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ISSUE 16  AUGUST 26, 1999


MONITOR INSECTS IN SEPTEMBER FOR CLUES
ABOUT NEXT YEARS PROBLEMS

    There are several common insect pests that can be monitored in the fall to obtain information on risk of infestations next season. Some of these pests are: white grubs, wireworms, grasshoppers, sunflower midge, wheat midge.

White Grubs

    White grub infestations can be a problem wherever coarse textured soils, grasses, and trees come together to provide suitable habitat for the insect. White grub survival will be greatest in coarse soils (sands to loams), they prefer to feed on grasses and tuberous crops, and the adult June beetles feed on the foliage of trees (especially willows and cottonwoods). The southern end of the Red River Valley, especially central Richland County, has been one area where white
grubs have persistently caused economic damage.

    In North Dakota, white grubs of economic importance have a three-year life cycle. Control, when needed, is generally during the second year of the cycle. At this time, grubs are small (3/4 inch or less long) and feed aggressively in the spring on seedling corn, sugarbeets, wheat, and others. In Richland county, the year 2000 will be the second year of the cycle for white grubs that dominate cropping sites.

    From late August to early October, white grubs can be found feeding on plant roots in the upper few inches of the soil. They remain there until a killing frost, when they will move down to below the frost line (about 4 feet) to overwinter. September is the best time to survey fields for white grubs to determine if populations are large enough to cause problems with next years crops. Waiting until spring to sample the soil to determine white grub populations is a poor choice for our northern
areas. The grubs do not return to the surface for reliable sampling until late May, well after susceptible crops are planted.

    Sample fields by digging square foot areas within the plant row to a depth of 6 inches . Research at NDSU has demonstrated that a parallel line of 30 samples dug at a distance of 45 yards from shelterbelts can provide a quick assessment of whether grub populations are large enough to justify control next spring with insecticide at planting time. Rather than counting white grubs, decisions can be made based on the percent of samples with at least one grub present.

 

Interpreting percent white grub infested samples to make decisions at a sample distance of 45 yards from shelterbelts
% Infested                      
Samples at                           IPM Recommendation
45 yards                            
Less than
40%
Larval populations are expected to be below the treatment threshold of 1 larva per square foot. Take a series of samples at 10 to 20 yards to estimate infestation levels closer to the shelterbelts.
Between 40
and 60%
Larval populations are expected to be close to the treatment threshold of 1 larva per square yard. Expect injury to susceptible crops from the shelterbelt to 45 yards. A planting time insecticide would be recommended, if available for the crop. Consider taking a series of samples at 65 yards to estimate infestation levels beyond the original sample distance.
Greater than
60%
Larval populations are expected to be greater than the treatment threshold of 1 larva per square yard. Expect injury to susceptible crops from the shelterbelt to 65 yards. A planting time insecticide would be recommended, if available for the crop. Consider taking a series of samples at 90 yards to estimate larval populations at the outer limits of expected dispersal.

 

Wireworms

    The same difficulties that we have with sampling white grubs in the spring apply to wireworms. In fields where there has been a history of wireworm problems or high risk crops are going to be planted (e.g., potato), consider placing bait stations to attract wireworms this fall. Bury 1 to 2 cups of a 1:1 mixture of corn and wheat to a depth of 4 to 6 inches. Presoaking the whole grain bait one day prior to baiting increases the bait’s attractiveness to wireworms by promoting seed germination and release of CO2. Mound the soil over the top in a dome shape so rainwater runs off. Cover the mound with a piece of black plastic (3 square feet) to promote warming of the soil. Mark the site with a surveyor’s flag. Soils must be moist and at least 45EF. There should be about one baiting site for each acre. Distribute traps randomly through the field. However, consider field history and other conditions that may influence the presence of wireworms, placing more traps in high risk locations. Dig up baits and surrounding soil after one to two weeks or leave until the spring. In the spring, wireworms should be concentrated below the station. Count the number of wireworms per station and calculate an average per station.

    Use Table 1 to interpret wireworm counts when making treatment decisions for potatoes. For other crops, if the average is greater than one wireworm per bait station, the risk of crop injury is high. In this case, a soil insecticide applied at planting to protect sugar beets and corn is recommended. Seed treatments alone may not be adequate to protect other crops. If the average is one or fewer wireworms per station, seed treatment for small grains, corn, and sunflower should be used. If no wireworms are found in the traps, risk of injury is low; however, wireworms may still be present but were not detected by the traps.

 

Table 1. How to Interpret Wireworm Counts from Bait Stations for Making Potato Treatment Decisions *

Average # Wireworms/ bait station

Risk of economic damage

   IPM recommendation

0                         Low               Control notneededorverify
                                                through soil sampling

up to 0.5           Moderate

Sample soil and use decision card

up to 1.0           Less than
                        50:50

up to 2.0           Probable

up to 4.0            High              Apply insecticide at planting

more than 4.0     Extreme       Do not plant potatoes

* Source: Bechinski, et. Al. U of Idaho Circular. 1994.


Grasshoppers

    The adult grasshoppers are now moving around, seeking out green vegetation for feeding. The females will also be laying their eggs in the soil. The areas where they feed, are also the sites where they will deposit eggs. The grasshoppers frequently insert the egg pods at the base of plants, among the roots. Egg pods are only one to two inches deep in the soil. To begin planning for effective grasshopper management programs for next year, note those areas where grasshoppers are congregating and laying their eggs. These sites should be monitored early next season to detect when emergence begins and the size of the population. Controlling young grasshoppers in these emergence sites is the most economical way to deal with outbreaks.


Sunflower midge

    Inspect fields for deformed flower heads damaged during larval infestations in July. If damage is found, such as reported in Webster and Starkweather, ND area, consider your management options for next season.

    Planting Date: Grower observations and university tests have shown that significant reductions in midge damage can be achieved when sunflowers are planted late. When practiced, sunflower development lacks synchrony with the midge life cycle by delaying the early bud stage of the plant until after most of the adults have emerged. In some years, delayed planting will result in much smaller yield losses from midge damage when they are present in large numbers. Unfortunately, the optimal planting time to reduce midge damage will vary between years, depending on spring temperatures and the timing of adult midge emergence.

    A better strategy is to spread out budding dates over a wider window, since risk of infestation and damage should differ by planting date. The bud stage, the growth stage most attractive and susceptible to midge injury, lasts approximately 21 days. Unfortunately, for most hybrids, the time from seedling emergence to early bud stage is the same, regardless of relative maturity rating. Therefore, staggering planting dates to produce a real difference in budding times may require a seven to 10 day waiting period between planting dates.

    Hybrid Selection: Selection of a hybrid sunflower seed which is tolerant to midge damage can also help reduce losses. Several commercial hybrids that have tolerance at light to moderate midge infestation levels are available, but no present hybrid can tolerate heavy midge infestation levels. Check with your local extension agent for a list of evaluated hybrids rated during the most recent midge resistance screening tests.


Wheat midge

    If midge larvae were found at numbers greater than 13 per head, economic infestations occurred this season. Risk of infestation will likely be high for next year. Also, the fall soil survey conducted by NDSU will provide some idea of risk for regions. The survey map will be available in February, 2000. To reduce risk of infestation, plant wheat early, rotate, and scout to determine if treatment is warranted next year.

Phillip Glogoza
Extension Entomologist


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