Trichogramma ostriniae

Learn more about Trichogramma ostriniae

Also known as "Tost"

Release rates and timing for European corn borer control

Because Tost is able to disperse and reproduce in the field, some of the first research in corn examined the potential for a single inoculative release of 30,000 per acre to reduce damage, with the expectation that the population would grow in the field and colonize adjacent plantings.  Damage reduction was significant in sweet corn but not field corn using the single inoculative release approach (Wright et al., 2002, Hoffmann et al., 2006). Later trials and on-farm demonstrations looked at multiple, higher rate releases per field, timed to coincide with the European corn borer (ECB) flight and egg-laying activity. The following release rates were used in a multi-state on-farm demonstration project conducted 2007-2009 (Seaman et al., 2010).

Highest rates of parasitism have been found to be within a 100 ft. radius of release points (Wright et al., 2001). Release-point spacing in the on-farm demonstrations referenced above assumed an effective dispersal radius of 50 ft. Release points were typically spaced 100 ft. apart in the field, at least 50 ft. from field edges. 

Soon after the on-farm demonstrations referenced above were conducted, ECB populations began to drop across the U.S. (Hutchison, et al., 2010). Populations of ECB still persist in some areas, so anyone considering releasing Tost for ECB management should confirm that damage and larvae found in sweet corn or peppers is ECB (instead of a different pest), and that populations are still present in your area.

Sweet corn

European corn borer 

Because the wasps disperse so well in the field, data directly comparing European corn borer (ECB) damage plots in which Trichogramma ostriniae (Tost) was released with untreated plots has been difficult to collect because parasitized egg masses are soon found in the untreated plots. Most measures of effectiveness have been collected from on-farm trials without untreated controls. In general, releases of Tost reduce damage from ECB. Whether the reduction in damage is useful compared with alternative methods depends on ECB pressure, farm resources (e.g. access to a sprayer for applying insecticide), relative cost, and grower preferences. Weather can influence levels of control by Tost. Their ideal developmental temperature range is 82-91°F (Wang et al. 2004). Seaman (anecdotal) has observed reduced levels of parasitism during prolonged cool, rainy conditions.

High rates of parasitism have been observed in the field, but even when parasitism levels are in the 80-100% range in sweet corn, damage levels in harvested ears can be unacceptably high when ECB pressure is very high (Seaman et al., 1997). In situations with lower pressure, damage levels at harvest can compare with those achieved by insecticide applications. Unusually high pheromone trap catches can be an indicator of high pressure. In sweet corn, on-farm demonstrations releasing wasps in early season corn that is in the four-leaf or larger stage during the first-generation ECB flight, combined with scouting and the use of a 15% of plants infested threshold at the tassel emergence stage resulted in 57-79% of fields being under threshold and not needing a spray at tassel emergence (Seaman et al. 2010). These demonstrations were conducted between 2007 and 2009 before ECB populations dropped, and at that time, most early season sweet corn fields were over threshold at tassel emergence (Seaman, anecdotal observation). Tost can reduce insecticide use in early season sweet corn when ECB is the only pest present, and can reduce damage overall in fields where insecticide treatments are not an option. Higher rates and repeated releases have tended to result in higher levels of control.

Other sweet corn pests

Three other Lepidopterous pests in sweet corn, fall armyworm (Spodoptera frugiperda), corn earworm (Helicoperva zea), and western bean cutworm (Striacosta albicosta) are often present concomitantly with ECB mid-late season. We have seen no evidence of substantial levels of parasitism of either corn earworm or fall armyworm eggs in our trials.

Releases targeting western bean cutworm (WBC) in sweet corn and field corn (90,000/A for three or four consecutive weeks during the WBC flight) found an average 59% of egg masses were parasitized, with an average of 64% of eggs in an egg mass parasitized. These levels of parasitism did not reduce damage in either sweet corn or field corn in areas with very high populations, but in an area with lower populations, damage levels were acceptable in two sweet corn release fields (Seaman, 2017). More work is needed to confirm optimal release rates and WBC population levels at which acceptable control levels can be expected.

Pepper and potato

Kuhar et al. (2004) conducted trials including untreated controls replicated by location in pepper and potato in four states over two growing seasons. The table below summarizes their release approach and results:

Summary of efficacy

Trichogramma ostriniae can reduce damage from European corn borer in a number of crops. Combining Tost releases with scouting and the use of thresholds at tassel emergence in early season sweet corn during the first-generation ECB flight has been shown to reduce the number of fields needing a spray to control larvae on the emerging tassel. In general, Tost can reduce damage from ECB in a number of crops, but unfavorable weather conditions can result in reduced levels of control compared with insecticides, making Tost a more attractive option in situations such as certified organic production, or on farms lacking an effective sprayer, where conventional insecticide use is not an option.