Our Vision for Sustainable Pest Management – Part 4: Biological control action levels–examples from the field

by Ron Whitehurst, PCA and co-owner Rincon-Vitova Insectaries, Inc.

Pest Control Advisors (PCAs) make their decisions based on monitoring to determine an “action level” or “action threshold”. In other words, they look for signs that it is time to do something to prevent a serious pest problem. To align PCAs with the SPM goals, it is important that they understand the big difference between action levels for the conventional chemical input-based farms most of them are familiar with compared to farm systems that are either biological input-based, such as most organic acreage, or biodiversity-based. 

Treatment action levels on chemical input-based farms, of course, do not apply when chemicals are not an option. A new framework is needed for such farms. PCAs need training in determining “biological action levels”.  Entomology professor David Headrick asks his students at Cal Poly San Luis Obispo to think about two separate thresholds, one for chemicals and a different one for biological inputs. The following slide from his Biological Control class helps illustrate the need for early regular monitoring at low pest densities in order to time a natural enemy release to maintain the pest population at a low density. The timing of applications has to be carefully thought through. It is clear that the Economic Injury Level and the Chemical Control Action Threshold happen at a significantly higher pest density. 

Biological action threshold graph, Professor David Headrick, Cal Poly SLO Biological Control Course Lecture

As Dr. Headrick further explains, “Maintaining pest densities at low levels is most easily and effectively done with biological control agents. That is what they evolved to do – find prey when they’re scarce. It is also the most economically sustainable approach.”

Readers of “ACRES USA – A Voice for Ecological Agriculture” have been informed for decades about the potential for insects to find food and mates through subtle phenomena happening at low population densities. Dr. Philip S. Callahan, a regular contributor to ACRES USA, published Tuning In To Nature in 1975 describing experiments demonstrating insect behavior in response to low electromagnetic energies. He wrote, 

“A sick plant actually sends forth a beacon, carried in the infrared, attracting insects. It is then the insect’s role to dispose of this plant deemed unfit for life by nature…. Early in my career, I studied pesticides, as did all entomologists. But the findings I released…taught me that attempting to poison insects was at cross purposes to nature and would, in the end, prove futile.”

Biological control practitioners would never consider a biological action, such as releasing a few green lacewing larvae, when pest densities are high. Biological action levels must be earlier, at the first sign of a key pest in the season, when successful biological control is achievable. Consideration is also given to various cultural practices that minimize disruption of biological control. Long-range planning for habitat enhancement is another consideration.  Enhancing habitat in the long term can maintain pest levels at such low densities that monitoring does not need to be as in-depth or as frequent as field scouting shows no sign of reaching a biological action level. The focus of field scouting evolves to be more about continuing to enhance and monitor natural biological control.

Biological control entomology intersects not only with agroecology, including soil ecology, conservation biology and population dynamics, but also increasingly with molecular biology and insect-insect and plant-insect communication. Farmers and their Pest Control Advisors will need to be observant of population dynamics at the landscape scale and how insect and plant volatiles affect plant defenses and insect behavior. 

Dr. Joseph Patt with the USDA-ARS received doctorate degrees in both entomology and botany. His research on releasing parasitoid wasps for control Colorado potato beetle in eggplant led him to measure the accessibility of nectar in different potential habitat plants to make sure there was enough space in the floral architecture for the large heads of the wasps that the New Jersey State Insectary produced. Without nectar, the number of required wasps was unaffordable. By comparing 15 different plants and choosing to plant seed dill and coriander that have many flowers with open nectaries, he ensured adequate nutrition for wasp searchability and reproduction. This minimized the number of wasps that had to be mass-produced for a cost-effective program of one row of floral habitat every tenth row. Unfortunately farmers dropped the biological program when the EPA registered a new chemistry with Colorado potato beetle in eggplant on the label. Research funding in this area also disappeared. 

