The European corn borer is known to infest potato in several areas of the United States. In areas where both pheromone types occur, the proportion of E type is higher in spring-planted potato than in corn. Early-season adults deposit eggs on the underside of young plant leaves. First instars will bore into stems but also feed on the leaf and in the leaf veins. Second and third instars enter potato vines by tunneling through the stems, usually in the leaf axil at the node. The remainder of larval development occurs in the potato stem. The presence of fecal material, or frass, at leaf axils and wilting or dying leaves are good indications of European corn borer larvae. A later generation of moths may lay eggs before harvest, but the resulting larvae will not reach maturity due to termination of the potato crop as it becomes senescent.

Figure 48. Potato stem that has been broken due to tunneling by a European corn borer larva (M. R. Graustein).

Larval damage to potato can be found throughout the plant; however, most damage occurs in the lower third of main stems. Larval damage to the conductive tissues results in wilting and eventual death of leaves in the affected portion of the plant. Tunneled stems (Figure 48) are weakened and often break during heavy rain and wind. Studies have shown that larvae create new tunnels at the same rate throughout their development and ultimately produce an average of 4.7 tunnels per larva (B. A. Nault).

Research shows that European corn borer does not reduce marketable yield in the absence of disease (G. G. Kennedy and B. A. Nault). In studies in North Carolina, where heavy infestations reached as much as 38 European corn borer tunnels per plant, no significant yield loss was found. However, European corn borer damage may predispose the potato plant to the bacterial soft rot pathogen Erwinia carotovora and blackleg disease, caused by several Erwinia species, under wet growing conditions. Studies have shown that blackleg disease occurs more often in fields that are infested with European corn borer. Natural inoculation is likely to occur because late instars tunnel in the bottom third of the plant where the bacteria are found. Greenhouse studies have shown that European corn borer larvae may transmit the soft rot bacterium when they move between stems.

Because soft rot diseases reduce marketable yields to levels that can be economically devastating to growers, the threshold for European corn borer damage has been set at 10 to 25 percent of the stems infested. Fields are scouted early in the season for first instar entry holes, and if infestation reaches threshold level, an average of one to two insecticide applications should be made. European corn borer larval movement between stems could increase the chance that they contact lethal doses of the insecticide residue.