Figure 25. Adult lady beetle, Coleomegilla maculata (M. E. Rice).

Figure 26.Lady beetle larva, a predator of European corn borer eggs (M. E. Rice).

All immature stages of the European corn borer are attacked by natural enemies that contribute to reducing its densities. Of these, predators and some parasitic insects usually are seen easily on plants, whereas diseases and small parasitic insects are difficult to find without a microscope. Interestingly, the natural enemies that are most difficult to see may play the greatest role in controlling the European corn borer.

Predators include lady beetles, minute pirate bugs, and predaceous mites that feed on the eggs and young larvae, as well as other insects and birds that feed on other European corn borer life stages. A native lady beetle, Coleomegilla maculata (Figure 25), can be an important egg mass predator, particularly in the eastern United States during second-generation European corn borer egg laying, when the beetle often is abundant on corn and both adults and larvae (Figure 26) prey on egg masses. Picnic beetles sometimes will enter the European corn borer tunnels to feed on plant sap, injuring larvae and crowding them out of the tunnels. The downy woodpecker, ringneck pheasant, and other birds dig overwintering larvae out of their chambers in cornstalks and plant debris.

Figure 27. Average consumption of European corn borer egg masses for each adult Coleomegilla maculata present in tasseling corn based on regression analyses of field data (C. E. Mason and T. Zheng).

Figure 28. Lydella thompsoni adult fly, which parasitizes European corn borer (R. G. Weber).

Figure 29. Lydella thompsoni puparium next to the cadaver of a fifth instar European corn borer in potato (C. E. Mason).

Figure 30. Eriborus terebrans female, a parasite of European corn borer larvae in the north central region of the United States (D. A. Landis).

Figure 31. A Macrocentrus grandii female preparing to insert an egg into a European corn borer larva (S. Udayagiri).

Figure 32. Cocoons of Macrocentrus grandii that have formed and are nearly covering the remains of a European corn borer larva (S. Udayagiri).

In locales with abundant C. maculata, these predators should be considered during cost-benefit analyses. Research indicates that at an egg mass density of 0.25 per plant, each adult C. maculata will consume an average of 0.27 egg mass per day before pollen is shed and 0.16 egg mass during pollen shedding (Figure 27). Fewer egg masses are consumed during pollen shedding because the beetles eat some pollen also. An egg mass is available to predators for 4 days because it takes approximately 4 days to hatch. Therefore, when C. maculata is present, the number of egg masses can be reduced by four times the daily consumption rate for each beetle seen on the same plants as those sampled for egg masses during scouting. The following example will help illustrate this reduction.

A scout samples 100 corn plants and finds 26 European corn borer egg masses and 15 C. maculata adults on the same leaves. The corn has emerging tassels but is not shedding pollen. Because the number of egg masses per plant is very near 0.25, this figure is used in Figure 27 to determine that the consumption rate per beetle is 0.27 egg mass per day. Over a 4-day period, each beetle will consume a total of 1.08 egg masses. Since 15 C. maculata adults were found, 1.08 egg masses times 15 beetles, or 16.2 egg masses can be subtracted from the 26 egg masses found. This leaves 10 egg masses that will not be eaten by the predator and will potentially produce an infestation of European corn borer larvae. As the density of egg masses increases, the consumption per C. maculata adult also increases (Figure 27). At a density of 2 egg masses per plant, the daily consumption is 2.2 egg masses per adult beetle before pollen shedding. At higher egg mass densities, each beetle consumes more egg masses per day because it can find them more often.

