Aphids On Flax

The potato aphid, Macrosiphum euphorbiae, is the main aphid pest on flax and is featured in this article.  

Appearance

Potato aphids are about 5 mm when fully grown, are pale green (darker along the centre line) and have prominent, dark tipped tail tubes. Viewed through a hand lens, the tip of the tail tubes have a netlike appearance. A pink phase of the potato aphid may also occur. Potato aphid colonies are made up of adults and their offspring, closely clustered together.

Life Cycle

Potato aphids overwinter as eggs on roses, raspberries and strawberries. Wingless females hatch from eggs in the spring. These females give birth to nymphs which develop into both winged and wingless adults. In western Canada, the winged females migrate from the winter hosts to summer hosts such as flax in late June to early July when the crop is flowering and developing seed. A number of generations of wingless females develop on flax until early August, when winged females and males appear in response to diminishing daylight. These winged adults migrate back to the winter hosts, resulting in rapid drops in aphid populations on the summer hosts. Adults on the winter host mate and females produce eggs for overwintering.

Effect On Crop

Adults and nymphs of potato aphids damage flax by extracting plant fluids from the stems, leaves and developing bolls. Most of the yield reduction potato aphid can cause is due to fewer seeds being produced. The weight of individual flax seeds can also be reduced, but this results in relatively little of the yield loss compared to the reduction in the number of seeds. Potato aphids have no effect on the oil quality of flax. Plants under drought stress may be prematurely desiccated by high aphid densities. Potato aphids can cause yield losses of 20% or more in oilseed flax when it reaches densities of 50 or more aphids per plant.

Scouting Techniques

The easiest way to detect aphids in flax is to sample the upper portions of the plant with an insect sweep net when the crop is in full bloom. If aphids are found, fields need to be more closely inspected by randomly collecting plants.

To inspect plants, sever stems at the base, and lightly tap the severed plant on a white surface, such as a tray, to dislodge the insects. Inspect a minimum of 25 plants at full bloom and 20 plants at early green boll randomly in the field to provide an accurate estimate of aphid density. Additional plants may need to be sampled if aphid levels are close to the economic threshold (see sequential sampling decision plan shown below). Record total number of aphids and calculate average per plant.

If control is not warranted at full bloom, aphid densities should be assessed again at the green boll stage.

Economic Thresholds

The economic threshold for potato aphids in flax is three aphids per main stem at full bloom and 8 aphids per main stem at the green boll stage. See the sequential sampling decision plan shown below for more specific economic thresholds.

Sequential Sampling Decision Plan For Potato Aphids In Oilseed Flax

Upper and lower cumulative counts of potato aphids are used to make control decisions. Aphid densities below the lower limit should not be sprayed, while counts between the upper and lower limits should continue to be sampled until a decision on whether or not to spray can be made. If cumulative aphid numbers exceed the upper limit control measures should be applied within 48 hours.

A single application of a foliar insecticide at full bloom or the green boll stage will control the potato aphid until harvest.

The yield loss of flax is 0.021 t/ha per aphid per plant for crops sampled at full bloom and 0.008 t/ha per aphid per plant for crops sampled at the green boll stage.

The potato aphid is highly susceptible to attack by fungi (especially in years of high rainfall and humidity in late June and July). Aphid populations sampled at full bloom that have many diseases insects should be sampled again at the early green boll stage to determine the effect of the disease on aphid densities.