Flax may be infested from the time of emergence to maturity by various insect pests. Fields should be examined regularly, and controls applied when infestations reach the economic threshold. The following species are potentially damaging but often occur in too low a number to cause economic loss.

GRASSHOPPERS

Grasshoppers have been the number one threat to North Dakota flax in recent years. Young grasshoppers may attack young plants and cause damage. However, more damage is done to the crop before harvest by the older, larger grasshoppers. They can quickly cause large numbers of bolls to drop by chewing through the more succulent portions of the stem below the bolls. Growers need to be aware of grasshopper activity in the vicinity of flax fields well before adult migration begins in July. Because of the limited availability of insecticides to control insects in flax, attempts to reduce grasshopper populations in neighboring crops and non-crop areas is advisable.

CUTWORMS

Two subterranean species of cutworms, the redbacked, Euxoa ochrogaster, the pale western, Agrotis orthogonia, attack flax. The adult moths of these species lay eggs on the soil surface in weedy summer fallow fields during late summer. These eggs overwinter and the young larvae feed on flax seedlings in the spring. Cutworms usually remain below ground, cut off the young plants near the soil surface and draw them down where they are eaten. An average population of 10 cutworms per square yard can cause a 10% reduction in the yield of flax, and control should be considered.

ARMY CUTWORM

Larvae of the army cutworm, Euxoa auxiliaris, damage flax and many other crops by feeding on foliage in the spring, and to a lesser degree, in the fall. It can be an important pest in southwestern North Dakota. Populations of 9 per square yard can cause significant damage.

BERTHA ARMYWORM

The bertha armyworm, Mamestra configurata, was a regular pest of flax before canola and mustard were grown on the Prairies. However, since their widespread introduction, the bertha armyworm rarely causes economic damage to weed-free flax fields. If bertha-armyworm-infested canola fields are swathed and green flax fields are nearby, the flax can suffer significant damage from invading larvae. When abundant, bertha armyworms cause serious damage by chewing through the stems below the bolls causing them to drop to the ground. Young bertha larvae are green but larger larvae are usually velvet-black.

ASTER LEAFHOPPER

The aster leafhopper, Macrosteles quadrilineatus, can damage flax. This insects feeds by sucking juices from the flax plants. More importantly, aster leafhoppers can carry the aster yellows mycoplasm and the crinkle virus, and can infect the plants with these diseases while feeding. The damage from these insects is most serious on late-seeded crops.

WIREWORMS

Wireworms, although often serious pests of cereal grains in the seedling stage, seldom damage flax.

Insecticides Registered for controlling insects in Flax
 

INSECTICIDE	DOSAGE IN LB AI/ACRE	PRODUCT PER ACRE	RESTRICTIONS ON USE
carbaryl (Sevin)	0.5 - 1.5	rate varies by formulation	Do not apply within 42 days of harvest for seed or straw.  Do not apply more than a total of 3 lbs of active ingredient per acre per crop.

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North Dakota State University
NDSU Extension Service