Many parasitic insects have been imported from Europe to the United States in an effort to control the corn borer. However, very few have become established. A tachinid fly, Lydella thompsoni (Figures 28 and 29), and two wasps, Eriborus terebrans (Figure 30) and Macrocentrus grandii (Figures 31 and 32), are the major parasitic insects in the United States. European corn borer larvae are consumed and killed by these parasites, which, when combined, attack an average of about 7.5 per-cent of European corn borer larvae in the eastern and midwestern United States. Each species tends to have an area of concentration. Recent studies in the east central United States have found that L. thompsoni is most abundant in North Carolina. M. grandii prevails in a band from northeastern Pennsylvania to eastern Virginia. And E. terebrans is prevalent in Ohio and Michigan. Levels of parasitism in some fields in any state can occasionally be quite high (up to 80 percent). The percentage of European corn borer larvae attacked by parasites may depend upon the shape and size of the field and its proximity to natural or wooded areas.

Studies have indicated that L. thompsoni disappeared from the United States in the 1960s; but, it has been re-established in several states through periodic releases during the past 15 years. L. thompsoni often needs an alternate host in the spring before the European corn borer larvae develop to a stage that can be parasitized by this fly. A variety of caterpillars can be alternate hosts for L. thompsoni, including Papaipema nebris, the stalk borer (pictorial key, Figure S). A native fly, Lixophaga spp., parasitizes European corn borer larvae in several states, especially in the mid-Atlantic and southeast regions.

Figure 33. European corn borer egg mass much darker than the blackhead stage typical of egg parasitism by a tiny Trichogramma species (C. E. Mason).

Figure 34. Egg mass of European corn borer being parasitized by a tiny Trichogramma nubilale wasp (C. E. Mason).

A group of tiny wasps, Trichogramma spp., attacks European corn borer eggs (Figures 33 and 34). Several species are used in Europe and Asia to control European corn borer or its close relative Ostrinia furnacalis, the Asian corn borer. A native U.S. species, Trichogramma nubilale, has been found parasitizing eggs in Delaware (Figure 34). In Minnesota, this wasp has been released experimentally in small plots of sweet corn to control European corn borer. However, the quality of released wasps is still too inconsistent for reliable use in U.S. cropping systems. A closely related wasp, Trichogramma ostriniae, has been recently introduced from China and has been tested in the eastern United States. An average of more than 97 percent of the European corn borer eggs in a field population was parasitized when experimental releases were made at a density of 202,000 wasps per acre in sweet corn in Delaware. This indicates that the species has a great promise for augmentative releases in biological control of European corn borer. Improved technologies for the rearing and release of Trichogramma for control of European corn borer are being developed to make this strategy more economically feasible.

Two types of diseases are very common in European corn borer populations. A fungus and two microsporidia can collectively infect 10 to 90 percent of the larvae in a field, depending on environmental conditions. However, few larvae are killed by these pathogens during whorl-stage and tasseling-stage corn development.

Figure 35. Dead larva of a European corn borer that was killed by Beauvaria bassiana, a pathogenic fungus (M. E. Rice).

The fungus, Beauvaria bassiana, can kill many overwintering larvae (Figure 35) following a season with environmental conditions favorable for the pathogen. Most epidemics of this beneficial fungus occur during and after periods of ample rainfall late in the season with temperatures in the mid-80os F (about 28 degrees C). Infective spores of B. bassiana naturally exist in the soil and on or in plant debris throughout the United States. The predominant microsporidium that infects the European corn borer is Nosema pyrausta. This protozoan-like microbe reduces egg laying, kills some larvae, and increases over-wintering mortality. Mortality caused by this disease increases when European corn borer larvae are stressed by other factors, such as harsh weather. An unclassified microsporidium in the genus Nosema is not as common as Nosema pyrausta, but its prevalence is increasing in Iowa and Illinois. It is more virulent, thus causing greater mortality than N. pyrausta.

Bacillus thuringiensis subspecies kurstaki (Berliner), commonly known as Bt, is a bacterium that produces spores and protein crystals that are toxic to European corn borer larvae. Commercial formulations, both granular and liquid, are effective against whorl, sheath, and collar feeding on corn plants, when applied by conventional methods or through pivot irrigation. Bt especially is useful in European corn borer control because, other than to caterpillars, it is virtually nontoxic to other organisms, including mammals, birds, fishes, and beneficial insects.