Effective Ways to Control Caterpillars in Alfalfa Fields

Alfalfa is an important forage crop grown widely for feeding livestock. However, various insect pests can damage alfalfa stands and reduce yields. Caterpillars are a major pest problem in many alfalfa producing regions. Severe caterpillar infestations defoliate plants, lower hay quality and cause significant economic losses. This article discusses different methods for effective caterpillar management in alfalfa fields.

Identifying Caterpillars in Alfalfa

The first step is accurate pest identification. Look for these signs of caterpillar activity

• Visible caterpillars on foliage – Look for green, brown or black worms feeding on leaves
• Chewed or skeletonized leaves – Caterpillars eat leaf tissue between veins
• Frass deposits – Greenish-black excrement pellets on leaves
• Webbed nests – Tent caterpillars build silken shelters in forks of stems

Common alfalfa caterpillars include the alfalfa looper, armyworm, cutworms, webworms and alfalfa caterpillar. Know your pest to better target control measures. Contact extension agents for help with identification.

Cultural and Mechanical Controls

Certain agronomic practices can help reduce caterpillar damage:

• Maintain field sanitation – Clear weed hosts and destroy crop residue after harvest
• Early cutting – Harvest alfalfa in pre-bloom stage to avoid infestations
• Apply nitrogen – Split applications of 40-60 lb N/acre promotes plant vigor and recovery
• Use tolerant varieties – Some varieties have antibiosis resistance against alfalfa caterpillar
• Irrigate correctly – Prevent moisture stress and promote rapid regrowth after cuttings
• Mow borders – Eliminate tall grass surrounding fields to reduce migration into alfalfa

Manual removal of caterpillar nests and egg masses can also help reduce pest pressure. But cultural practices alone don’t provide adequate caterpillar control in most cases.

Biological Control Options

Natural predators and parasites can be augmented to reduce caterpillar populations:

• Parasitic wasps like Copidosoma floridanum parasitize eggs of armyworms and loopers. Release trichogramma wasps that parasitize eggs of many caterpillars.

• Predatory insects like ground beetles, lady beetles, lacewing larvae, and insidious flower bugs prey on eggs and small caterpillars.

• Entomopathogenic bacteria like Bacillus thuringiensis (Bt) and Bacillus sphaericus are effective for early instar caterpillars when ingested.

• Entomopathogenic nematodes like Steinernema spp. and Heterorhabditis bacteriophora infect and kill caterpillar larvae.

Often, multiple natural enemies working together provide the best biological suppression of caterpillars. Avoid broad-spectrum insecticides that kill beneficial insects.

Chemical Control Options

Insecticides should only be used when caterpillars reach economic threshold levels:

Foliar insecticides

• Pyrethroids like lambda-cyhalothrin provide quick knockdown of larval populations.

• Spinosyns like spinetoram give extended residual activity against foliage feeding caterpillars.

• Diamides like chlorantraniliprole and cyantraniliprole have excellent efficacy against a wide range of caterpillar pests.

• Bacillus thuringiensis (Bt) insecticides specifically target the larval stage while sparing most beneficial insects.

Systemic insecticides

• Neonicotinoid seed treatments like thiamethoxam provide early season protection against young larvae.

• Foliar sprays with flubendiamide, chlorantraniliprole and cyantraniliprole are absorbed into plant tissues, killing caterpillars systemically.

Always follow label directions carefully for dosage, timing, preharvest intervals and grazing restrictions when using insecticides. Avoid spraying during pollinator foraging periods. Combine chemical and biological controls for best results.

Integrated Pest Management

The most effective long-term approach for managing caterpillars is implementing IPM practices:

• Set up pheromone traps to monitor adult moths and determine caterpillar activity periods.

• Scout fields weekly and treat only when economic thresholds are exceeded. For armyworms, a threshold of 20-30 larvae per square foot justifies insecticide application.

• Target small larvae before extensive damage occurs. Large caterpillars are harder to control.

• Rotate between insecticide classes to minimize resistance.

• Release trichogramma wasps or Bt to enhance control and reduce pesticide use.

• Continue monitoring after treatment to check control effectiveness. Retreat only if populations rebuild.

It takes extra time and effort to implement IPM compared to routine spraying. But combining multiple tactics maximizes control while minimizing environmental impact. The payoff of protecting alfalfa yield and quality makes IPM worth the investment.

Preventing Future Occurrences

Along with controlling current infestations, implementing preventive measures is key to avoiding recurrent caterpillar outbreaks:

• Maintain proper cutting schedules – Late harvest allows caterpillar populations to explode.

• Eliminate weed hosts and volunteer alfalfa – Weeds serve as reservoirs for pests.

• Consider trap cropping – Plant strips of attractive host plants like buckwheat to aggregate pests for easier control.

• Use semiochemicals – Deploy pheromone dispensers for mating disruption of pest moths.

• Enhance biodiversity – Increase plant diversity within fields to support more natural enemies.

With some extra management effort, these practices create an agro-ecosystem less prone to caterpillar flare-ups. Prevention works hand in hand with control.

Caterpillars can certainly take a heavy toll on alfalfa production. But implementing science-based IPM using cultural methods, biological control, targeted insecticide use and preventive measures provides effective, long-term control. The key is diligent field scouting, proper pest identification and timely intervention. By integrating multiple tactics, alfalfa growers can protect their fields from caterpillar damage and avoid unnecessary losses. With the right IPM approach, these pests don’t have to limit alfalfa yields.