sealPurdue News

April 8, 2002

Entomologist: 'Nonstarring' pests can bring down curtain on crops

WEST LAFAYETTE, Ind. – Rootworms often claim the starring roles on the crop production stage, but farmers should be just as wary of the supporting cast.

White grubs, wireworms and seedcorn maggots are capable of show-stopping performances, as well, said John Obermeyer, Purdue University Cooperative Extension Service entomologist.

"What's going to bring to light that there are problems within a field is certain areas of the field will be delayed in their emergence," Obermeyer said. "The plants may be stunted or have a reddish-type discoloration. As one starts digging up these plants to investigate, they may find these critters entered into the seed. The root systems may have been pruned back and also there may be boring into the hypocotyl of the young plant."

White grubs attack corn and soybeans from April to mid- or late June, before emerging as Japanese beetles and feeding again. Wireworms feed from April through June, with seedcorn maggots a threat from planting to mid-June.

There are no insecticide treatment options available once pest damage has begun. Farmers may have to replant fields if damage is excessive.

Early-planted fields are at a greater risk of insect feeding, Obermeyer said.

In recent years, planting dates have inched up the calendar. Eleven percent of the 2001 Indiana corn crop, for instance, was in the ground by mid-April. Nearly all the crop was planted by May 10 – the earliest on record.

Although corn yields averaged an all-time high 156 bushels per acre, Hoosier farmers should not be lulled into thinking even earlier planting dates will produce better results, Obermeyer said. Soil temperatures are cooler at the start of spring, slowing seed germination and inviting pests.

"The seed is going to sit there a longer period of time, and these other critters will be near or move toward that seed source and eventually start nipping at it if the seedling is delayed long enough that it doesn't get up and growing," Obermeyer said. "Eventually, these seeds could be stunted, which could affect yield, or they may be killed outright."

Other factors that contribute to insect infestation include no-till planting into dying cover crop, fertilizing with animal manure and the presence of rotting weeds.

Conservation tillage offers some benefit to reducing damage by non-rootworm pests, Obermeyer said.

"The interesting thing about no-till is we have found with our predominant species of white grub – which happens to be the Japanese beetle – that for the most part that grub wants to feed on nothing more that dead and organic material in the soil," he said.

"It's kind of a trade-off. Conservation tillage is going to increase the amount of that organic matter in the top few inches of the soil, which is going to attract the grubs to sit there and feed. It just so happens if there's a corn seedling or soybean seedling nearby, the grub will also nip at that."

Manure – both the animal and plant kind – attracts pests to crop fields. Once there, crop damage is possible.

"Where winter annuals or cover crops have been killed and brought down into the seed slot with the planter creates a 'green' manure," Obermeyer said. "This invites seedcorn maggot, which is nothing more than a fly that looks much like a housefly. They get down into those seed slots, lay their eggs near the seed, then feed on the seed and destroy it."

Farmers can improve their odds of avoiding white grubs, wireworms and seedcorn maggots, Obermeyer said. He recommends producers:

• Plant no earlier than mid-April in southern Indiana counties, later in northern counties.

• Observe insect population cycles. In the case of Japanese beetles, insect numbers dropped in many parts of Indiana from 1997-99, jumped dramatically in 2000 and fell again in 2001.

• Scout fields for insects. Total Indiana acres inspected has increased almost annually since 1977, to about 3 million acres in 2000.

• Treat seed with insecticide before planting, or use pretreated seed. "This can be done by the producer, by stirring it into the seed boxes. Other treatments are applied by the seed companies. These treatments definitely help against these other critters in the soil."

For more information on white grubs, wireworms and seedcorn maggots, log onto the Purdue Department of Entomology corn pests Web page at or the soybean pests page at

Purdue News Service: (765) 494-2096;

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