Blue alfalfa aphid, Acyrthosiphon kondoi Shinji

The blue alfalfa aphid, Acyrthosiphon kondoi Shinji, was first identified by entomologists in the spring of 1975 in the Imperial Valley of California. Since then it has become widespread throughout the state and has become established in Arizona, Nevada, New Mexico, and as far east as Kansas and Oklahoma.  Both adults and nymphs feed on photosynthetic fluids from the leaves and stems of the alfalfa plant. Low to moderate populations of blue alfalfa aphid may cause little to no visible yellowing of plants. Blue alfalfa aphid feed in the new growth, at the tips and young leaves. After prolonged feeding, leaves will eventually turn yellow, starting at the veins, leaves will curl and wilt, and turn necrotic (dead).  Heavily damaged plants may become entirely defoliated except for a few leaves at the end of the stems a result of the aphids moving higher into the plant. Under high aphid densities, growth is slowed after cutting. Infestation by weeds and general susceptibility to stress is increased; stand  longevity may be decreased after damage by aphids.                    

Cooler weather is ideal for pea and blue alfalfa aphid population growth.  Distinguishing between the pea and blue alfalfa aphid is critical for proper treatment (see Godfrey’s blog here, //ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=9639).  This can be done with a hand lens and using the ‘aphids found in alfalfa’ key located here, http://ceriverside.ucanr.edu/newsletters/December_201034750.pdf.  Both aphids are blue-greenish in color (the pea aphid has a “pinkish” biotype that is less common).  The aphids are easily separated by looking at their antennae.  The pea aphid has 3 to 4 narrow dark bands (annuli) along the antennae which are tan in color whereas the antennae of the blue alfalfa aphid are uniformly brown.

A significant IPM “tool” is that of host plant resistance which is bred into many alfalfa varieties. The most effective means of controlling pea and spotted alfalfa aphids is planting resistant varieties; CUF 101 is highly resistant (>50% resistant plants) to spotted, pea, and blue alfalfa aphids. UC Cibola is highly resistant to spotted alfalfa aphid, resistant (31-50% resistant plants) to pea aphid, and has a low resistance (6-14% resistant plants) to blue alfalfa aphid. The resistance differs by variety and this information is summarized here, http://www.alfalfa.org/pdf/2013%20NAFA%20Variety%20Leaflet.pdf. 

At temperatures of 15°C (59°F) plant host resistance is only partially effective against aphid pests, but at 20°C (68°F) plant resistance is fully “functioning”. Once the conditions get to 85°F daily high and 70°F night temperatures, resistance should be 100% active. So, in cooler weather plant host resistance is minimal while development of pea and blue alfalfa aphid is optimum. The development “window” of blue alfalfa aphid is 27.1 °C (80.8°F) and the lower is 3.5°C (38.2°F). Once we reach an average day/night temperature of 27.1 °C (80.8 °F) we should see blue alfalfa aphid populations drop off and plant host resistance fully “functioning”.

It is not uncommon to see some build-up of blue alfalfa aphid and pea aphid in the spring before the temperatures warm. Alfalfa aphid infestations are not consistent from year to year with some fields heavily infested with aphid, well above threshold, and other fields have low numbers of aphids. The economic infestations of blue alfalfa aphid are higher this year than in recent years. Insecticide applications have been warranted in many fields this year, and in some cases additional products have been used to manage aphids (See Godfrey’s blog here, //ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=9639). An alternative to applying chemical insecticides is early harvest of the field as long as yield is not significantly compromised.  Do not treat alfalfa with insecticides until the economic treatment level for a specific pest has been reached and the predator and parasite populations have been assessed for their potential role in controlling the pest.  Use of broad spectrum insecticides like pyrethroids should be avoided as their use can actually exacerbate the aphid problem by removing many critical natural enemies that manage aphids (predator-prey population dynamics).

If you are assessing fields in the southern half of the state, watch for aphid mummies which indicate the presence of parasitoids which aid aphid management, http://www.ipm.ucdavis.edu/PMG/C001/m001epaphidne.html.  Several species of lady beetles, green lacewings, bigeyed bugs, damsel bugs, and syrphid fly larvae also play a role and should be conserved. Several species of parasitic wasps are found in alfalfa: Hyposoter exigua is a parasite of beet armyworm and western yellowstriped armyworm, Cotesia (Apanteles) medicaginis is a parasite of alfalfa caterpillars, and Trichogramma spp. are egg parasites of various caterpillar pests. Parasitic wasps that attack aphids in alfalfa include Aphidius spp., Diaeretiella spp.,

Aphid Mummy

and Lysiphlebus spp. Bathyplectes curculionis is an important parasite of the alfalfa weevil. See the table of relative toxicities of insecticides and miticides to find out which pesticides are most compatible with natural enemies, http://www.ipm.ucdavis.edu/PMG/r1900211.html.

Currently there is a working group (Pete Goodell, Larry Godfrey, Eric Natwick and Vonny Barlow) that are gathering information to address the blue alfalfa aphid issue across the state (see Goodell’s blog here, //ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=9786).  We are putting together a “road show” that will be presenting a synopsis of the blue alfalfa aphid issue to date.  Look for it in your area soon.