ISSUE 3 May 18, 2000CEREAL APHIDS LOOM LARGE ON GRAIN GROWERS MINDS
Small grain growers have been anxious all winter about the outlook for aphids in their wheat, barley, and oats because of the prevalence of Barley Yellow Dwarf Virus in 1999. Reports from the winter wheat areas about high incidence of BYDV are an early alert of the potential problem since aphids from these regions will make there way here soon.
Well, aphids were found in 2 to 3 leaf wheat just east of Fargo in Minnesota late last week. The good news is that the aphids were Corn leaf aphid. All were wingless nymphs, indicating that they had overwintered as eggs and hatched recently, not flown in from southern areas. There numbers were averaging around 1 to 2 per 10 plants, well below a treatment level of 85% infested stemsThese local aphids would have a very low risk of being infected with BYDV. The mild winter and wheat following corn probably contributed to this winter survival of eggs and the early establishment on the wheat.
There are four cereal aphids we expect to see each season. It is important to recognize them.
Corn Leaf Aphid - bluish green with black legs, cornicles and antennae.
Bird Cherry Oat Aphid - olive green, brownish patch at the base of cornicles.
Greenbug - pale green with darker stripe down back.
English Grain Aphid - bright green with long black cornicles.
The greatest risk of yield loss from aphids feeding on grains is in the vegetative to boot stage plants . Significant yield reductions after the onset of flowering could not be demonstrated in research published from South Dakota in 1997 (Voss et al., 1997. J of Economic Entomology 90: 1346-1350). Reasons for these conclusions were that after heading, the only major yield component aphids can affect is seed weight and that aphids are unable to sustain the very large populations necessary to achieve significant impact on this factor. Other components of yield are determined earlier (number of spikelets - determined
Therefore, to protect yield loss due to aphid feeding, the treatment threshold is 85% stems with at least one aphid present, prior to complete heading.
Trying to protect wheat from BYDV however is much more difficult.
Last year it was suggested that treatments be considered if 50 to 75% of the stems have at
least one aphid present. It is difficult to assess populations of aphids at these low
levels. The most abundant aphid last year was Bird Cherry Oat aphid, the aphid most
efficient at moving BYD and which vectors the most virulent strain, PAV. In most years,
the English Grain aphid has tended to be the most abundant aphid in our fields,
particularly at heading. EGA also transmits BYD but is less efficient at it. Also, with
late planting last year, BYD
infections occurred in vegetative stages. Infections in younger plants have greater impact on yield than infections in older wheat.
In the next few weeks, migrating aphids are likely to be detected in the region. Weather plays a major role in how the populations increase once they arrive. Temperatures from 72 to 80oF with moderate humidity favor increases in the population. Sporadic rain showers promote the development of pathogenic fungi that infect the aphids and kill them. Numerous beneficial predators and parasites establish in fields along with the aphids.
If an aggressive program is taken to manage aphids, effects of treatments in younger wheat should be scouted closely. Early treatments delay the predator and parasite populations from increasing, giving the aphid populations opportunity to increase rapidly, perhaps back to threshold levels prior to heading. I believe, if Bird Cherry Oat aphids or Greenbugs are the most abundant aphid found, an aggressive approach would be more warranted than if the dominant aphid were English Grain.
OTHER INSECTS AROUND THE VALLEY
Reports of wireworm and white grub feeding are coming in from the
southern end of the Red River Valley. They have affected sugarbeets and corn. The White
grub problems have been in traditional areas of Richland County on coarser textured soils.
Rescue treatments after the crop has emerged are not very successful. The wireworm damage
is distributed throughout the field; the white grub injury is adjacent to shelterbelts.
A label for the use of Asana to control SBRM flies was issued this week. The rate is 5.8 to 9.6 fl oz per acre (0.03 to 0.05 lb AI per acre). It can be applied by air or ground. The user must have a copy of the label in their possession.
This use is still being evaluated by NDSU researchers. The product will control adult flies but the verdict is still out on the overall impact it may have on preventing larval infestations. Field trials this year should provide additional information on the overall effectiveness of the product at preventing losses from SBRM.