Corn and soybean pests

By Jim Hoorman, Hoorman Soil Health Services

A week of good weather is welcomed as farmers finish planting and replanting. The past several weeks have been cold and wet, and crop growth has been slow. Warm weather increases plant growth and warms microbes, releasing abundant nutrients. A number of agricultural pests, including insects, pathogens, and weeds, are now slowing down crop growth.

Among insects, seed corn maggot and wireworms are among the worst Midwest seed insects. Cold, wet, soggy fields allow these insects to thrive. Seed corn maggots (Delia platura) are pale, larvae that burrow into and hollow out germinating seeds and young seedlings. Infestations cause poor crop stands, stunted growth, and plant death and are a problem in cool, soggy, and manured fields.

Wireworm (Melanotus communis Gyllenhal) is the most common wireworm attacking corn and soybeans and is the larval form of the click beetle. Wireworms have a slender, elongated body and are dark brownish orange. They have a distinct head and projecting mouthparts, with small true legs near the head. Wireworms feed on the germ of seeds and the growing point of corn, creating tunnels that cause twisting and stunting, and no ears.

Corn flea beetle adults overwinter in grassy areas and migrate to corn after emergence, where they attack corn seedlings. Harsh winter conditions may reduce the number of corn flea beetle adults and associated injury. Corn flea beetle harbors the pathogen Stewart’s wilt, a bacterial pathogen, that can damage corn.

Other adult insects or insect larvae, including seed corn beetle, billbug, true white grub, stink bugs, and cutworm, may feed on seeds or seedlings and cause damage; similar to disease symptoms. Insecticides can help control both pest species, but firefly larvae, black ground beetles, and certain wasps are natural enemies of soft-bodied insects.

For corn and soybean pathogens, several bacteria and fungi cause poor crop growth, especially when soils are wet and soil temperatures are cool (< 55° F). Six species of Fusarium cause root rot, with reddish-brown to dark-brown lesions. Multiple Pythium variantscause slimy lesions, whileRhizoctonia causesbrown lesions on roots, progressing to reddish-brown, sunken cankers on the crown and brace roots.

Phytophthora sojae is one of the most damaging diseases in soybeans, causing stand reductions. It prefers wet, soggy fields that are warmer than 60°F. Seeds and seedlings can be infected and killed at any time after the seed has absorbed moisture. The pathogen can also infect plants later in the season following periods of heavy rain, resulting in stem and root rot.

Phytophthora infection appears as soybean gaps in a row (rotted seed), with plants emerging but dying after emergence. Examine the stem at the soil level for a pinched appearance. Pythium and Phytophthora symptoms are very similar. In the stem rot phase, Phytophthora can be diagnosed by a distinct chocolate-brown lesion moving up the stem from the soil.

Corn and soybean pathogens are often managed with fungicides and seed treatments, but there are several natural enemies of these pathogens. Soil with good soil health and microbial diversity has a natural ability to suppress pathogens. Trichoderma is a type of fungi that suppresses Fusarium, Rhizoctonia, Pythium, Phytophthora, and even soybean cyst nematode and scleotina (white mold in soybeans). Levulinus brevis is a Gram-positive, rod-shaped lactic acid bacterium that suppresses Fusarium pathogens and is a natural repressor of diseases and pathogens. Lentoactobacillus rhamnosus strains inhibit the growth of pathogenic fungi and bacteria in the root zone, thereby minimizing the need for chemical pesticides. Unfortunately, many natural pathogen-suppressing fungi and bacteria die out when fungicides and seed treatments are used.

Due to the wet weather, many weeds have gotten a head start on the crops. The best time to kill weeds is when they are small or less than 4 inches tall. Cover crops like cereal rye have been very effective at suppressing weeds and helping dry out the soil. However, the longer they grow, they can also suppress corn growth without some extra nitrogen to break down the cereal rye residue.

In general, these herbicides (i.e., glyphosate, Gramoxone, Liberty Ultra) have a broader spectrum of weed control. Certain herbicides with residual activity will also have some burndown activity. When weeds are taller than 4 inches, higher application rates may be needed. Gramoxone is a burndown that desiccates everything and has to be used pre-plant. Most farmers now are planting corn and soybeans with enhanced genes for herbicide compatibility, like glyphosate and Enlist.

It has been another rough start to the growing season. With warmer weather and less rain, farmers should finish up planting soon.