selective medium for Thielaviopsis basicola, the causal agent of black root rot of cotton. With this medium we were able to quantify pathogen populations and derive an inoculum density / disease severity relationship for T. basicola in the San Joaquin Valley. By understanding this relationship we were able to more accurately predict disease potential and evaluate potential controls based on inoculum densities of T. basicola. In this project we also examined the effects of summer flooding on populations of T. basicola. Summer flooding of cotton fields has become a fairly common practice for farmers in the Tulare Lake Basin area of the San Joaquin Valley. As many as 40,000 acres have been flooded to reduce black root rot . Because the soil in this area is finely textured and does not drain easily, it can be flooded for several weeks without using excessive amounts of water.

In an extensive three year survey of Kings County, T. basicola was found to be widely distributed, disease severity was positively correlated with inoculum density, and significant black root rot developed in most fields only when populations of T. basicola were greater than 50 colony forming units per gram of soil (cfu/g). Inoculum densities greater than 50 cfu/g of soil may be considered optimal for disease development in the San Joaquin Valley. There was a positive correlation between inoculum densities of T. basicola and the number of years in continuous cotton. Populations were lower in fields where crop rotation or summer flooding had been practiced. Disease severity was less in fields which were flooded, and significantly more bales of cotton per acre were produced from fields which were flooded when compared to fields which were not. Populations of T. basicola increased in the presence of cotton and decreased in soil planted with non-hosts or fallow.


Ground Mealybug Studied

Population dynamics and damage potential of ground mealybugs in alfalfa.

Principal Investigators: L. D. Godfrey, Entomology, Davis; C. Pickel, UC IPM Project, Sacramento Valley

Summary of Accomplishments: The ground mealybug (Rhizoecus kondonis Kuwana) is an emerging pest of alfalfa, prunes, and probably other crops in the Sacramento Valley. This insect spends its entire life cycle in the soil; in alfalfa, feeding by this insect results in reductions of yield and plant density.

A Yuba County alfalfa field showing areas of stunted, chlorotic plants was sampled every two to four weeks from July 1992 to June 1994 for ground mealybugs. Ground mealybugs were found to have three generations per year with adult population peaks in July, January, and April. The highest levels recorded during this study were 39.7 mealybugs per 75 in3 soil core. There was a trend for more mealybugs at a 6 to 12 and 12 to 18 inch soil depth than at 0 to 6 inches in the soil. Soil moisture levels less than 13% appear to inhibit populations, but some individuals were found in extremely dry soil (1.7% moisture).

Ten alfalfa varieties, four with root knot nematode resistance and six with no resistance, were evaluated in a greenhouse study for susceptibility to ground mealybug. Nematode resistance did not appear to influence survival by ground mealybugs. Greenhouse studies, over a 4-month period, showed no effects of ground mealybug infestation on alfalfa growth. Interactions with other pests or with unfavorable environmental conditions probably account for the stress to alfalfa from ground mealybugs in the field.

Survival of ground mealybugs were evaluated on 11 crop plants in a greenhouse study: potato, tomato, safflower, dryland rice, alfalfa, cantaloupe, sugar beets, cotton, wheat, dry beans, and field corn. Some survival occurred on all plants, but there were differences among these hosts.

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