6.2 Pest Biology, Control Methods & Why Applications Fail
Key Takeaways
- The six pest management methods are biological, chemical, cultural, genetic, mechanical or physical, and regulatory control.
- Biological control uses predators, parasitoids, and pathogens, and is applied through conservation, augmentation, or classical importation.
- Regulatory control is government action such as quarantine, inspection, and eradication programs, and in New Hampshire is a distinct certification category, I.
- Growing degree-day accumulation predicts the vulnerable stage of a pest far more reliably than the calendar date.
- Common causes of application failure are misidentification, wrong timing or life stage, inadequate coverage or rate, unfavorable weather, equipment error, and resistance.
6.2 Pest Biology, Control Methods & Why Applications Fail
Pes 304.09(b)(4)c requires knowledge of "pest development and biology relevant to problem identification and control." That phrase contains the whole idea: biology tells you when a pest is vulnerable, and a pesticide applied outside that window is a legal application that accomplishes nothing.
Biology determines the treatment window
Life stages differ in susceptibility
- Eggs are usually the least susceptible stage; most insecticides do not penetrate the chorion. Horticultural oils are a notable exception, which is why dormant oil works on overwintering mite and scale eggs.
- Early instar larvae and nymphs are the most susceptible stage - small body mass, thin cuticle, and active feeding.
- Late instars are far harder to kill and have already done most of the damage.
- Pupae are essentially untreatable.
- Adults may be mobile, protected by wax or armor (armored scale), or simply not feeding.
For scale insects, the only broadly susceptible stage is the mobile crawler, a window of days. For borers, the window is the period between egg hatch and entry into the wood; once inside, a contact spray cannot reach them. For annual weeds, the window is preemergence or the seedling stage.
Degree-day models beat the calendar
Insect development is driven by accumulated heat, not by dates. Growing degree days (GDD) accumulate the daily heat above a species-specific base temperature - commonly base 50 degrees Fahrenheit - from a fixed start date. Because a cool New Hampshire spring pushes development back by two or three weeks relative to a warm one, GDD accumulation predicts egg hatch and crawler emergence far better than "the second week of June."
Plant phenological indicators serve the same purpose without a weather station: growers time treatments to the bloom stage of common landscape plants that track the same heat accumulation as the target pest.
Reproductive capacity sets the pace
Aphids reproduce parthenogenetically and viviparously - females produce live young without mating - so a population can go from undetectable to damaging in a week under good conditions. That biology is why scouting interval matters as much as scouting technique, and why a threshold crossed on Friday can be a crop failure by Monday.
The six pest management methods
+------------------------------------------------------------------+
| 1. BIOLOGICAL ... natural enemies: predators, parasitoids, |
| pathogens |
| 2. CHEMICAL ..... pesticides, including biopesticides and |
| pheromones |
| 3. CULTURAL ..... changing the practices that favor the pest |
| 4. GENETIC ...... resistant varieties; sterile insect technique |
| 5. MECHANICAL / traps, barriers, exclusion, tillage, heat, |
| PHYSICAL ..... hand removal |
| 6. REGULATORY ... quarantine, inspection, eradication programs |
+------------------------------------------------------------------+
Biological control
Natural enemies fall into three functional groups:
- Predators consume many prey over a lifetime - lady beetles, lacewing larvae, predatory mites, ground beetles, spiders, birds.
- Parasitoids develop in or on a single host and kill it - Trichogramma wasps in caterpillar eggs, Encarsia formosa in greenhouse whitefly, braconid wasps in hornworms.
- Pathogens infect and kill - Bacillus thuringiensis, Beauveria bassiana, entomopathogenic nematodes, granulosis viruses.
Three strategies deploy them:
| Strategy | What it means |
|---|---|
| Conservation | Protect the natural enemies already present - selective chemistry, refuge plantings, avoiding broad-spectrum sprays during peak activity |
| Augmentation | Release purchased natural enemies to boost existing populations, common in greenhouses |
| Classical (importation) | Import and establish a natural enemy from the pest's native range, a government program rather than a grower activity |
Cultural control
Changing the growing practice so the environment favors the crop rather than the pest: crop rotation to break host-specific soil pests, sanitation (removing culls, pruning out cankers, eliminating standing water), planting and harvest date adjustment, irrigation timing and method to shorten leaf wetness, plant spacing and pruning for air movement, and fertility management, since excess nitrogen produces the succulent growth that aphids and many pathogens prefer.
