Values for resistant standards are totals for ratings
1 to 3. Percentage of plants surviving may be higher but
may include many with little or no resistance.
DISTRIBUTION AND SEVERITY OF
Blue Alfalfa Aphid, Acyrthosiphon
Click on the map above for a larger version. See also the KEY.
CORRELATION TO FIELD REACTION
Although there have not been extensive
comparisons of greenhouse and field results, it appears that levels of
resistance are comparable in both situations.
Although biotypes of blue alfalfa aphid are
not proven to exist, there is evidence of differential reactions to
resistant plants in different locations.
Use of cultivars for rearing blue alfalfa
aphids that have resistance to spotted alfalfa aphid and pea aphid may
help to prevent colony contamination (PA-1). The best procedure for
collecting aphids from fields for colony establishment is tapping from
infested stems. Fewer will be injured and chances of including natural
enemies will be much reduced compared to sweeping. Field-collected
aphids should be held in isolation for 2 to 3 weeks to check for
presence of parasites.
A field-cage technique has been used for
evaluating blue alfalfa aphid resistance in the Southwest. Rows of PA- 1
are planted in plots during fall. In spring, plots are caged and alfalfa
is infested by sprinkling aphids over plants. As the aphid population
increases, test entries and additional rows of PA-1 are planted between
established rows of PA-I. As the test entries emerge, seedlings are
counted and estab lished rows of PA-1 are cut back to force aphids onto
seedlings. When rows of spring-sown PA-1 have been killed, the
infestation is terminated by spraying with malathion, and the numbers of
surviving plants in the test entries are counted and compared with
original plant counts to calculate % resistant plants.
Ellsbury, M.M., and M.W. Nielson. 1981. Comparative host plant range
studies of the blue alfalfa aphid, Acyrthosiphon kondoi Shinji,
and the pea aphid, A. pisum (Harris) (Homoptera: Aphididae). USDA
Bull. No. 1639.
2. Lloyd, D.L., B.A. Franzmann, and T.B.
Hilder. 1983. Resistance stages of plant growth to spotted alfalfa aphid
and blue-green aphid. Aust. J. Exp. Agric. Anim. Husb. 23:288-293.
3. Nielson, M.W., W.F. Lehman, and R.T.
Kodet. 1976. Resistance in alfalfa to Acyrthosiphon kondoi. J.
Econ. Entomol. 69:471-472.
4. Zarrabi, A.A., J.L. Caddel, and R.C.
Berberet. 1985. Blue alfalfa aphid resistance evaluation as affected by
seedling age. Proc. Central Alf. Imp. Conf. 19:26-27.