Information provided is from the Manitoba Insect and Disease Update for the week of July 13th at: http://www.gov.mb.ca/agriculture/crops/seasonal-reports/insect-report-archive/insect-report-2016-07-13.html
You will often find aphids clustered on some plants; adults with the young nymphs nearby. Aphids develop through their stages very rapidly, so when monitoring all aphids are counted, not just larger ones. Parasitized aphids (Fig. 2), known as aphid mummies will look swollen and discoloured and are not counted.
Fig. 1. Aphids on wheat
Fig. 2. Aphid mummy
The economic threshold on cereals is 10-15 aphids /stem prior to the soft dough stage. Research has found that after the early-dough stage insecticide treatments were not cost-effective.
Scouting tips: It is quite normal to find some plants with 10-15 or more per stem, but if there are a lot of plants around these with none or very few per stem, than the field would be below the economic threshold. If you can randomly pick plants (not looking specifically for plant with aphids to do counts on), and on average be above 10-15 per stem, and not yet at the soft dough stage, that is the point when insecticides become economical. Aim to sample at least 20 stems for aphids; selecting 5 stems from each of at least 4 areas of the field is a good sampling plan.
Factors regulating aphid populations: Sometimes aphid populations will increase, but other times populations will stabilize or decrease due to predators and parasites, diseases, or heavy rains which can reduce a population. So if a heavy rain goes through, sometimes it is good to reassess a population. And if natural enemy levels are high, it is good to assess whether levels of aphids are possibly declining.
Soil moisture levels and aphids in cereals: As another consideration, research on English grain aphid in winter wheat found that there was a greater yield reduction for a given aphid density when plants were grown under water stress versus nonstress conditions. They concluded that “therefore, the water status of the plant should be considered to determine an economic injury level for S. avenae infesting winter wheat” (Environmental Entomology. 1988. Pp. 596-602). This may be difficult to do, but does show that a given level of aphid feeding on cereals is likely to reduce yields more when plants are growing under water deficits, which is generally not the case this year.
Submitted by: John Gavloski, Entomologist, Manitoba Agriculture
Visit the Insect Pages of Manitoba Agriculture’s website at:http://www.gov.mb.ca/agriculture/crops/insects/index.html