By Faith Hillsden, Survey Tech Summer Student and James Tansey PhD, AAg, Provincial Insect and Vertebrate Pest Management Specialist
July 2024
Pea aphids are a potential threat to pulse crops and perennial legumes throughout Saskatchewan, especially when large numbers arrive early. Reports of significant populations were received from American colleagues earlier in the year; pea aphids started showing up around a month ago (mid-June) in Saskatchewan. Populations were first reported in alfalfa but are now being reported in multiple pulse crops.
Pea aphids pierce plant tissue and suck plant sap from leaves, petioles, stems, flowers and young pods. Host crops include peas, lentils, faba, chickpeas, dry beans, alfalfa and clover. Pea aphids feed on pea and lentil crops initially but will colonize faba when other hosts dry down. Feeding on the growing points of the plant drains nutrients and causes decreased seed formation and size, deformed leaves, stunted growth and leaf dropping. Pea aphids can also vector viruses.
Pea aphids reproduce asexually through most of the year and the young are clones of its mother. Males are produced late in the summer and mating occurs then. Pea aphids overwinter as an egg on leaves and stems of perennial legumes such as alfalfa and clover. However, they do not overwinter well in Saskatchewan and populations here are usually the result of an influx of aphids from the mid-west United States.
As growth resumes in the spring, eggs hatch into female nymphs; these go through four nymphal instars before developing into adults. Adult females, called stem mothers, reproduce without mating and give birth to the next generation. Winged and wingless aphids are produced throughout the year, with winged aphids migrating into other legume host crops. It takes 5-50 days for aphids to mature, with an optimal temperature of 23-28 C. Juvenile mortality has been shown to be higher at greater temperatures. There can be 15 generations per year, with a single female producing 50-150 nymphs, resulting in rapid population growth.
Scouting for pea aphid should occur when pulse crops are 50-75 per cent flowering. Choose at least five spaced out locations and take 10, 180-degree sweeps, or check five plant tips. Count the number of aphids and find the average per tip or sweep. A variety of locations within the field should be assessed including field edges, because damage will appear first along the crop edges. It is important to scout fields often, as populations can double or triple in one week or be held or reduced by natural enemies. The economic threshold for pea aphids varies by crop.
- Lentil: the economic threshold varies depending on environmental conditions, and ranges from 20 to 66 aphids per 180° sweep with a 38 cm diameter (15 in) sweep net. This provides a 7-day lead time before aphid populations reach the economic injury levels of 78 aphids per sweep. When a high potential yield is expected, an economic threshold of between 20 and 30 aphids per sweep should be considered. A higher economic threshold of 46 to 66 aphids per sweep is recommended for hot and dry years with lower yield potential. If management action can be taken immediately, the economic threshold for pea aphids in lentils is 70 aphids per sweep.
- Peas: the economic threshold is two to three aphids per 20 cm (8 in) plant tip, or nine to 12 aphids per sweep, at flowering. This assumes $0.21/kg ($5.71 per bushel) and an average control cost of $16.63-$22.86/ha ($6.73-$9.25/acre). Tolerance will decrease with greater commodity prices and increase with greater control costs.
- Faba: aphids usually hide under the leaves and cluster on areas of new growth. Monitor pea aphids frequently from the end of July to mid-August. The economic threshold for pea aphids in faba beans is 34 to 50 aphids per main stem of a plant. This provides a 7-day lead time before aphid populations are expected to exceed the economic injury level of 96 to 142 aphids per faba bean main stem. This economic threshold is applicable to both tannin and low tannin varieties of faba beans. Pollination by bees is important for good yields in faba beans, and faba beans can have a long flowering period. Pollinators should be considered if aphid control is needed while faba beans are flowering.
Monitor frequently. If populations are steady or decreasing, natural enemies are having an effect and control may not be needed. Natural enemies include lady beetles, lacewings, hoverflies, minute pirate bugs, damsel bugs and parasitic wasps.
Tyler Hartl with Agriculture and Agri-Food Canada (AAFC) and the University of Saskatchewan has created a field survey for pea aphids accessible to producers throughout Saskatchewan. This quick survey aims to map populations of pea aphid throughout the province and will be implemented into a species distribution model to investigate migration patterns.