Texas Boll Weevil (Anthonomus grandis grandis Boheman) Susceptibility Status to Malathion in Support of the Texas Boll Weevil Eradication Program, 2008

A boll weevil eradication program has been ongoing in Texas since 1994.  The only insecticide used by this program is Ultra Low Volume (ULV) malathion.  A monitoring program evaluated the susceptibility of adult boll weevil populations to malathion using the adult vial test.  Cotton fruit with weevil egg-laying punctures were collected from the field and incubated for 6-7 days until pupae were extracted and kept over vermiculite.  To assess mortality, upon adult emergence, 2-3 d old weevils were exposed to glass vials coated with different concentrations of malathion diluted in acetone.  Data were collected from three Texas counties and analyzed for LC₅₀ and LC₉₀ resistance ratios and maximum likelihood tests of equality and parallelism using the program PoloPlus.  Results were compared with data obtained in 2000 from a susceptible laboratory colony from Mission, TX.  In 2008, Cameron, Medina, and Zavala Co. LC₅₀ resistance ratios were 4.7, 14.9, and 10.9, respectively, and all three populations were significantly more resistant than the Mission population based upon maximum likelihood tests for equality.  The levels of resistance observed this season are greater than in recent years, however, they are not as high as levels observed in 1999.   The higher resistance ratios observed could explain poor control of weevils in the counties under study if observed, and further tests will be needed to determine if malathion will continue to be a successful insecticide to use for the Texas boll weevil eradication program in these counties.