ISSUE 9 July 2, 1998
EUROPEAN CORN BORER UPDATE
Corn borer moth activity is increasing in Richland County in the southeast corner of the state. This increase in activity is due to the moth emergence of the one-generation type corn borer. These moths represent the portion of the population that has caused most of our infestation problems during July these past four seasons. Due to the warmer temperatures, emergence is getting underway a little earlier.
This activity should serve as our wake up call to begin monitoring corn for the presence of eggs, larvae, and feeding injury.
Yield losses due to European corn borer infestations are primarily due to stalk tunneling that results in physiological stress. With persistent autumn winds and dry conditions, tunneling in stalks and ear shanks increases the risk of stalk breakage and dropped ears.
Management - Natural Control
Heavy rains that occur before borers are able to burrow into the plant may kill the worms by drowning them or by physically removing them from the plant. The high humidity will promote disease outbreaks affecting the larvae.
Management - Chemical
The challenge of the crop manager is to distinguish when egg laying and larval populations can be tolerated or they need to be controlled. Corn should be monitored weekly for at least five weeks once plants exceed an extended leaf height of 17 inches. At this point, corn borer larvae will be able to survive on the plant. Inspect plants for the presence of egg masses, whorl feeding, and active larvae. Observing moth activity around field margins or within the field may alert you to developing infestations.
Field scouting for corn borers:
Whorl stage corn . . . . Pull the whorls from 10 plants at 5 locations across the field. Select whorls at random, avoiding damaged plants. Unwrap the whorl leaves; count and record the number of live larvae found.
Use the following worksheet to help make decisions about the profitability of treating an individual field.
Worksheet for Corn borer in whorl stage corn . . . You fill in the blanks
1.__% of plant infested
2.__borers per plant
3.__percent yield loss
4.__bushel loss per acre
5.__loss per acre
x__Avg. no. borers/plant
x__percent yield loss per borer*
x__expected yield (bu. per acre)
x__price per bushel
___cost of control per acre
= __Borers per plant
= __percent yield loss
= __bushels per acre loss
=$__loss per acre
*5% for corn in the early whorl stage; 4% for late whorl; 6% for pre_tassel
**80% for granules; 50% for sprays.
EMERGENCE UNDERWAY -WHEAT MIDGE UPDATE
Midge emergence is occurring as expected. All counties in the central, northeast and north have or will reach the 1300 degree days for female emergence to occur. Scouting for the presence of midge is critical now.
From the Carrington area, Jim Harbour, Area Plant Protection Specialist, reports that midge have been detected using emergence and sticky traps. Evening scouting found midge levels at 1 to 2 adults per 4 wheat heads which are threshold levels for adults in fields near the Research and Extension Center.
In the Devils Lake-Leeds area, reports of midge in emergence cages were received on Tuesday, June 30. In the Minot area, Jan Knodel, Area Plant Protection Specialist, is reporting detection of midge in emergence cages and on sticky traps. In Walsh County, Brad Brummond, Extension Agent, is reporting low levels of adults in fields. Evening monitoring of fields is critical in these regions. Peak emergence (1475 DD) will be occurring through the weekend and early next week. Degree days are accumulating at about 24 per day around the region.
The following map provides Wheat Midge degree day accumulations from weather station sites (NDAWN) around the region.
DD Wheat Midge Event
10% of the females will have emerged
about 50% of the females will have emerged
about 90% of the females will have emerged.
TIME TO SCOUT SUNFLOWERS FOR SPOTTED STEM WEEVIL
The Spotted stem weevil population has been small in North Dakota for the past several seasons. Spotted stem weevil larvae have been found to cause serious stalk breakage when 25 to 30 larvae are present in a stalk, weakening it when larvae make their overwintering cells in the stalk's base. Breakage is most likely to occur during drought stress or high winds. The spotted sunflower stem weevil is 3/16 inches in length, and grayish-brown with varying shaped white spots on the wing covers. The weevils emerge in mid to late June. Eggs are deposited in epidermal tissue of the stem. If controls are directed at the adults in order to minimize egg laying, treatments should be initiated during the first few days in July. About 50% of the eggs will be deposited by this weevil by mid July.
Treatment for sunflower stem weevils is recommended when scouting determines that an average of 1 adult per three plants is found.
Products registered for adult stem weevil control include Asana XL, Scout X-tra, Warrior, carbaryl, Furadan 4F and Lorsban 4E.
CEREAL APHIDS IN SMALL GRAINS
Concerns over aphids in headed wheat continue. Last weeks recommendations to not treat headed wheat just for aphids is still accurate. Some consultants in the southeast have reported declines in the aphid population in fields, likely due to disease and insect predators. Where insecticides have been applied, multiple insects (ex., grasshoppers, aphids, armyworm, and wheat midge) are being considered in making that final decision.