As part of a continuing effort to establish natural enemies of the European corn borer, Ostrinia nubilalis, in the USA, the egg parasitoid, Trichogramma ostriniae Pang et Chen (Hymenoptera: Trichogrammatidae), an important native parasitoid of the Asian corn borer, Ostrinia furnacalis (Guenée) was selected for introduction from China. This was accomplished through efforts of entomologists at the University of Massachusetts and USDA-APHIS. Although naturally occurring populations of T. minutum Riley, T. pretiosum Riley and T. nubilale Ertle and Davis, have been reported to parasitize European corn borer eggs on corn in the USA, they generally contribute little to control. In an effort to establish T. ostriniae in the northeastern USA, several million female T. ostriniae were released in New York from 1991 to 1998. Additional releases were made in Massachusetts, Delaware and other locations. Although, European corn borer egg masses were collected each spring following summers of release, no T. ostriniae have been recovered thus far in New York, indicating that it does not overwinter.
Because of its effectiveness against O. furnacalis in Asia, Trichogramma ostriniae was also identified as a possible candidate for augmentative biological control of European corn borer in the U.S.
Like others species of Trichogramma, the adult T. ostriniae is minute (<0.5 mm in length). The body is relatively compact and the antennae are short. This species is light tan in color and the eyes are red. Host eggs that have been parasitized by Trichogramma will turn solid black after about four days.
Because this species apparently does not overwinter in New York, it is only found during the season in which it is being released. Inundative and inoculative releases have been made exclusively in sweet corn and this is the only habitat from which it has been recovered.
Pests Attacked (Host Range)
Most effort to date with T. ostriniae has been directed against European corn borer in sweet corn. There have also been limited trials against diamondback moth, Plutella xylostella in crucifers and against European corn borer in peppers. These are the only two pests known to be attacked under field conditions.
In laboratory studies, eggs of 13 species of Lepidoptera were parasitized by T. ostriniae (Hoffmann et al., 1995). Eggs of the Noctuidae, Pyralidae, and Plutellidae experienced higher levels of parasitism than others tested.
In the absence of food, female wasps live about 2.7 days and produce 22 progeny. If they have access to honey, their lifespan increases to 13.9 days and the number of progeny increases to 86. This implies that the provision of food (e.g., flowers with nectaries) under field conditions may improve the effectiveness of releases.
Typically, 80% of the progeny are female. T. ostriniae can successfully parasitize eggs of the European corn borer until the "blackhead" stage (larvae are within 24 hrs of emerging). This relatively broad window of susceptibility to parasitism increases the likelihood of successful attack and control in an augmentative biological control program.
Trichogramma ostriniae has been shown to be effective against the Asian corn borer, O. furnacalis (Wang et al., 1984; Chiu & Chen, 1986; Zhang, 1988).
In the first field trials with T. ostriniae in the USA, Mason et al. (unpublished), recorded 97.3% parasitism of naturally occurring European corn borer eggs in sweet corn. Additional inundative release studies have shown that this species is capable of parasitizing high levels (>80%) of European corn borer egg masses (Seaman et al. 1997). However, despite the high levels of parasitism, damage to sweet corn was still too high.
More recent work (Hoffmann 1998) has demonstrated that early season inoculative releases show promise. Releases of 30,000 T. ostriniae per acre have resulted in season-long parasitism of corn borer egg masses. The relatively few released wasps, successfully reproduced and dispersed in sweet corn. In 1997, we recorded parasitism levels of 50-80% two months following an inoculative release, even in fields treated with insecticides for control of worm pests. Results in 1998 were similar to those in 1997. Whether or not these releases reduced damage to the crop is not known.
Wang et al. (1997) reported that releases of T. ostriniae made in sweet corn in Massachusetts, were adversely affected by high or low temperatures and that more European corn borer eggs were parasitized in the lower two-thirds of the sweet corn plant than in the upper one-third. They also reported that parasitism was similar on day two or three following a release, suggesting a limited life span for wasps in the field. If T. ostriniae is to be used for inundative release, they recommended that releases should be made every two to three days. Wang and Ferro (1998) have also investigated the functional response of T. ostriniae to various densities of European corn borer egg masses.
In contrast to the recent research with inoculative releases in New York (discussed above), the results reported by Wang et al. (1997) suggest that T. ostriniae does not persist very long in the field. It is possible that the persistence observed in New York is due to this species now overwintering and not due to the inoculative releases. Additional study is required to determine if T. ostriniae is now successfully overwintering.
If Trichogramma is being released, the use of broad-spectrum insecticides should be minimized. Pest-selective insecticides would be preferred.
Adult wasps are at great risk from insecticide applications and most will be killed by applications of broad-spectrum materials. However, wasps developing within host eggs are somewhat protected from insecticides and will develop and ultimately emerge.
Small-scale commercial production has occurred in the US. Wasps were produced by Beneficial Insectary, Redding, CA.
Hoffmann, M. P. 1998. Early season establishment of Trichogramma ostriniae for season-long suppression of European corn borer in sweet corn. pp. 143-146. In 1997 New York State Vegetable Project Reports Relating to IPM. NY IPM Publication 123.
Hoffmann, M. P., D. L. Walker and A. M. Shelton. 1995. Biology of Trichogramma ostriniae (Hymenoptera: Trichogrammatidae) reared on Ostriniae nubilalis (Lepidoptera: Pyralidae) and survey for additional hosts. Entomophaga. 40: 387-402.
Chui Shui-chen and Chen Chien-chung. 1986. Biological control of the Asian corn borer in Taiwan. Plant. Prot. Bull., (Taiwan) 28, 23-30.
Seaman, A., M. Hoffmann, J. Gardner, and S. Chenus. 1996. Pilot testing of Trichogramma ostriniae releases in fresh market sweet corn for control of European corn borer. pp. 149-154. In 1996 New York State Vegetable Project Reports Relating to IPM. NY IPM Publication 121.
Wang, B., D. N. Ferro and D. W. Hosmer. 1997. Importance of plant size, distribution of egg masses, and weather conditions on egg parasitism of the European corn borer, Ostrinia nubilalis by Trichogramma ostriniae in sweet corn. Entomologia Experimentalis et Applicata. 83: 337-345.
Wang, B. and D. N. Ferro. 1998. Functional response of Trichogramma ostriniae (Hymenoptera: Trichogrammatidae) to Ostrinia nubilalis (Lepidoptera: Pyralidae) under laboratory and field conditions. Environ. Entomol. 27: 752-758.
Wang Cheng-lun, Wang Huixian, Gui Chengming & Lu Hong. 1984. Studies on the control of the Asian corn borer, Ostrinia furnacalis (Guenee), with Trichogramma ostriniae. In: US. National Academy of Sciences Joint Symposium on Biological Control of Insects, (P. L. Adkisson & S. Ma, eds) Science Press, Beijing, China, 268-273.
Zhang Zhili. 1988. Trichogramma spp. parasitizing the eggs of Asian corn borer Ostrinia furnacalis and its efficacy in Beijing suburbs. In: Trichogramma and other egg parasites, Les colloques de l' INRA, 43, Paris, 629-632.