Diagrammatic representation in lateral view of the floral architectures on which E. puttleri and P. foveolatus were evaluated showing position of the nectar glands (in black) in relation to the other floral parts: (1) Umbels with exposed nectaries; (2) Cyanthia with exposed nectaries; (3) Umbels with partially hidden nectaries; (4) Cup- shaped flowers with partially hidden nectaries; (5) Capitula with hidden nectaries. Wasps are drawn to scale and are 3 mm long. Patt, et. al. 1997.

Many observations go into determining the presence of effective natural biological control. However, Pest Control Advisor training has been nested within a Production Agriculture curriculum, isolated from the sciences that explain population dynamics, and insect and plant physiology, biochemistry and electromagnetic communication to enhance biological control. To be aligned with SPM, the curriculum for PCAs must be equally nested within agroecology and the sciences that explain plant defenses and insect and mite behavior.

Dr. Headrick motivates his students to learn how to manage pests on regenerative organic farms by reminding them of the unsustainability of conventional chemical control. He tells them, “Chemicals are great for instant gratification, but not for long-term success in pest management.” This fact leads his students into the whole subject of pesticide resistance.

To be able to forecast whether population densities are approaching action levels, there is much to learn. Then, they have to be able to help farmers understand these concepts. Much research is needed for both areas of pedagogy. Scientists in France are developing learning models and games that teach decision-making about biological action levels. With such limited current training for PCAs, an entry level field scout requires at least two years of mentored field experience to learn basic skills to recognize action levels. Scouting in a variety of crops and farming systems is more challenging. It takes more years to be able to perceive the population dynamics and consider alternative cultural practices and cost-effective, manageable habitat enhancements and communicate with farmers to understand the options.

I enjoy those experiences when someone buys a rundown chemical farm and contacts me wanting to be organic. We start early in the fall to plan. I now know that measuring the upper and lower levels of compaction levels in the soil is critical to deciding on tillage. What characteristics are needed in a ground cover? Is good quality compost available?  Are ants likely to interfere with biological control? Where should this farm start with permanent habitat installations?

Early in the growing season there might be indications that one or more colonizations in perennial crops might help. Pest populations often stay so low that a biological action threshold is never reached and there is no need for augmentative releases or “treatments” with natural enemies. 

Here are a few examples to illustrate how biological control scouts determine a biological action level. 

Farm & location: Sanford, Santa Rosa Rd, Buellton

Size & farmscape: 12 acres between road and steep hillside, across the road from organic farm and Santa Ynez River, east-west river valley 16 miles from coast, diurnal breeze

Farming system, prior crop(s) & years in transition: at least a decade of chemical input-based lima beans, year 1 transition to organic

Crop(s) and key pest(s) & economic threshold: lima beans, two-spotted spider mite, in past would defoliate if not sprayed at least once, usually 3-4 times with conventional miticides

Cultural adjustments: none

Habitat enhancements: two interplantings ‘Beneficial Blend” with 20+ species plus weedy alfalfa, successional sweet corn, sorghum and sunflowers, perpendicular to prevailing westerly wind, 1) 30 ft from west end, 10 ft wide X 40’ long, 2) middle of 12 ac block, 10’ wide X 80’ long, 

Natural enemy colonizations: none

Monitoring method(s) and frequency: visual appearance of necrosis from spider mite damage, live mites and eggs, weekly across in 3-4 places and along perimeter 

Biological action level: monitoring showed the biological control from the interplantings protected most of the block except the south border on the east end  edges along the hill and drive road becoming infested; without natural enemy release, if there were hot, dry weather the mites could spread into the middle of the field protected by the biological control from the interplantings; mites could blow up requiring a spray to protect the whole block 

Action & result: two weekly releases of Galandromus occidentalis and green lacewing along the south border of the eastern half of the block brought the pest mites under control 

Farm & location: Dairy barn outside of Gunnison, Colorado

Size & farmscape: 10 cows, 1,000 sf open front, 3-sided, free-stall barn; manure moved daily to compost yard

Farming system, prior crop(s) & years of transition: organic cows over ten years

Crop(s), key pest(s) & economic threshold: houseflies annoy cows, reduce milk output

Cultural adjustments: more frequent clean-out, bucket trap near compost

Habitat enhancements: n/a

Natural enemy colonizations: monthly releases 10,000 fly parasites beginning at first sign of flies

Monitoring method(s) and frequency: 3X5 index “spot cards” counted weekly. Start with 4 cards and reduce to as low as 2 cards if counts are within 10%. Place one on the warm side and one on the cool side, one upwind and one downwind if there are differences.

