Manitoba-North Dakota Zero Tillage Farmer's Association Brandon, Manitoba, Canada January 1997

Phillip Glogoza

Extension Entomologist

North Dakota State University

Fargo, North Dakota

There has been considerable emphasis placed on determining the need for an insecticide application before it is made. A common phrase used in this connection is economic threshold or action threshold. Very simply stated this means, is there a high enough insect infestation level to warrant the cost of an insecticide application. Many recommended economic thresholds are stated in terms of a specific number of insects per some unit of measure such as square foot, stem, row feet. Many thresholds may actually vary when one considers the value of the commodity, interactions with other pests that are present, yield expectations, cost of control and more. However, determining these thresholds is more involved and experimental determination is often lacking.

A key to using any threshold is knowing what insects are present and at what level. The next step is to estimate how much can be spent profitably to reduce the damage and what would be the most efficient approach to the problem. Certain situations will occur where the insect infestation level is moderate or located around field margins, but expected to increase and move into fields. In such an instance, it would be desirable to control the infestation early in the interest of avoiding greater cost at a later date. This strategy is often most useful for grasshoppers.

SMALL GRAIN INSECTS

Currently, there are four insects causing management concerns within the region. These insects are: wheat midge, grasshoppers, cereal aphids, and the wheat stem sawfly. Other insects that can also be problems include barley thrips, armyworms, Hessian fly, and wheat stem maggot.

WHEAT MIDGE

This insect is currently an economic concern in Saskatchewan, Manitoba, and North Dakota. Following wet seasons in 1992, 1993, and 1994, the 1995 infestations were greater than normal. In 1996, wheat midge were detected in all areas east and north of the Missouri River in these areas. A contributing factor to the 1995 and 1996 outbreaks was delayed planting of wheat due to excessively wet soils.

The adult midge is active from late June to early August. Peak activity is from early to mid July. A model using daily temperatures to calculate degree day accumulations allows for a more accurate prediction of local adult emergence. Wheat is attractive for egg laying by midge from the time the head emerges from the boot through flowering. Insecticides for the control of midge are effective on the adult and impact the egg stage; however, control of the orange larvae, which feed on the developing kernels, has not been demonstrated due to protection within the glume.

THRESHOLDS FOR WHEAT MIDGE:

Examine wheat heads at dusk (9 pm and later when temperatures are above 60ÔF and wind speed less than 6 mph). The orange colored adult midge can be seen laying eggs on the wheat heads. Plants are susceptible as the head emerges from the boot. In general, if 1 or more midge are observed for every 4 or 5 heads, treatment is warranted. Treatments after 50% of the first heads have flowered are not recommended due to reduced levels of efficacy and for the protection of a parasitic wasp that attacks the midge eggs. Detecting adult midge:

Sticky traps may be used to capture adult midges active in wheat fields. A simple trap design would be a white Styrofoam plate, attached to the top and bottom of a surveyors flag. The trapping surface can be coated with Tanglefoot foot or vegetable oil. The trap can alert an individual to the presence of midge and their identity, but it does not provide information about the need to treat.

GRASSHOPPER

In the northern plains, grasshopper egg hatch normally begins in late April to early May. Peak hatch occurs about mid June. Heavy infestations typically occur in areas of low rainfall or during drought years. Outbreaks are usually preceded by several years of hot, dry summers and warm falls. Cool, wet weather increases disease occurrence and delays development of grasshoppers, reducing the overall population.

Thresholds for Grasshopper:

INFESTATION RATINGS
	Nymphs (Young hoppers) per square yard		Adults per square yard
Rating	Margin	Field	Margin	Field
Light	25-35	15-23	10-20	3-7
Threatening	50-75	30-45	21-40	8-14
Severe	100-150	60-90	41-80	15-28
Very Severe	200+	120	80+	28+

Whenever scouting indicates that populations are in the threatening category, management efforts should be considered. Managing grasshoppers while they are still nymphs is generally less costly, may be limited to the hatching sites, and has benefit s in preventing later injury from older nymphs or adults.

