By Scott Hartley, PAg, Provincial Specialist, Insect/Vertebrate Pest Management
Aphids are starting to be reported, primarily in lentil. In most cases, they have been noted as part of regular field scouting and numbers were not at economic threshold levels. However, humid conditions are good for development of aphid infestations.
It is often pea and lentil where aphids can be a problem. However, in canaryseed, aphids are almost an annual occurrence in July. In 2015 there were also widespread reports of the orange or red morph of the normally green English grain aphid in wheat.
Being soft-bodied, aphids are very sensitive to physical disturbance. A heavy rain or severe wind can dislodge aphids and reduce their numbers in a crop. Regular monitoring is essential in identifying changes in population densities.
Since aphids suck plant sap, the sap has to be actively flowing for the purpose of filling seeds. If seed filling is complete and the crop ceases to be lush and actively growing, the next generation of aphids may emerge in a winged form and migrate to other greener crops.
High numbers of aphids feeding at the bases of flowers and developing pods may result in abortion of flowers and reduced filling and possibly fewer seeds per pod. However, if there is sufficient moisture the plants can compensate for some of the fluid loss to the aphids. A crop under moisture stress may be more adversely affected by aphids.
Timing and necessity of insecticide applications should be treated on a case-by-case basis. Application of insecticide too early will be unlikely to provide a yield response but will affect beneficial predators (e.g. lady beetle larvae and adults) and wasp parasites. Late application would have no beneficial result and will be an unnecessary expense, as the aphids cannot damage crops that have completed seed filling. Research has shown that the best result from insecticide application for economic infestations of aphids on peas is at late flowering to early pod. From this stage until soft dough, there will be a gradually diminishing yield response by the plants as a result of insecticide application.
Beneficial insects and disease can play a major role in managing aphid populations. Aphids have many predators aside from lady beetles, including ambush bugs, minute pirate bugs and hover fly larvae. Several species of tiny wasps lay their eggs in aphid nymphs, killing the nymph. The result is gray or tan “bloated” but empty aphid bodies, referred to as “mummies”. There is a larger exit hole in the mummy, from which the adult wasp has emerged.
In humid conditions, an entomophthoran disease can kill large numbers of aphids. These natural control agents may not be effective in reducing large infestations, but when aphid numbers are around threshold levels, bio-control such as disease or beneficial insects may be a better alternative than insecticides. Because of their rapid ability to increase numbers, aphids have been known to rebound to high levels several weeks after insecticide application. Regular monitoring for the presence of these diseases and beneficial insects, as well as for the aphids, is important and can save considerable expense and labour.
Aphids have specific host crop preferences. Although some species may feed on more than one crop, not all aphids will attack all crops. Aphids noted in pea and lentil will be predominantly the pea aphid. However, in canola there are two or three species that could be encountered. The species feeding on canola leaves are generally not a concern. However, the turnip aphid tends to cluster on stems, often near pods and flowers, and can affect pod filling.
Recommended economic thresholds (ET) for aphids
Keep in mind that there are several factors affecting the economic thresholds and control decisions, including the market value of the crop, cost of control, moisture conditions and crop stage.
- Field Peas – Currently, the recommended ET is two to three per 20 cm plant tip at late flowering to early pod. This is based on an older pea variety (Century pea), and newer varieties may be able to tolerate more aphid feeding. This is based on the number of aphids on the plant tip, though there are also likely aphids on the rest of the plant. The chart below is from research on aphids on peas from Dr. Bob Lamb (AAFC, Winnipeg) and provides an estimate of yield loss related to aphid numbers.
Aphids per tip
% yield loss
- Lentil - Information on aphids in lentil from North Dakota suggests an ET with three qualifiers:
- Thirty to 40 aphids per 180 degree sweep; and
- Few natural enemies are present; and
- Aphid numbers do not decline over a two-day period.
- Canaryseed – based on U.S. and Australian research, the current recommendation for considering insecticide application for aphids on lentil is 10 to 20 aphids on 50 per cent of the stems.
- Cereals (e.g. wheat, barley) - 12 to 15 aphids per stem prior to soft dough.
- Canola – As a guideline, researchers have suggested that if 10 to 20 per cent of the stems have clusters of the turnip aphids, control is likely warranted.