Yield and Stand Reduction Caused by a New Biotype of Blue Alfalfa Aphid in Oklahoma

A.A. Zarrabi, R.C. Berberet, A.D. Bisges, and J.L. Caddel Oklahoma State University, Stillwater, OK 74078, U.S.A.

In 1995, the existence of a new blue alfalfa aphid (BAA) biotype designated as "BAOK90" was confirmed in Oklahoma (Zarrabi et al. 1995). The purpose of research reported here was to describe results of infestation by BAOK90 on yield and stand retention in alfalfa as observed in several types of field experiments. Field experiments were planted in September of 1995 and 1996 with entries having varied levels of resistance to BAA arranged in an RCB design with 6 replications. The experiments were naturally infested by BAOK90 in February /March 1997. Aphid population density and forage yield were estimated in both experiments. In first year stand, peak population densities of BAOK90 ranged from 43/stem in the most resistant entry "54H55" (Pioneer) to 93/stem in "OK08" the susceptible check on 31 March, 1997. Although the yield of 54H55 was significantly higher at first harvest than for two other entries developed in Oklahoma for resistance to BAOK90 ("OK 206" and "OK 207"), seasonal yields were not significantly different. In the second-year stand, peak population densities ranged from 88/stem (OK 207) to 195/stem (OK08) on 26 March, 1997. The entry from Pioneer (54H55) was not included in this experiment. The highest yields at both first harvest and for the season were in OK 206 and OK 207, significantly greater than in the susceptible check (OK08).

Insecticides have been evaluated annually for control of BAA and alfalfa weevil larvae (AW) since 1987. The results for 1987 (the original biotype of BAA was prevalent) and 1997 (the predominant biotype was BAOK90) allow for comparison of the potential for yield reductions. Population densities (ca 4.0/stem) and damage attributable to AW were similar in the 2 years. However, in 1987, the untreated plots yielded nearly 3800 Kg/ha at first harvest with a population of 89 BAA/stem, while, in 1997, BAA numbered 53/stem and damage by the more virulent biotype resulted in virtual destruction of the first crop (167 Kg/ha).

Research was conducted from 1995-97 to evaluate spring-grazing vs. early cutting for insect and weed control, and profitability in forage production. Harvest treatments included cleanly grazing stands with cattle for a 3-4-week period during March; prebud harvest in early April near the time of peak occurrence of BAA and AW; and early bloom harvest in May. On subplots for each harvest option, there were sprayed (insecticide and herbicide as needed) and unsprayed treatments. In 1997, the lowest yields occurred in the unsprayed prebud and unsprayed early bloom harvest treatment combinations. Forage yield was 350 Kg/ha at the first early bloom harvest (unsprayed), primarily as a result of damage by BAOK90, which caused a highly significant loss of stand, as well. Results obtained confirm that since 1991, alfalfa lines selected for resistance to the new BAA biotype BAOK90 tend to have lower aphid populations and produce higher yields in the field evaluations. Evaluations of insecticides and harvest schedules have shown that management programs which do not provide for effective and timely control of BAA result in dramatic reductions in yield and loss of stand in alfalfa.

Reference
Zarrabi, A.A., R.C. Berberet, and J.L. Caddel. 1995. New biotype of Acyrthosiphon kondoi (Homoptera: Aphididae) on alfalfa in Oklahoma. J. Econ. Entomol. 88: 1461 1465.