Biological action level: average 100 spots/card, over 65’F so flies are active

Action & result: balEnce Fly Spray (beneficial fungus Beauveria bassiana) on surfaces; average spots/wk below 20.

Farm & location: Anonymous, Edna Valley, San Luis Obispo County

Size & farmscape:  Two fields separated by a seasonal creek: 40 acres and 35 acres, sandy loam soil.

Farming system, prior crop(s) & years of transition:  Standard, previously farmed as vegetables, conventional production, but no synthetic pesticides used.  I was hired to manage the crop start to finish using only biological control.  

Crop(s), key pest(s) & economic threshold:  Hemp for CBD, key pests:  western flower thrips, spidermites, noctuid caterpillars, botrytis.  CBD products are supposed to be made from plants without any pesticide residues and with as few contaminants as possible.  In this case having biological control agents on the plant surfaces at the time of harvest was deemed acceptable for the CBD extraction process.

Cultural adjustments:  Typical row crop approach, plastic mulch on beds, 40 inches on center, transplants at 2 foot spacing.  Transplants grown in a greenhouse from certified seeds.

Habitat enhancements: None.

Natural enemy colonizations:  In the greenhouse setting, the following natural enemies were released at standard rates so that they were actively foraging and reproducing on plants before they were placed in the field – a “pre-transplant inoculum”: Orius releases were made for thrips, Stratiolaelaps scimitus (Hypoaspis miles) was inoculated onto the transplant container soil surface for fungus gnats, lacewing eggs for whiteflies and small lepidoptera and Aphidoletes aphidimyza for aphids.  

In the field, subsequent releases were made based on monitoring.  Spot treatments of Neoseiulus californicus was made for spidermite control.  Bacillus thuringiensis was applied as a spray for caterpillar control.

Monitoring method(s) and frequency: Greenhouses were monitored with visual inspections, tap method and yellow sticky cards.  Monitoring was done once a week until plants reached about 8 inches tall, then twice a week until transplanted.  

Fields were monitored with visual inspection and beat sheet.  

Field monitoring was conducted once a week along rows, every 6th row but different rows each time, and always checking the first three upwind rows and two downwind rows each time.  

Biological action level:  All biological control agent releases (greenhouse and field) were made only if the target pest was present.  Thresholds were set “at first sight of pest”, with the idea that pests at low population densities are more easily controlled.  Most mite issues started on the upwind rows, predatory mites were applied as spot treatments.  N. californicus was chosen due to the hot, dry conditions and its ability to feed on prey other than T. urticae.  Bt sprays were applied to the entire field as soon as adult moths were observed being disturbed by the beat sheet monitoring methods.  Lepidopteran eggs were impossible to locate on the dense and trichome-laden foliage and flowers and waiting until feeding damage was readily observed was too late to gain control of the caterpillars.  The concern with caterpillar feeding was not so much the foliage, but the flowers.  When caterpillars fed in the dense flower clusters, they were virtually impossible to see and the feeding damage resulted in Botrytis infections.  Botrytis is a devastating fungal pest and will ruin a crop because it negatively affects the terpenoid extractions.  Closer to harvest, Bt sprays were conducted once a week as per the growers request.  

