Two pheromone types, or strains, of European corn borer occur in the United States. They correspond to the dominant component in mixtures of E and Z isomers of 11-tetradecenyl acetate produced by a gland at the tip of the abdomen of the female. Females of the E type produce predominantly the E isomer of the pheromone (99 percent E, 1 percent Z), and females of the Z type produce predominantly the Z isomer (3 percent E, 97 percent Z). Males are usually attracted to females of their own type, although some hybridization occurs. In Delaware, where both pheromone types co-exist, about 20 percent of the population is made up of hybrids.

Both types are present in the eastern United States from Massachusetts to South Carolina. However, west of Pennsylvania, the Z type is predominant, and the E type is rarely found. Researchers believe that the Z-type European corn borer is closely associated with corn, whereas the E type is found on other crops, particularly potato, wheat, and weeds, as well as corn. The E type appears to exhibit a higher survival rate on some noncorn hosts than the Z type. In Delaware and some other locations, the E moths terminate diapause sooner and appear 2 or 3 weeks earlier in the spring than Z-type moths. Consequently, in the first generation, the E type is more commonly associated with early hosts, such as potato, wheat, and some early weeds. The Z type emerges later, when corn is usually large enough to allow survival of the first instars. The E type has two or more generations each season, whereas Z-type European corn borers may occur either in one-, two-, or multiple-generational areas. No E-type European corn borer is known to occur in the univoltine, northern ecotype.

Wire mesh pheromone trap with 20-inch base diameter with the base placed below the top of vegetation in an action site. A wire mesh inward skirt at the base has an opening 12 inches in diameter where the lure us centered (C. E. Mason).

Pheromone traps with synthetic lures can be used to monitor European corn borer populations and determine first appearance and generational duration of male moths. In the mid-Atlantic region, both pheromone types commonly occur together. Typically in the southeastern United States, coastal populations are E-pheromone responsive; but, the moth strains may be mixed and then become predominately Z-pheromone responsive when progressing about 300 miles west. In the north central United States, European corn borer moth sampling can be done by using only the Z pheromone lure. When moth pheromone strains are mixed or unknown, both E and Z lures should be used in separate traps placed at least 50 feet apart and maintained throughout the season to determine the strain composition for future reference. It is important to obtain synthetic pheromones from a reliable source because the chemical purity and dispensing quality can significantly influence pheromone trap effectiveness. Very pure and fresh pheromones, obtained by researchers from a chemical supply company and placed in a dispensing septum in the appropriate mixtures of isomers, trapped significantly more (2 to 5 times more) moths compared to some commercially prepared and pre-packaged pheromone dispensers labeled for European corn borer. Pheromone companies are continuing to improve their products.

Pheromone traps can be useful for European corn borer management to project when egg masses or larvae will be present. An advantage of pheromone traps as opposed to light traps is that the catches are predominately European corn borer male moths, making it easier to count them without having to separate them from a mixture of other insects. A reduced-in-size version of the Hartstack-style wire mesh inverted cone trap, with a base 20 inches in diameter and a wire mesh inward skirt at the base with an opening of 12 inches (Figure 5), has been shown to be more efficient in capturing European corn borer moths than a larger commercially available design of this trap (with a 30-inch base diameter), usually sold as a Heliothis trap. Water pan pheromone traps also have been used successfully in some locations.

Pheromone traps should be placed in vegetation that has the highest likelihood of harboring moths. Trap placement can be on the field edge, in grassy moth action sites, or within the field. In soybeans, potato, and cotton, which also serve as action sites, within-field trap placement is preferred. In corn, trap placement within the field at ear height is effective for the second generation in the eastern United States where field sizes and habitat are variable. In the central Corn Belt, traps are typically placed with the base below the top of the vegetation in moth action sites along grassy areas adjacent to corn fields. Putting pheromone traps in open areas without much vegetation usually is not productive. Traps are most effective when the base is placed at least 6 inches below the top of the canopy of the vegetation. The use of single traps is also much less reliable than small groupings of three or more traps placed at least 50 feet apart on a farm, where average catches from the combined traps are used to estimate the initiation of moth flight.

Interpretation of pheromone trap catches must be done carefully. A sudden, large increase in moth catch, following low moth numbers, obviously indicates the beginning of a significant moth flight. However, gradual increases over a period of time could be recognized as a moth flight initiation only in retrospect. Some experience is required to learn how to interpret trap catches. Cool or windy nights will reduce the effectiveness of pheromone traps. Researchers are continuing to develop improved pheromone trap techniques. Until pheromone trap effectiveness has been further evaluated and proven reliable, use light traps in conjunction with pheromone traps.