The blue alfalfa aphid (BAA), Acyrthosiphon kondoi Shinji, has become an important pest in several western states, including Oklahoma where the highly virulent biotype ‘BAOK90’ has been identified (Zarrabi et al. 1995). Stern at al (1980) reported that the original BAA biotype prefers to feed on the tender succulent parts of the alfalfa (Medicago sativa L.) plant and aphids cluster on the terminal growth. With higher populations, they feed on upper parts of stems and undersides of leaves. In contrast, Berberet et al. (1991) demonstrated that spotted aphids, Therioaphis maculata (Buckton), show a consistent preference for feeding in lower portions of the plant canopy. Since the biotype BAOK90 is known to cause severe losses in productivity and stand density of alfalfa, it is critical that accurate sampling procedures be available to assist in decision-making relative to use of insecticides. The purpose of this research was to determine the within-plant spatial patterns of BAOK90 to improve sampling procedures.

The study was conducted in a greenhouse at 22C with 16 h photophase in two experiments, each consisting of eight plants of the susceptible cultivar ‘OK08' grown in 15 cm diameter pots. The plants were cut back and when stems had 4-5 nodes, all except three stems per plant were clipped at the crown. Three adult BAA were then placed on the mid-portion of each stem. Aphid numbers ( nymphs and adults) were then recorded for each petiole, leaflet, axillary branch, and internodal stem section of each node per stem at 2-day intervals up to 10 days postinfestation. Because the number of nodes varied from stem-to-stem and plant-to-plant, stems were divided into bottom, middle, and top regions for purposes of data analysis. As closely as possible, nodes were apportioned equally among stem regions.

Total numbers of aphids per stem varied from 93 to 1,333 at 10 days postinfestation. Significant differences were determined for total aphid numbers among stem regions and aphids on node parts (ie. petioles, leaflets, axillary branches, and internodal stem sections) by stem regions for each day postinfestation. Interactions among stem regions (bottom, middle, and top), postinfestation days, and nodepart were assessed and simple effects of each factor were examined in the presence of significant interaction. The within-plant aphid dispersal validated that the biotype BAOK90 differs from the original BAA biotype. This biotype prefers to feed on bottom and middle stem sections of the plant. Internodal stem sections were favored over the petioles, leaflets, and axillary branches. Leaflets were only favored when they were near the terminal buds. Results show that accurate assessments of BAA (BAOK90) population density can be obtained only when entire stems are sampled.

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Plant Patterns of Therioaphis maculata on Resistant, Tolerant, and

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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(5):1461-1465.