Action & result: I achieved excellent results with a 100% harvestable crop.  The greenhouse inoculation program was an effective and cost-efficient approach to having natural enemies evenly spread throughout the field and working on pest populations before full exposure to field conditions and new pest populations.  The approach of applying biological control agents on mature plants in the field can often lead to losing many of them during the process.  The Bt sprays were effective, but caterpillar control needs to be re-evaluated and diversified to avoid resistance.  Additionally, making spray applications on the dense flowers can itself lead to conditions that aid fungal growth.  Consistent, systematic, monitoring from crop onset and application of appropriate biological control agents when pest populations were extremely low was the recipe for success.  

Farm & location: Millennium Grove, Santa Paula, CA

Size & farmscape: 5 acres, landfill on long side

Farming system, prior crop(s) & years of transition: biological input-based;  sandy rocky, not organic

Crop(s), key pest(s) & economic threshold: Haas avocado (flowers Feb-May), persea mite; 8% leaf damage can cause defoliation

Cultural adjustments: 3-6 inches mulch, seaweed+high quality compost extract foliar 5X between Feb & June (flowering period), no artificial nitrogen or mineral fertigation 

Habitat enhancements: one “predator food station” every 8-12 trees, 1-2 stations/acre (with 12-20 plants of corn/acre (early, middle, late varieties planted monthly in April, May & June with Johnson grass and/or native creeping ryegrass or other grass with summer through fall flowering); maintain by watering each monitoring visit, cutting some bloom from grass patches when flowers are done to stimulate new flowering for continuous production of pollen blowing onto surrounding trees to maximize reproduction of predator mites

Natural enemy colonizations: none

Monitoring method(s) and frequency: spring & summer every other week, fall & winter monthly. Machlitt method: number of random leaves with one or more persea mite.  Number of Euseius hibisci  mites feeding on Persea mites on 50 random leaves 

Biological action level: Release N. californicus (Nc) predator mites by blower. Number depends on month, heat, humidity: 

  • April-June below 85’F, 25 leaves out of 50 w 1+ Persea, <10 Euseius: 100 Nc/tree first release
  • July-Sept below 85’F,  same levels as above: 150 Nc/tree first release
  • Forecast of Santa Ana winds (<10% humidity):same levels as above:  200-250  Nc/tree if first release
  • Forecast of heat wave over 100’F for 3+ days: no release since Persea die

Action & result: One June release of 100 Nc/tree resulted in <2% leaf damage; monitoring in August showed 15 Euseius/50 leaves; some corn and grass still producing pollen; Persea stayed below action level 

Millennium Grove. Trials of cover crops, grasses, weeds, and occasional corn hills to supply pollen to increase reproduction of predatory mites.

Farm & location: Christmas tree farm, Decatur, Illinois

Size & farmscape: 50 acres edge suburb, riparian native woodland east side, monocropped farm blocks three sides 

Farming system, prior crop(s) & years of transition: biological input-based Christmas trees for 20+ years

Crop(s), key pest(s) & economic threshold: 30 acres in Mugu and Scotch pine trees, pine needle scale, average 5 covered scales per needle on 10% of needles after pruning out the current year’s infested needles 

Cultural adjustments: Pruning infested branches

Habitat enhancements: permanent border of pine trees, mowed grass cover crop, one strip native flowering annuals per ten acres east-west

Natural enemy colonizations: none; Chilocorus lady beetles well established

Monitoring method(s) and frequency: double-sided tape around branch on warmest (south) side of tree, red nymph crawlers stuck on tape or on white paper on a clipboard when branch is hit over the paper once

Biological action level: average more than 1 nymph per tape or on white paper, release Lindorus lopanthae predatory beetles with 40/ac 1st release, 30/ac 2nd release two weeks later, and 1 or 2 more releases if crawlers continue to appear

Action & result: 2022 released total of 100 Lindorus per acre in four releases over 5 weeks during crawler emergence prevented development of noticeable armored scale

REFERENCES

Patt, Joseph, George Hamilton, James Lashomb, 1997. Foraging success of parasitoid wasps on flowers: Interplay of insect morphology, floral architecture and searching behavior  Entomologia Experimentalis et Applicata, vol 83

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