No. 3 Article 1/April 23, 2010

Secondary Corn Insects: How Significant Is the Threat?

For the past 10 days, corn planting has begun in earnest across much of southern and central Illinois. Seedling corn and concerns over secondary insect infestations go hand in hand. So, not surprisingly, I've noticed quite a few articles in industry newsletters and the popular press on secondary insect infestations. How justified are these concerns? How often do secondary insects become an economic threat?

These same questions were raised well over half a century ago by entomologists with the Illinois Natural History Survey. J.H. Bigger and H.B. "Pete" Petty began a 10-year on-farm survey in 1954 to address these questions. Many will remember Pete, who was the first extension entomologist in Illinois and originator of the "Spray School." Throughout the 10 years (through 1963), 452 cornfields in all areas of the state (though mostly in the northern two-thirds) were examined for insect injury. Only portions of fields where no soil insecticide had been used were sampled. Six groups of insects were most commonly found: the cornfield ant, the corn-root aphid, corn rootworms, the grape colaspis, white grubs, and wireworms. At the start of the survey, the standard sampling unit was a five-hill sample per field. Because of changing agricultural practices, over the course of the survey, a five-plant sample unit per field was used.

The incidence of secondary insect infestations can be significantly affected by crop history. Bigger and Petty took a close look at rotational practices and insect injury and offered the following observations: "During the 10-year period, it was found that wireworms are most likely to be important as a pest of corn following grass, clover, or alfalfa. Cornfield ants and corn-root aphids are most likely to appear on corn following grasses; there is a tendency for white grubs to be more prevalent on corn following soybeans or grass; rootworms are more important on corn grown for three or more years in succession in the same field; and the grape colaspis is noticeably more abundant on corn following clover."

Over the past 50 years, we have witnessed the transition to one primary rotation in Illinois, corn following soybeans. I find it interesting that Bigger and Petty indicated that there is a tendency for white grubs to be more problematic in this rotational practice. Many producers would agree that Japanese beetles have become an established and perennial problem in corn and soybeans throughout much of Illinois. As we see reductions in the acreage devoted to alfalfa, clover, or pastures and their subsequent rotation with corn, we likewise should see reductions in the threat posed by wireworms and grape colaspis. Other significant changes since this survey in the mid-1950s and early 1960s include the movement away from broadcast soil insecticide applications toward banded applications. Producers during that time frame also were using persistent chlorinated hydrocarbon insecticides, such as aldrin and heptachlor, each spring, often without any scouting information or knowledge of pest densities.

White grub injury to corn.

Wireworm injury to corn.

Grape colaspis injury (right) to corn seedlings.

Results from the survey show that wireworm infestations were generally found in over 50% of producers' cornfields during the 10-year study (Figure 1). In contrast, grape colaspis and white grubs occurred in 10% and 23% of fields, respectively. The percent of corn plants infested with wireworms was 20% over the 10-year investigation (Figure 2). On average, very few corn plants were fed on by grape colaspis (4%) or white grubs (7%).

Figure 1. Percentages of Illinois farmers’ cornfields infested by wireworms, white grubs, or grape colaspis from 1954 to 1963.

Figure 2. Percentages of plants infested by wireworms, white grubs, or grape colaspis in Illinois farmers’ cornfields from 1954 to 1963.

Because of the increasing popularity of transgenic Bt corn hybrids, we are witnessing the widespread use of two nicotinoid insecticidal seed treatments across the Corn Belt. These seed treatments are Cruiser Extreme 250 (active ingredient thiamethoxam, 0.25 mg ai/seed) and Poncho 1250 (active ingredient clothianidin, 1.25 mg ai/seed). Manufacturers of these products indicate that insect protection is provided for a large cross-section of secondary soil insects, including wireworms, white grubs, and grape colaspis. Both of these insecticidal seed treatments offer some contact activity (protecting the corn seed) and systemic action (distributed throughout the seedling corn plant).

The survey results from Bigger and Petty would suggest that most cornfields do not support economic infestations of secondary insects. One could argue that today's less diversified rotational practices might even result in fewer economic infestations of some soil insects, such as wireworms and grape colaspis. Even though the use of nicotinoid insecticidal seed treatments would appear to offer significant environmental advantages compared with the chlorinated hydrocarbon insecticides used by farmers 50 years ago, prophylactic and large-scale use characterizes both insecticidal groups. Are there potential unwanted consequences to this insurance pest management approach? With the benefit of hindsight, we now know many of the unintended consequences of the use of aldrin and heptachlor in producers' cornfields, such as resistance development by western corn rootworms, declines in predator bird populations, and the biomagnification of chlorinated hydrocarbons in the food chain. Time will tell if any significant unwanted consequences begin to emerge from the use of the nicotinoid insecticidal seed treatments. For now, these popular insecticides are in high demand by corn producers.--Mike Gray

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