Genetic control
Resistant varieties are the cheapest control there is - scab-resistant apple cultivars, rust-resistant turfgrasses, nematode-resistant rootstocks. The sterile insect technique releases sterilized males to reduce reproductive success in the wild population.
Mechanical and physical control
Exclusion (row covers, insect netting, screening, rodent-proofing), barriers (tree guards, sticky bands), trapping (mass trapping, mating disruption with pheromones), tillage to bury pupae and uproot seedlings, and thermal control - soil solarization, steam sterilization of media, heat treatment of stored products.
Regulatory control
Government action to prevent the entry or spread of a pest: quarantines, inspection of nursery stock and firewood, certification programs, and eradication campaigns. In New Hampshire this is important enough to have its own certification category - Category I, Regulatory Pest Control - limited to government employees controlling invasive species as defined in RSA 430:52, VII, or pests on the USDA APHIS Regulated Pest List.
Suppression, prevention, eradication
Three possible goals, and picking the wrong one wastes money:
- Prevention - keeping a pest from becoming a problem where it is known to be a recurring threat. Appropriate when the pest is predictable and the crop is high value.
- Suppression - reducing a pest population to an acceptable level. The normal goal in most outdoor settings.
- Eradication - destroying an entire pest population. Realistic in enclosed environments (a greenhouse, a food facility, a dwelling) and in public health and regulatory programs, but rarely achievable and rarely necessary outdoors.
Why pesticide applications fail
When a treatment does not work, resistance is the last explanation to reach for, not the first. Work through this list in order:
- Misidentification. The product was effective against a pest that was not there.
- Wrong timing or life stage. Applied to eggs, pupae, or late instars; applied after the borer entered the wood; applied postemergence to an established perennial that needed a fall systemic.
- Inadequate coverage. Too little carrier volume, wrong nozzle, wrong pressure, insufficient canopy penetration, undersides of leaves untreated where mites and whiteflies live.
- Wrong rate. An under-calibrated sprayer delivering less than the label rate is both an efficacy failure and a resistance driver.
- Weather. Rain before the rainfast interval, temperature outside the product's effective range, a heavy dew diluting the deposit, wind carrying the spray off target.
- Product selection. A contact material chosen for a pest sheltered inside a rolled leaf, a gall, or a burrow; a stomach poison chosen for a sap feeder.
- Equipment problems. Worn nozzles past the 10 percent replacement threshold, a plugged screen, a failed agitator leaving product settled in the tank, incompatible tank mix.
- Resistance. Genuine, and confirmed when a previously effective product fails at label rate, with correct timing and coverage, repeatedly, against a population with a history of exposure to that mode of action.
Worked example
A grower sprays an insecticide for a wood-boring beetle in mid-August after seeing fresh exit holes, and sees no improvement the following season.
- Exit holes are evidence the adults have already left. By August the damaging generation has completed development inside the wood.
- The treatable window was the period between egg hatch and larval entry, predicted by degree-day accumulation in early summer, not by the appearance of exit holes.
- A contact insecticide cannot reach a larva inside xylem; the options are a preventive bark spray timed to egg hatch or a systemic applied early enough to be translocated.
- Before repeating anything, the grower should confirm the species, because the emergence timing and the registered options differ.
- Cultural and mechanical measures - removing and destroying infested wood, maintaining tree vigor so trees resist attack, avoiding movement of infested firewood, which is also a regulatory matter in New Hampshire - are likely to accomplish more than any repeat application.
Which of the following is an example of regulatory pest control?
Why do growing degree-day models predict insect treatment timing better than calendar dates?
A fungicide that worked well for several seasons no longer controls a disease. What should the applicator evaluate first?