Stalk borers are sporadic insect pests that are native to North America and widely distributed east of the Rocky Mountains, from southern Canada to the Gulf Coast states. Stalk borer moths lay eggs in late August and early September on a wide variety of plants, including smooth brome grass, quackgrass, orchard grass, woolly cupgrass, wirestem muhly, and some broadleaf weeds, especially giant ragweed. The eggs (brown) overwinter on these weed hosts, and larvae begin to hatch usually in late April and early May. Stalk borers can utilize a wide range of hosts that include as many as 176 plant species from 44 families. Infestations are most commonly associated with infestations of broadleaf weeds such as giant ragweed, cocklebur, giant burr-elder, docks, and burdock. Stalk borer larvae typically have seven to eight instars; however, as many as 16 instars have been documented on plants with poor nutritional quality. Larvae may infest corn stems near the soil surface or move from whorl tissue and tunnel downward into stalks. Whorl injury results in plants with tattered leaves. Tunneling activities of the larvae may result in stunting, tillering, delays in plant development, and potentially barren corn plants.
Stalk borer larva.
Early instar stalk borer larva.
Stalk borer moth.
Poor weed management programs most often result in interior infestations of stalk borers within cornfields. Most infestations are adjacent to field borders, ditch banks, waterways, and terraces that are infested with grasses and broadleaf weeds. Stalk borer larvae typically penetrate no further than 15 to 20 rows into cornfields from these noncrop areas. Continuous corn appears to be more susceptible to infestations.
Stalk borer infested corn plants.
Good management of stalk borers begins with sound weed management practices. In addition, knowledge of the timing of stalk borer dispersal from weed hosts in the spring can improve the effectiveness of an insecticide treatment. Dispersal of stalk borer larvae (generally fifth to seventh instars) from weed hosts begins when approximately 1,100 heat units (base 41°F) have accumulated since January 1. By the time 1,400 to 1,700 heat units (base 41°F) have accumulated, from January 1, 50% of stalk borer larvae have abandoned their weed hosts and started their search for new corn hosts. Decisions regarding the necessity of an insecticide treatment need to be made within the heat unit accumulation range of 1,400 to 1,700 (base 41°F). Figure 2 provides the actual degree-day accumulations (base 41°F), from January 1 through May 12, 2003. Based on these heat unit accumulations, some limited movement of stalk borers into corn may be occurring in the southern tip of Illinois. Growers should begin their scouting efforts for this insect pest within the next week in these southern counties.
Some insecticides are labeled for use in tank mixes with "burndown" herbicides. As stalk borer larvae leave dying weed hosts, they come into contact with the insecticide and are killed. Several insecticide products are labeled for stalk borer control (Table 2). Economic thresholds for stalk borers have been developed and published by Iowa State University. These thresholds (Table 3) are based on six corn-leaf stages, three corn prices, control costs of $13 per acre, and a control level of 80%. The information in the table reveals that, as corn prices increase, the economic thresholds decline. The economic thresholds for smaller plants are lower than they are for taller plants. Also, recognize that these are only guidelines. If you can treat for a lower price or if you can achieve better control than 80%, the thresholds can be adjusted. Entomologists at Iowa State have reported that Bt corn "suppresses" or "slows down" stalk borer injury. So, if finding sufficient time for scouting for stalk borers is a problem, concentrate scouting efforts in non-Bt fields.--Mike Gray and Kevin Steffey