In Oklahoma, damaging populations of aphids can occur in alfalfa nearly any time of year; however, infestations are most common during March and April as the first crop is growing. Two of the three key aphid pests in alfalfa, the pea aphid (Acyrthosiphon pisum) and the blue alfalfa aphid (A. kondoi), are found frequently during this period. The spotted alfalfa aphid (Therioaphis maculata) has generally been described as a pest during hot, dry weather. In Oklahoma, however, this species tends to be most abundant during fall, winter, and spring when weather is mild and dry.  

The spotted alfalfa aphid is much smaller than the other species and has light green to tan coloration. Using a magnifying glass, several rows of dark spots can be seen on the back of this aphid. This species prefers to feed on leaves in the lower portions of the plant canopy. As damage to the lower leaves increases, the aphids move onto leaves in the middle and upper parts of plants.  

The spotted alfalfa aphid has the greatest damage potential of the three species. This aphid causes a severe toxic reaction in susceptible alfalfa that often results in discoloration (yellowing) along veins of leaves near the plant terminals. This characteristic symptom of spotted alfalfa aphid infestation is called 'veinbanding'. Heavily infested plants turn yellow, and some leaves often have reddish discoloration.  

Foliage and entire plants may be killed quite rapidly. During fall and winter, damage in new seedlings may be especially severe due to susceptibility of small plants to this aphid.  

Aphid populations in alfalfa may be sampled either by using a sweep net or by pulling stems and estimating numbers per stem. The latter method is encouraged for better precision; however, be certain to cradle the stem as its picked, to avoid dislodging aphids.  

Sampling should not be conducted within 24 hours following hard rains or high winds, as most aphids will not be on plants and an accurate estimate of population density cannot be obtained. Treatment guidelines are indicated in the following table.  
  

Several types of predators aid in limiting aphid numbers in alfalfa. Most important among these are lady beetles, aphid lions, and damsel bugs. A limiting factor on the value of predators in early spring is that aphid numbers often increase much more rapidly than predator numbers.  

In Oklahoma, cultivar evaluation is conducted constantly to determine the alfalfa best suited for production in the state. Several cultivars have adequate levels of resistance to spotted alfalfa aphid. Resistance to aphids greatly reduces chances of serious damage and limits the need for chemical insecticides.  

Throughout the year chemical insecticides play a vital role in an integrated control program. General recommendations should be followed to achieve optimum control with insecticides. These guidelines include:  
 

• Apply chemicals at proper gallonage (at least 10 gal./acre by ground and 2 gal./acre by air);  
• Do not apply insecticides when temperatures are below 50F and/or wind exceeds 8-10 mph.