Trichogramma releases in North Carolina cotton: Why releases fail to suppress heliothine pests

Field studies were conducted in 1996 and 1997 to determine the fate of natu rally oviposited F3 heliothine eggs in cotton plots treated with augmentati ve releases of Trichogramma exiguum Pinto & Plainer and nontreated plots. F our cohorts of newly oviposited eggs (<24 h old) were followed in 1996 and two cohorts in 1997. In 1996, mean +/- SD percent parasitism, estimated by in-field studies following the fate of naturally oviposited eggs, ranged fr om 7 +/- 7 to 61 +/- 8% in T. exiguum release plots and 0 +/- 0 to 35 +/- 1 3% in control plots. The mean I SD percent of eggs hatched in T. exiguum re lease plots ranged from 1 +/- 2 to 11 +/- 4% and 7 +/- 4 to 28 +/- 10% in c ontrol plots. In 1997, mean +/- SD percent egg parasitism ranged From 27 +/ - 4 to 40 +/- 3% in T. exiguum release plots and 15 +/- 18 to 25 +/- 8% in control plots. The mean +/- SD percent of eggs hatched in T. exiguum releas e plots ranged from 7 +/- 3 to 12 +/- 2% and 18 +/- 6 to 28 +/- 8% in contr ol plots. Despite increased parasitism and reduced egg hatch in T, exiguum release plots, overall, there was no significant difference in larval densi ty (all instars combined) between T. exiguum release and control plots. Com bined analysis of the heliothine larval populations and egg fate data revea led that the additional egg mortality produced by released T. exiguum was o ffset by lower larval mortality in release plots. Because of the occurrence of compensatory mortality, the egg stage of heliothines is not an appropri ate target for biological control using Trichogramma wasp releases.