Outcrossing of non-Bt cotton (Gossypium hirsutum (L.)) in refuges by transgenic Bt cultivars could
reduce the efficacy of refuges for delaying resistance in seed-feeding
pests. Based on reports that outcrossing decreased as distance from Bt cotton
increased in small-scale studies, we hypothesized that increasing refuge
width or distance from Bt fields would reduce outcrossing. In a large-scale
study in Arizona, we quantified Bt seed in refuges of experimental and
commercial fields, comparing outcrossing between in-field (narrow) and
external (wide) refuges and among rows of refuges at various distances from
Bt fields. Some refuges, including those in tightly controlled experimental
plots, contained up to 8% adventitious Bt plants. Some, but not all, Bt
plants likely resulted from Bt seed in the non-Bt seed bags. We did not detect a
difference in outcrossing between in-field and external refuges. However,
statistical power was low because outcrossing was low (< 0.4% of seeds)
in both treatments. Higher outcrossing levels (≤ 4.6% of
seeds) were observed in the studies measuring outcrossing at various
distances from Bt fields, yet outcrossing did not decrease as the distance
from Bt fields increased. We hypothesize that Bt plants in refuges
cross-pollinated surrounding non-Bt plants, overshadowing the expected
association between distance from Bt fields and outcrossing.