High Yield Grass Seed Production and Water Quality Protection Handbook

Management of Insects, Slugs and Related Pests

Slug damage in grass seed fields has
increased under reduced tillage,
full-straw load management practices.
Slugs are most effectively controlled in
the fall after rains trigger surface activity.

Use of bait stations can help growers
monitor slug presence and determine if
treatment is necessary.

Monitoring male moth populations is a
key facet of successful sod webworm
control. If over 75 moths are detected
per trap in a two to three week period,
the field will likely sustain sufficient
damage from the larvae of sod webworm
to justify their control.

Monitoring for western orchardgrass
billbug should be conducted in the
spring. When adult damage to blades is
significant (such as pictured below),
treat to kill adults before eggs are laid.

Larva of sod webworm.

Adult western orchardgrass billbug.

Integrated pest management (IPM) can dramatically reduce losses from many insect, mite, slug and symphylan pests. Rather than relying solely on one practice, such as application of a pesticide to control a pest, IPM relies on three principles: prevention, monitoring and selection of most appropriate controls.

IPM in grass seed crops begins with knowledge of the potential and actual pest species and numbers present prior to seeding. This is accomplished by knowing the history of the field, crops grown and past pest problems. Soil core samples prior to planting indicate actual pests present. Healthy and vigorously growing seedlings in the absence of damaging pest numbers ensure targeted plant density and stand vigor for optimum seed yield. Today this must be accomplished without the use of pre-plant, soil persistent, chlorinated hydrocarbon insecticides.

Where row-crop ground is also used to produce grass seed, wireworms and symphylans should be controlled by using appropriate insecticides during the row-crop cycle. Less effective methods (but at times the only alternatives available) for managing these soil pests include a mandatory fallow period after the last crop and/or tillage in preparing the seed bed. Plowing or disking can be used to kill the pests and disrupt their avenues of travel through the soil to the seedling roots. Increased seeding rates are also used to accommodate minor stand loss from soil pests.

Slug control

Slugs are becoming an increasing problem in western Oregon grass seed fields where open field burning and soil tillage is no longer practiced. The increase of straw residue on soil surfaces creates a moister, more diverse habitat that, in the absence of cultivation, greatly favors large slug populations.

Sampling for slugs well before planting a grass seed crop can help growers gauge potential problems before they occur. Damage to the previous crop and the use of bait stations placed throughout a field just before planting (if soil moisture is sufficient and slugs are active on the soil surface at night) can help predict areas where damage from slugs is likely to occur.

The most effective way to control slugs is through cultivation. Plowed and disked fields have virtually no problems with slugs except along field borders where migration occurs. At the very least, strip tillage and preparing a firm seed bed can delay slug damage until after seedling grasses have attained sufficient growth to withstand some feeding.

Metaldehyde baits are quite effective at controlling slugs. They must be applied appropriately, however, to achieve the desired effect. In dryland grass seed production, slugs are most effectively controlled in the fall after rains have caused soil surfaces to be sufficiently wet to allow for surface activity. Most slugs at this time are ready to feed and mate. They readily accept the mini-pellet formulations of bait. Ten to fifteen pounds of bait per acre in the fall can usually provide adequate control that not only kills most of the slugs present but also prevents them from laying eggs.

Monitoring

Timely detection of pest insects and understanding the relationship between their numbers and grass seed yield is essential in optimizing profit under IPM. The sod webworm, for example, can be effectively monitored and easily controlled in grass seed crops if supplemental irrigation water is available to carry an insecticide into the root zone where the pest occurs. In controlling sod webworm, pheromone traps are placed in fields during the middle of June. During late June, the male moths are attracted to and are caught in these traps. If over 75 moths are detected per trap in a two to three week period, the field will likely sustain sufficient damage from the larvae of sod webworm to justify their control.

Soil samples

Larvae levels of sod webworm can be determined by taking soil samples in early September. Depending on the vigor of the field and presence of other pests needing control, an insecticide such as chlorpyrfos (Lorsban) can be applied and watered in to effectively control the larvae. Because Lorsban binds tightly to organic matter in the soil, virtually no lateral movement of the insecticide occurs after it dries and adsorbs to organic matter.

Another example where monitoring can greatly facilitate control is the western orchardgrass billbug. Larvae of the billbug can destroy large acreages after a couple of seasons of infestation. By monitoring for adult feeding damage in the early spring and applying an insecticide only when necessary to kill the adult beetles before egg laying occurs, serious economic damage can be avoided.

By using a combination of cultivation, thermal residue management, appropriate selection of grass species and variety and naturally occurring biological controls, grass seed growers can control insects, mites and related pests with a minimal amount of pesticides. Reducing pest loads through prevention and knowing when to apply the pesticides through monitoring are vital components of successful grass seed production.

Glenn Fisher and Sujaya Rao; Oregon State University Extension Service