Corn and Soybean Pests

A week of good weather is welcomed news 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 up the microbes to release abundant nutrients. A number of agricultural pests including insects, pathogens, and weeds are now slowing down crop growth.

On insects, seed corn maggot and wireworms are two of the worst Midwest seed insects. Cold, wet, soggy fields allow these insects to thrive. Seed corn maggots (
) 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 true legs that are small near the head. Wire worms feed on the germ of seeds and also on the growing point of corn, creating tunnels that cause twisting and stunting, and no ear.

Corn flea beetle adults overwinter in grassy areas and migrate to corn after emergence and attack the corn seedling stage. Harsh winter conditions may reduce corn flea beetle adults and associated injury. Corn flee beetle harbor 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 larva, black ground beetles and certain wasps are natural enemies to soft bodied insects.

For corn and soybean pathogens, there are a number of bacteria and fungi that cause poor crop growth, especially when soils are wet and the soil temperature is cool (LT 55OF). Six species of Fusarium cause root rot with reddish brown to dark brown lesions. Multiple variants of Pythium cause slimy lesions while Rhizoctonia causes brown lesions on roots changing 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 60OF. 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 looks like soybean gaps in a row (rotted seed) with plants emerging but dying after emergence. Examine the stem at the soil level, looking 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 treated using fungicides and seed treatments but there are a number of natural enemies to these pathogens. Soil with good soil health and microbial diversity have a natural ability to suppress pathogens. Trichoderma are a type of fungi that suppress Fusarium, Rhizoctonia, Pythium, Phytophthora, and even soybean cyst nematode and scleotina (white mold in soybeans). Levilactobacillus brevis is a gram positive, rod shaped, lactic acid bacteria 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 and minimizes 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 stat 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 they also help dry out the soil. However, the longer they grow they also can suppress corn growth without some extra nitrogen to breakdown 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 burndown that desiccates everything and has to be used pre-plant.

Most farmers now are planting corn and soybean 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.