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ISSUE 9   JULY 1, 1999

 

CEREAL APHIDS AND BARLEY YELLOW DWARF

    There is still valid concern about aphids in small grains and Barley yellow dwarf viral (BYDV) infections. Much of this concern is going to focus on the later planted grains. If infected aphids move into younger fields and infection occurs, the impact on the plant will be greater. Incidence of infection should be a function of number of aphids present and percent carrying the virus.

    If looking for BYDV in fields, look for yellowish to reddish stunted plants growing singly or in small patches surrounded by normal plants. Early infection results in stunting, reduced tillering, yellowing of leaves, delayed heading, and fewer, lighter kernels. Leaves with symptoms are shorter than normal.

    All four cereal aphids we find in our small grains are capable of transmitting BYDV. The Bird cherry oat aphid is the most efficient vector of the virus. This aphid is the darker one that is ofen found on lower leaves. Its color ranges from olive-green to almost black with a reddish-orange patch on its back by the cornicles, or "tail-pipes".

    A common question has been, "Should we adjust the treatment threshold lower to try and prevent BYDV infections?" I have not been able to find any information that answers the question directly. However, the earlier we detect aphids in a field at potentially harmful numbers, the sooner we can control them with the benefit of reducing the risk of infections in younger plants. Research conducted at NDSU by Boeve and Weiss found that when aphid populations average 9 to 15 aphids per stem, the percent infested stems in a field will range from 80 to 90%. We have used this information along with other statistical models to provide treatment guidelines that are based on simple scouting approach that uses percent infested stems, a minimum number of stem samples and classifying a stem as infested when a single aphid is present. Boeve and Weiss also proposed a dynamic or changing threshold based on wheat prices. The guidelines adjust the threshold upwards when wheat prices are low (< $4) or down when wheat prices are higher (> $5).

    If we want to reduce the risk of BYDV infection, one way of approaching it would be to use a lower treatment threshold during earlier growth stages timed to coincide with movement of aphids into the field. This approach may not keep BYDV out of a field, but could help in reducing secondary spread of the virus within the field before flag leaf stage is reached.

    When 75% of the wheat stems have at least one aphid present, the average number of aphids per infested stem will be close to 5 aphids. This density is about 40% of the current treatment threshold (13 aphids per stem). The expected benefit of using a lower treatment threshold should a reduced impact on yield from both aphid feeding and BYDV infection. Consider using the lower threshold of 75% aphid infested stems from seedling through tillering stages, a time when the BYDV impact will be greater. From flag leaf to boot stages, go back to using the higher, 85% infested stem level for aphids, as the potential impact from BYDV declines.

 

EMERGENCE UNDERWAY -WHEAT MIDGE UPDATE

    Female midge emergence should begin as expected in the central counties with the accumulation of 1300 degree days. Scouting for the presence of midge in heading fields is critical now.

    In addition to your own scouting, begin listening for local reports from your extension agents about activity reported from your area. The critical flight period usually lasts about two weeks. When we reach 1600 DD, 90% of the females have emerged, however the adults will still be active, just that very few new midge will be added to the population. Continue scouting fields that are at risk until 1800 DD have been reached. By that time, most of the midge should have reached the end of their life.

DD               Wheat Midge Event

1300

10% of the females will have emerged

1475

about 50% of the females will have emerged

1600

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. Jan Knodel, NDSU Plant Protection Specialist in Minot, observed spotted stem weevil last week. There are some concerns about stem weevil in Emmons and surrounding counties. Spotted stem weevil larvae cause 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 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.

 

EUROPEAN CORN BORER UPDATE

    Corn borer moth activity is increasing. Moth captures at Oakes and Carrington have been averaging around 15 to 20 moths per night. In the valley, numbers have been much lower. These moths are most likely the two-generation type borer. Later, the one-generation type corn borer will increase the numbers further. The one-generation borers represent the portion of the population that has caused most of our infestation problems during July these past five seasons.

    This activity should serve as our wake up call to begin monitoring corn for the presence of eggs, larvae, and feeding injury. Corn borer survival on corn shorter than 17 inches in height should be slight. Taller corn will sustain the borers. Shot-holing of the emerging leaves is the first indication that infestations are developing.

Phillip Glogoza
NDSU Extension Entomologist


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