Susceptibility of the boll weevil to Steinernema riobrave and other entomopathogenic nematodes

The susceptibility of the boll weevil (BW), Anthonomus grandis Boheman, to Steinernema riobrave and other nematode species in petri dishes, soil (Hidalgo sandy clay loam), and cotton bolls and squares was investigated. Third instar weevils were susceptible to entomopathogenic nematode (EN) species and strains in petri dish bioassays at 30 degrees C. Lower LC50's occurred with S. riobrave TX-355 (2 nematodes per weevil), S. glaseri NC (3), Heterorhabditis indicus HOM-1 (5), and H. bacteriophora HbL (7) than H. bacteriophora IN (13), S. riobrave TX (14), and H. bacteriophora HP88 (21). When infective juveniles (IJs) of S. riobrave were applied to weevils on filter paper at 25 degrees C, the LC50 of S. riobrave TX for first, second, and third instars, pupae, and 1-day-old and 10-days-old adult weevils were 4, 5, 4, 12, 13, and 11 IJs per weevil, respectively. The mean time to death, from lowest to highest concentration, for the first instar (2.07 and 1.27 days) and second instar (2.55 and 1.39 days) weevils were faster than older weevil stages. But, at concentrations of 50 and 100 IJs/weevil, the mean time to death for the third instar, pupa and adult weevils were similar (1.84 and 2.67 days). One hundred percent weevil mortality (all weevil stages) occurred 3 days after exposure to 100 IJs per weevil. Invasion efficiency rankings for nematode concentration were inconsistent and changed with weevil stage from 15 to 100% when weevils were exposed to 100 and 1 IJs/weevil, respectively. However, there was a consistent relationship between male:female nematode sex ratio (1:1.6) and nematode concentration in all infected weevil stages. Nematode production per weevil cadaver increased with increased nematode concentrations. The overall mean yield of nematodes per weevil was 7680 IJs. In potted soil experiments (30 degrees C) nematode concentration and soil moisture greatly influenced the nematode efficacy. At the most effective concentrations of 200,000 and 400,000 IJs/m2 in buried bolls or squares, higher insect mortalities resulted in pots with 20% soil moisture either in bolls (94 and 97% parasitism) or squares (92 and 100% parasitism) than those of 10% soil moisture in bolls (44 and 58% parasitism) or squares (0 and 13% parasitism). Similar results were obtained when nematodes were sprayed on the bolls and squares on the soil surface. This paper presents the first data on the efficacy of S. riobrave against the boll weevil, establishes the potential of EN to control the BW inside abscised squares and bolls that lay on the ground or buried in the soil.