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Several species of aphids are commonly found in alfalfa. Aphids are soft-bodied, small (1mm – 3mm) insects with piercing-sucking mouthparts. They feed on plant phloem, and exude excess carbohydrates in a sugary substance. Feeding damage results in yellowing and stunted plant growth along with transmission of different plant viruses. In addition, the sugary exudate attracts sooty mold that covers the leaf surface and limits photosynthesis. Aphids have long antennae that extend over the back of their bodies at rest. These insects tend to be light to dark green but there are species that have completely black or brown bodies. Wings are present in some aphids, especially when they are migrating to a new host or overwintering sites. Aphids have well-defined cornicles, a pair of tubes at the ends of their abdomen. Cornicles are involved in releasing warning pheromones and are one of the distinguishing features of aphids. Cornicles on some aphids may be greatly reduced to small nubs, and may not be as long and clearly visible as in other species.
Scouting for aphids in alfalfa should be done by collecting stems rather than sweeping because sweeping over-estimates the number of aphids. Many producers are often alarmed by aphids accumulating on their harvesting equipment during harvest, and are tempted to apply insecticides. It is important to remember that just as with sweeping, harvesting equipment collects aphids from vast areas of the field. For that reason, sampling individual stems is a much more accurate method of estimating aphid numbers.
It is important to mention that many aphid predators and parasites frequent alfalfa. Ladybird beetles, syrphid fly larvae, minute pirate bugs, and lacewings as well as many species of parasitoid wasps are usually abundant in alfalfa and provide sufficient control of aphids. Pesticide applications eliminate these predators, however, and should be avoided unless recommended thresholds for aphids (or other alfalfa pests) are reached.
Fields should be scouted weekly, especially in the spring and early summer. Count aphids on at least 30 stems collected from locations well away from the field border (more than 10 m). For larger fields, sampling in multiple areas of the field may be necessary. Identify the species of aphids, and use thresholds for each species described below to make management decisions. These thresholds are expressed in number of aphids per stem.
Pea aphid is by far the most common aphid in alfalfa in South Dakota, but several other species can also be present.
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