Cultural Control Methods:

Early seeding	allows for early establishment and vigorous growth of plants.
Crop rotation	avoid planting in areas of high egg deposits. Fields with late maturing crops or green plant cover attract adults which then lay eggs.
Tillage	Summer fallow will act as a trap crop, attracting females for egg laying. Spring tillage of these sites will reduce successful emergence of nymphs.

APHIDS

Aphids are pear-shaped in appearance and usually bear two projections known as cornicles located on the posterior end of the abdomen. The antennae are slender and are usually held along the sides of the body. Aphids have piercing-sucking mouthparts and will usually be found feeding in colonies, on plant stems, and leaves. On small grains they will begin moving into the heads after crops start to mature. The injurious grain aphids in North Dakota are yellowish-green to green in color and are about 1/3 - 3/16 of an inch long.

Thresholds for Small Grains: English Grain, Bird Cherry Oat, Greenbug:

Recent research with cereal grain aphids has provided scouting guidelines that use different thresholds based on the value of wheat. The thresholds are 9, 13, and 17 aphids per stem when wheat prices are > $5, > $4, and < $4 per bushel, respectively. Sampling plans designed to save time during field scouting are available. These plans require as few as 25 wheat stems to a maximum of 100 stems to make a treatment decision. Field scouting should begin at stem elongation and continue up to the heading stage of wheat. Aphid populations, at or above the thresholds, during these growth stages will result in economic injury to plants.

Natural Controls:

Lady beetles, Aphid lions, Syrphid fly, and parasitic wasps play a major role in reducing aphid populations. When natural enemies are present in large numbers, and the crop is well developed, farmers are discouraged from spraying fields.

WHEAT STEM SAWFLY

The adult wheat stem sawfly is wasp-like in appearance and has black and yellow rings on the abdominal segments. The larva is white and looks somewhat like a fly maggot. When full grown, it measures about 1/2 inch long. The head capsule is tarnish-brown and the rear end terminates in a short, blunt point. The larva feeds within the wheat stem, boring down through the joints, and by late summer has reached the lower parts of the plant close to the surface of the ground. Here it cuts a V-shaped groove entirely around and inside the stem, which causes the stem to break off.

Sawfly damage occurs annually in North Dakota. This insect primarily affects wheat in the central and western areas of the state. The larvae tunnel in the stem reducing grain yield by 10-14%. Additional loss occurs when infested stems lodge, rendering the grain unharvestable. Larvae overwinter in the wheat stubble making infested sites the source of next year's problems.

Managing Wheat Stem Sawfly:

Harvesting . . . Swath the most heavily infested fields at 30-35% moisture before significant lodging occurs. This strategy requires field surveys to determine infestation levels. Infested stems have a reddish-brown spot below the 2nd or 3rd node. Examine 50 consecutive stems in a drill row from at least two sites (one near the margin of the field, another near the center). Determine the percent of stems infested at each site. If >6% of the stems are infested, swath to reduce lodging losses within infested sites.

Fall tillage . . . A shallow fall tillage to dislodge stubble and leave it on the soil surface can result in 90% mortality of overwintering larvae. Tillage can be limited to areas where surveys indicated infestations within the field or strip.

Crop rotation . . . Plant non-host crops such as oats, flax, sunflower, legumes, and

to a lesser extent barley, rye, durum and winter wheat. Resistant wheat varieties . . . Solid stem varieties are unsuitable for sawfly development. The following table describes solid stem varieties available for sawfly management.

