Two recently prepared formulations (Fyfanon ULV and Cheminova CS) and a one year old formulation (3M MEC) of malathion, were each aerially applied at three different doses to cotton fields near Tucumcari, New Mexico for evaluation against boll weevil. An extensive bioassay utilizing field collected cotton leaves and laboratory reared boll weevils was used to compare the individual activity levels among the different treatments. Mortalities resulting from weevils exposed to treated and untreated leaves were also compared. Mortality data resulting from all treatments for residual ages of 0, 2, 3, 4, 6, 8, 10, 12 and 14 days are shown.

Two doses of the standard Fyfanon ULV formulation demonstrated significant activity for 6 days after treatment. The traditional 0.93 lb AI/acre dose produced 100% mortality compared to 15% mortality in untreated populations on day 0. At 8 days after treatment, the 0.93 lb AI/acre dose produced 11% mortality compared to 9% in the untreated populations which was not statistically different in this study.

All three doses of the Cheminova CS formulation produced significant activity for 8 days after treatment. The 0.93 lb AI/acre dose produced significantly higher mortality compared to mortality in the untreated populations, 66% vs 38%, at 12 days after treatment. This significant residual activity lasted twice as long as that resulting from the standard Fyfanon ULV treatment.

All three doses of the one-year old 3M MEC formulation demonstrated significant activity for 8 days after treatment. At 8 days after treatment, the 0.93 lb AI/acre dose produced 37% mortality compared to 6% mortality in the untreated population. At 10 days after treatment the 0.93 lb AI/acre dose produced 27% mortality compared to 38% mortality in the untreated population.

Significant rainfall impacted residual treatment activity and confounded analysis of residues 8 to 14 days old. However, the recently prepared Cheminova CS formulation demonstrated significant activity at 12 days following treatment.

When coupled with data gathered in similar studies in 1999 and 2000, data from this study strongly suggest that activity for two weeks after treatment may be achieved with an encapsulated formulation of malathion. Such improvements in treatment effectiveness could substantially reduce overall pesticide load, application costs and logistics for boll weevil programs.