ISSUE 7 JUNE 17, 1999
CEREAL APHIDS IN SMALL GRAINS
Aphids have arrived early and are doing quite well in local small grain fields. Bird Cherry-Oat aphid and English Grain Aphid are the primary aphids being found.
The critical time for making aphid treatment decisions is during stem elongation to heading. Treatment after heading has not been determined to be beneficial. Aphid impact on yield and quality is not great enough to justify the cost of control in headed wheat.
Treatment guidelines for controlling aphids in wheat is when 85% of the stems have at least one aphid present from vegetative to heading stages. Once heading is complete, treatment for aphids is not recommended.
Bird cherry-oat aphid tends to stay in the lower half of the plant, feeding on the leaves and lower stem. This aphid is most responsible for Barley Yellow Dwarf virus (BYDV) transmission. The English grain aphid is most often found on the developing wheat head. The growth rate, egg production, and rate of population increase is higher for English grain aphid when feeding on wheat heads than when they feed on leaves. This is why we tend to see this aphid's population build quickly once wheat heads.
Because aphids populations are increasing early, later planted wheat should be monitored closely. Aphids will move from older fields to younger ones. If BYVD infected aphids are allowed to establish, infections can result in significant yield reductions as were observed in northeast North Dakota last season.
ALFALFA BLOTCH LEAFMINER
This recently established insect pest is causing concern, particularly in the southeast quarter of North Dakota and neighboring Minnesota. Alfalfa fields with a whitish cast across the field have extensive infestations of this fly pest. The same appearance can be confused with alfalfa weevil feeding, however, you do not have the skeletonizing of the leaves.
Signs of an infestation are "pinholes" in the leaves, caused by the adult fly, and a comma shaped mine formed by the larva tunneling through the leaf. The larval damage will be white. The pinholes are easily visible if you hold the leaf up against the sunlight.
Currently, insecticides have not provided consistent or significant control. The best recommendation would be to cut a little early to avoid leaf loss due to their drying. Ian McRae, U of Minnesota entomologist, points out that early harvest in either the first or second cut should reduce the future generations in the season. Long term control of this insect should be achievable through natural enemies. In the eastern US where this insect arrived in 1968, the release of a parasite from Europe has reduced populations to insignificant levels. Hopefully we could have the same success.
CUTWORM ACTIVITY SIGNIFICANT
There are numerous reports concerning cutworm feeding through the valley region. Variegated cutworm have been the primary culprit. They have been found in sugarbeets, alfalfa, sunflowers, and more.
Variegated cutworm will take on a climbing habit, moving up onto the plant in the evening and feeding. Leaves will be chewed from the margins, smaller larvae may feed on the interior of the leaf, creating holes. Damage I have seen could easily be confused with grasshopper injury . . . except you don't find any.
You likely will not find anything above ground during the day. The cutworms retreat to the soil during the daytime. In the alfalfa checked Tuesday morning, a damaged plant or area was quite obvious when compared to surrounding normal plants. Digging in the soil at the base of the damaged plants, we could find larvae from 3/4 to 1 1/2 inches in length.Action Thresholds for Cutworms by Crop:
WHEAT MIDGE DEGREE DAYS
Degree day accumulations in the southeast corner of North Dakota are very close to 1300 DD, the point where female midge are beginning to emerge. Though this area has not had large overwintering populations, heading wheat should be monitored beginning this weekend for the presence of midge. The remainder of the valley will begin to see emergence beginning towards the middle of next week.
We should begin seeing the "lauxanid" fly, which has a plump body and is tan in color, compared to the wheat midge, which has a slender body and is bright orange, in the fields. The lauxanids usually show up about five days before midge. So, be sure people aren't seeing Lauxanids and thinking wheat midge.
There are always questions about tank mixing a fungicide with the Lorsban 4E-SG for midge control. Dow Agrosciences representatives inform me that compatibility should not be a problem with the wheat fungicides. They do recommend that the Lorsban be added to the spray tank last for best mixing results.
BLACK STEM WEEVIL IN SUNFLOWERS
Numerous calls and concerns about the large numbers of Black stem weevil in our fields. In general, this weevil does not pose a significant threat to sunflowers like the Spotted stem weevil does. The larvae complete development in the season and overwinter as adults, rather than overwintering as larvae in the stalk like the spotted stem weevil. This limits the impact on the stalk by not weakening it later and making it vulnerable to breaking.
Should we be concerned with the current activity? We really are not sure. Plans are underway to assess potential damage by the large populations that are present in young plants. We would discourage treatment of sunflowers strictly for controlling black stem weevil based on our current information.
Black Stem Weevil, Apion occidentale
POTATO LEAFHOPPER ADULTS IN POTATO
With all the adult leafhopper activity in the region, the treatment threshold information is difficult to interpret since most information is based on nymphs. In potato, it is suggested that an adult threshold based on sweep net sampling be used to make decisions. If an average of ½ to 1 adult leafhopper per pendulum sweep is found, treatment would be recommended. Asana XL and dimethoate have provided good control of the adults at reduced label rates.