WHEAT STEM SAWFLY RESISTANT HARD RED SPRING WHEAT VARIETY DESCRIPTIONS
Variety	Height	Straw Strength	Maturity	Stem Rust*	Leaf Rust*	Test Weight	Protein	Yield* *
Ernest	standard	med	med	R	R	high	high	High
Lew	standard	med	med	R	MS	high	low	High
Leader	standard	med	med	R	MS	high	high	Med
Tioga	standard	med	med	R	MS	high	avg	Low
AC Eatonia	standard	med	med	MR	MR	high	high	High
Cutless	semidwarf	med	med early	R	R	high	avg	Med
Rambo	semidwarf	strong	med early	R	R	high	avg	High
Glenman	semidwarf	strong	med	R	MS	avg	low	High
* R = resistant; MS = moderately susceptible; MR = moderately resistance **Yields are relative to sawfly resistant varieties.

Chemical control . . . None is currently recommended for the wheat stem sawfly.

BARLEY THRIPS

Adults are about the size of this undotted "I". They are dark brown to black and the females have wings fringed with long hairs. The wingless young are pale greenish-yellow. Barley thrips are usually a pest on barley at heading time but they have also been found on durum.

Female barley thrips fly to barley from overwintering sites during mid to late May. Sampling for thrips should begin when the flag leaf is first visible and continue until the head is completely emerged from the boot. Sample at least 50 feet in from field margins. Most thrips can be found under the top two leaf sheaths. The dark brown to black thrips can be found by unrolling the leaf sheaths away from the stem. Insecticide treatments are only effective when applied before heading is complete.

Threshold for Thrips:

Treat when thrips are equal to or greater than the number calculated by Threshold (Thrips/stem) = Cost of Control - Expected $ value per bushel 0.4

WHEAT STEM MAGGOT

The larval stage of the wheat stem maggot feeds within the stem of the developing wheat plant. Infested stems eventually die and the unfilled head is referred to as a white head. Besides wheat, this insect also can complete its life cycle on oats, barley, rye and other grasses.

The maggot tunnels in stems of wheat resulting in a white head that can be easily pulled out. This damage becomes evident after flowering. Infestations rarely exceed 2% and fail to become an economic concern. Crop rotation and destruction of volunteer grain are the most effective methods of reducing maggot populations. No chemical control is recommended.

HESSIAN FLY

The Hessian fly can be found throughout North Dakota, but seldom causes economic loss. Adult Hessian flies are small gnats about 1/10 of an inch long. They lay their eggs on wheat leaf blades. Eggs hatch and the larvae move down beneath the leaf sheath and begin feeding. After completion of the larval development, they change into the pupal stage that looks very similar to a flaxseed, and it is referred to as the flaxseed stage. Several generations are passed in a single season. Early infestations of wheat cause a stunting of the young plant, and later season infestations cause stem weakening and lodging before harvest. The Hessian fly will also attack barley, rye, and some wild grasses.

The Hessian fly overwinters as a maggot or pupa in winter wheat, volunteer grain, and wheat stubble. Overwintered maggots pupate and emerge as adults from April to May, infesting winter and spring planted wheat. By June, maggots pupate (flaxseed stage), emerging as adults in August to lay eggs for the overwintering generation.

Managing Hessian Fly:

Winter wheat planting date . . . Winter wheat will act as a bridge to get Hessian fly from one season to the next. Delaying planting should reduce infestations. Suggested planting dates for ND are: North - August 25 to September 15; South - September 1 to 20.

Tillage . . . Burying stubble and destroying volunteer grain after the first killing frost or early in the spring before fly emergence helps suppress adult populations.

Rotation . . . Rotate wheat with nonsusceptible crops (oats, corn, soybean, sunflower, flax).

Resistant varieties . . . Two South Dakota releases, Guard and Shield, are hard red spring wheat. They are semi-dwarf varieties. Guard is reported to be prone to shattering.

Chemical control . . . Thimet is registered as a planting time treatment for wheat. Population levels of this pest rarely warrant such a treatment, however.

SUMMARY

In conclusion, it is important to monitor fields for pest activity, whether its insects, weeds, or diseases. By becoming familiar with the pests that occur within your region, the farm manager can begin to plan monitoring activities when environmental conditions or crop development dictate. By using recognized thresholds the manager learns to recognize when pest levels seriously pose a threat to successful crop production.