|Orr, David - NC STATE|
|Van Duyn, John - NC STATE|
Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 16, 2000
Publication Date: N/A
Interpretive Summary: Trichogramma are minute wasps that primarily attack eggs of moths, some of which are considered agricultural pests. Most attempts using releases of Trichogramma wasps for suppression of cotton bollworm and tobacco budworm in cotton have failed. This study was conducted to determine why releases of Trichogramma in cotton fail to suppress cotton bollworm and tobacco budworm populations, despite high levels of egg attack. We found that man of the cotton bollworm and tobacco budworm eggs that were attacked by Trichogramma wasps would have been dislodged off plants by wind and rain or killed by predators and other factors. Secondly, although releases of Trichogramma wasps significantly increased mortality of cotton bollworm and tobacco budworm eggs, this increased egg mortality was offset by lower mortality of cotton bollworm and tobacco budworm larvae. Because of the potential for this trade-off (compensatory mortality), the cotton bollworm and tobacco budworm egg stage is not recommended as a suitable target for biological control efforts in cotton.
Technical Abstract: Field studies were conducted in 1996 and 1997 to determine the fate of naturally oviposited F3 heliothine eggs in cotton plots treated with augmentative releases of Trichogramma exiguum Pinto and Platner, and non- treated plots. Four 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 from 7 +/- 7 to 61 +/- 8 percent in T. exiguum release plots and 0 +/- 0 to 35 +/- 13 percent in control plots. The mean (+/- SD) percent of eggs hatched in T. exiguum release plots ranged from 1 +/- 2 to 11 +/- 4 percent and 7 +/- 4 to 28 +/- 10 percent in control plots. In 1997, mean (+/- SD) percent egg parasitism ranged from 17 +/- 4 to 40 +/- 3 percent in T. exiguum release plots and 15 +/- 18 to 25 +/- 8 percent in control plots. The mean (+/- SD) percent of eggs hatched in T. exiguum release plots ranged from 7 +/- 3 to 12 +/- 2 percent and 18 +/- 6 to 28 +/- 8 percent in control plots. Despite increased parasitism and reduced egg hatch in T. exiguum release plots, overall, there was no significant difference in larval density (all instars combined) between T. exiguum release and control plots. Combined analysis of the heliothine larval populations and egg fate data revealed that the additional egg mortality produced by released T. exiguum was offset by lower larval mortality in release plots. Because of the occurrence of compensatory mortality, the egg stage of heliothines is not an appropriate target for biological control using Trichogramma wasp releases.