Aphids Active in Nebraska Spring Alfalfa

Aphids Active in Nebraska Spring Alfalfa May 6, 2016

Examination of an alfalfa field in northeast Nebraska this week revealed some alfalfa weevil larvae (under threshold at just over one larvae/sweep) and a surprisingly high population of pea aphids, Acyrthosiphon pisum. This was somewhat unexpected due to the recent winds and rain which often dislodge aphids from plant stems, resulting in reduced pea aphid numbers.

A predominance of pea aphids was not surprising as this species overwinters in the egg form in Nebraska. The closely related blue alfalfa aphids (A. kondoi) were not noted in this field, as the latter species does not usually survive Nebraska winters and migrates up from southern states. The species is expected to arrive later this month

These two species can be easily differentiated by examining the antennae. The blue alfalfa aphid antennae gradually changes in color from the base to the tip, while the pea aphid has black bands at each antennal segment. Pea aphids are larger and light green compared to the blue alfalfa aphid which is smaller and slightly darker.

Both species have a needle-like mouth part which they use to pierce the alfalfa stem and suck juices from the plant. Occasionally they will be found on the tops of plants and on leaf bottoms.

During the spring, aphid populations are expected to be 100% female. They give birth to live aphids (rather than laying eggs) at the rate of approximately 35 aphids over their lifetime (five/day for a week starting at about six days of age). Aphid populations can increase quickly if not kept in check.

Biological control insects found in spring alfalfa
Figure 1. Scouting in one field in northeast Nebraska found a number of biological control insects along with spring alfalfa aphids. There are three ladybird beetle species (left to right): the convergent ladybeetle, Asian multicolored ladybeetle (each eating a pea aphid), and spotted ladybird beetles. Below the convergent ladybeetle and next to an aphid is a parasitic wasp and in the center, you can see a damsel bug (long and brown). (Photo by Michael Rethwisch)

Several factors should be considered when deciding how to manage aphids on established Nebraska spring alfalfa:

  • aphid numbers and species,
  • plant height,
  • rains,
  • numbers of biological control species (wasps, ladybird beetles, other predators),
  • weeds in the field, and
  • pre-harvest interval.

The most valuable and economic control measures are the biocontrol species. Rains often dislodge the aphids from the stems on which they are feeding, resulting in lower numbers.

Control of Aphids in Alfalfa

In the field visited biological control was evident as there were numerous specimens of three ladybeetle species: the convergent ladybird beetle, the red-pink colored spotted ladybird beetle and the larger Asian multicolored ladybeetle (Figure 1). While most ladybird beetles primarily eat aphids, the larger ladybird beetles in alfalfa (such as the seven-spotted lady beetle which was not noted in the field) have also been documented to eat alfalfa weevil. Ladybeetles, the best biocontrol agents of aphids, have been documented to eat as many of 20-30 aphids and weevil larvae per day.

Table 1. Treatment thresholds for spring alfalfa aphids.
Plant HeightPea AphidsBlue Alfalfa Aphids
Under 10 inches 40 to 50 per stem 10 to 12 per stem
10 to 20 inches 70 to 80 per stem 40 to 50 per stem
Over 20 inches 100+ per stem 40 to 50 per stem

Two other biocontrol agents were visible: a parasitic wasp (probably Aphidius sp.) and a few damsel bugs. The wasp lays eggs in aphids, which then hatch and eat the aphid from the inside, leaving a mummy when it pupates in about a week. Note: Aphids will continue to give birth for several days after wasp eggs are inserted. Damsel bug effects on aphid populations have not been well established as damsel bugs will also eat other insects larger than aphids.

If biocontrol species are not numerous, insecticides may need to be utilized if the stems are short and not close to harvest. While systemic insecticides are effective against aphids on alfalfa, be aware of blooming plants in the field, and treating the field may be prohibited by the insecticide label. Use of pyrethroid insecticides often can eliminate many of the biological control species, resulting in higher levels of aphid numbers, especially on the lower plant due to crop treatment intercept.

Stem heights and aphid numbers help determine treatment thresholds (Table 1), with taller stems supporting higher aphid numbers. Insecticides registered for Nebraska alfalfa, rates and pre-harvest intervals can be found in the 2016 Guide for Weed, Disease, and Insect Management in Nebraska (UNL130)

As some alfalfa fields have already been harvested, it is recommended to scout the regrowth which is at risk for aphid feeding damage.