THE AMAZING APHID
Aphids are tiny, soft-bodied sucking insects that feed on several host crops, such as pea, potato, flax, canola and soybean. Some species are carried north by wind, and usually begin appearing in June or early July. The pea aphid and potato aphid have been known to overwinter in the Prairies. If fields in a region were infested last year with these species, fields of susceptible crops may be at risk again the next year.
Because they are small and soft bodied, some species of aphids are susceptible to wind and rain knocking them from plants, helping to reduce numbers and negative impact on yield. Heavy rains can significantly reduce numbers of some species of aphids. Their soft bodies also make them susceptible to contact insecticides. Because aphids pierce the plant and suck the sap, systemic insecticides can offer effective control, especially in a crop such as canaryseed, where the aphids hide in the head of the plant.
Some species of aphid give birth to live young in the early part of the year but will lay eggs later in the season. An adult female can several offspring in her lifetime, and if conditions are right, a single aphid can go from live birth to procreating in one week.
Aphids, though prolific, have many enemies. Some species of ladybird beetles (A. K. A ladybugs) will eat up to 100 per day. Lacewings also snack on the little monsters, as do hover flies and pirate bugs. Several parasitic wasps attack them as well. The absence of natural predators, it’s been said, would mean an outbreak of aphid every year in almost every crop.
John Gavloski, extension entomologist with Manitoba Agriculture, Food and Rural Initiatives, says that those who have sprayed a broad-spectrum insecticide early in the year may want to be extra vigilant in scouting for aphid later in the season.
Perhaps most amazingly, adult aphids, when severely crowded, can actually spontaneously sprout wings and fly to a new field. This is of particular concern for the soybean aphid, whose numbers can get extremely high before spraying is economically warranted.
The aphid is a formidable foe. Crops can be attacked by several species, and it’s not always easy to tell one species from the other. Thresholds can vary from three per tip to 250 per plant, depending on the aphid and the crop.
Of most concern for Saskatchewan is the pea aphid. Canaryseed growers deal with aphids almost annually. Green peach aphids on potato and potato aphids on flax can occasionally reach levels of concern. Several species feed on canola though rarely causing economic damage. And there have been reports of an unconfirmed species on lentils, as well. Scott Hartley, insect specialist with the Saskatchewan Ministry of Agriculture, says that aphids are not a consistent threat for much of his province, but they are a growing problem, especially in the eastern half of the province.
“Our biggest concern is the pea aphid on peas,” Hartley says. “Aphids have sucking mouthparts and pierce the leaves and stem of the plant to suck sap.” Leaf feeding is of secondary concern to stem feeding, as sucking the sap from the stem at flowering and early pod fill significantly impacts yield.
While several sprays are registered for aphid control on pea, thresholds are based on work done in the U. S. and Manitoba only. “The only definitive pea aphid work done on the Prairies was done some time ago in Manitoba by Dr. Bob Lamb on Century peas, an older variety,” Hartley says. He’d like to see new work done on more contemporary varieties to get a more accurate picture of what is an economic threshold with modern varieties in different growing regions. As it stands, the given threshold of two to three adults per plant tip may be too low depending on the health of the plant stand and, more importantly, the maturity stage of the crop.
“Ideally, you need to get in and spray during flowering to protect the plant at early pod fill,” Hartley says. Heavier infestations later in the season may not warrant a spray as the damage is either done, or the feeding isn’t likely to cause significant damage.
SCOUT EARLY AND OFTEN
Aphids are a tricky pest. Several species, such as those that attack soybean crops, blow up from the U. S., while others, such as the pea and potato aphid, can overwinter. John Gavloski, extension entomologist with Manitoba Agriculture, Food and Rural Initiatives, says that it’s this species-specific behavior that makes scouting so important.
Scouting for aphid is crop specific. Sweep nets work well for pea crops, but flax stems should be either snipped and tapped into a bucket or onto a plate, or tapped while in the field. For aphids on peas, time is of the essence if high numbers are found. “If you have critical numbers early in flowering, it’s important to spray before plants get too far into the podding stage,” Gavloski cautions.
Hartley says that aphid populations can be random throughout a field. “It’s impossible to know where wind currents have carried the aphids, so it’s important to scout entire fields, not just field margins,” he says. He adds that it is possible to have half a field infested and the other relatively clean, meaning that thorough scouting should ensure spraying is only done where it’s necessary.
Farmers don’t like seeing insects on their crops, and some get rather antsy when plants are caked in them, but Gavloski warns that when it comes to soybean aphid, the threshold is incredibly high. “The average population needs to hit 250 per plant and rising before spraying is considering economic,” he says. “This means scouting has to happen several times to determine if the population is growing. It can be hard to look at that many insects on a plant and not want to spray.”
In the case of pea or potato aphid the threshold is much lower, but the trick is to accurately determine the average number of aphids per plant. “When you’re scouting, it’s important to be random, but our eyes are naturally drawn to damaged or insect-infested plants,” Gavloski says. “When scouting flax for potato aphid, it’s just as important to count all the stems with zero aphids as it is to count those with 10. It’s the average number, not the total that matters.”
REGISTERED SPRAYS FOR APHID
Several insecticides are registered for aphid. Only one active ingredient, dimethoate, works through contact, ingestion and systemically — a major plus for canaryseed growers in particular. Sold as Lagon or Cygon, Hartley says the systemic action is critical for those very-hidden aphids on canaryseed.
Gavloski adds that all other registered sprays, such as Matador and malathion, work through contact and ingestion only. “Contact and ingestion may be just as effective as systemic insecticides on certain crops, such as soybeans, where the aphids are very exposed and in high numbers,” he says. The ingestion aspect of sprays will provide some short-term residual insecticidal properties, however weather conditions significantly impact the length of time from a few to several days.
As always, for complete crop-specific registration and rates, consult the most recent Guide to Crop Protection.
While aphids don’t necessarily cause significant yield damage in cereal crops unless in very high numbers, some species can be vectors of disease, such as barley yellow dwarf virus. “Aphids can spread disease as they inject saliva into the plants while feeding. For some diseases, the aphids’ dirty mouthparts can move the disease from plant to plant,” Gavloski says. Greenbug aphid feeding can also leave dead spots on stems and leaves, decreasing the overall amount of photosynthesizing potential of the plant.
As for predicting where aphids will attack and at what level, both Hartley and Gavloski say that the wind is the biggest determining factor and not one you can plan for. “From what we currently know, soybean aphid is carried into Manitoba mainly by wind, making it difficult to predict infestation levels ahead of time,” Gavloski says. Some species of aphids may be of economic concern only in localized areas in a given year. For example, in 2008 potato aphid on flax was very localized, causing an issue only in Manitoba’s south central area near Pilot Mound. Hartley says that the western half of Saskatchewan need not worry — it’s only the eastern half of the province who should be on the lookout for the pest.
Lyndsey Smith is a field editor for Farm Business Communications. She’s based in Regina.