Corn Soil Insect Primer

Early season corn insect pests can significantly impact stand establishment. Below are the main corn seedling pests along with scouting technique, action thresholds and management strategies. (OMAFRA Publication 811, Agronomy Guide, Chapter 13 and Publication 812, Field Crop Protection Guide)


Description: Wireworms are 7 – 35 mm (1/4 to 1-1/3 inches) long, cylindrical, copper-brown coloured and hardened. (Figure 1) Due to their long life cycle, the wireworms can damage several successive crops by attacking roots, seeds and germinating corn seedlings.

Figure 1 - Wireworm
Figure 1 – Wireworm

Risk Factors: Damage is usually worse in the second year after sod, following years with high grassy-weed pressure, or when corn and cereals are frequently in the rotation. They are more common in sandier soils, especially on knolls.

Scouting: Scout in mid-April using bait stations, a few weeks before planting.

Management: The action threshold is one wireworm per bait station. Use insecticide seed treatments in fields that have reached threshold, have a history of wireworm, or are following grassy sods. No rescue treatments are available. Controlling grassy weeds in crops previous to corn will help with prevention. 

Black Cutworm

Description: Black cutworm larvae are greyish-black, about 3.5 cm (1-1/4 in.) long when mature. (Figure 2) In early spring, larvae will first develop on weeds and then move to corn seedlings. A corn plant attacked by cutworm may suddenly wilt because the stem has been hollowed out or fed on underground. Larger larvae cut off the plants at or just below ground level.

Figure 2 - Black cutworm larva, Pinholes
Figure 2 – Black cutworm larva, Pinholes

Risk Factors: These include a history of cutworm damage, presence of winter annual weeds such as chickweed and volunteer wheat before planting, no-till and heavy crop residue.

Scouting: Start once every 5 days as soon as corn emerges, looking for leaf-feeding pinholes by young climbing larvae as the first sign of damage.

Management: If more than 10% of corn plants show leaf feeding / pinholes, treating with a foliar insecticide will give nearly 100% control. The risk has passed when corn reaches the five-leaf stage and begins to produce roots at the base of the plant, Using seed treatments specifically for black cutworm control is not recommended, unless the field has had a continuous history of cutworm injury. For fields with a history of cutworm injury, consider planting Bt corn hybrids specifically for black cutworm control. In no-till corn fields, it is important to remove green vegetation that could attract the moths in early spring. Fall burndown of volunteer crops and weeds is recommended. Fields should be bare for at least 2 – 3 weeks before planting. 

Corn Rootworm

Description: There are Western and Northern corn rootworm (CRW) in Ontario. (Figure 3) CRW eggs are deposited in the soil from July until a killing frost. Eggs over-winter and begin hatching in early June. Larvae feed on corn roots from mid-June to mid-July. 

Figure 3 - Western Corn Rootworm Adult
Figure 3 – Western Corn Rootworm Adult

Risk Factors: These include fields which were in corn the previous year and had high CRW beetle populations, or were the latest planted corn field the previous season.

Scouting: Monitor 20 plants in five different locations in your field weekly from when the CRW adults emerge at the end of July to the end of August.

Management: Crop rotation is the best strategy. If crop rotation is not practical and there was more than one beetle per corn plant throughout the month of August, it may be necessary to either: plant transgenic CRW Bt hybrids, plant corn seed treated with high rate insecticide, or apply a soil insecticide to control CRW.

European Chafer

Description: Various types of grubs can attack field crops. (Figure 5) European chafer is the most common. Proper identification of the species is important because their different life cycle influences the management strategies needed. Grubs prune roots, causing plants to become stunted and eventually die. Fields near turf (lawns, golf courses and pasture) are particularly prone. Spring feeding damage by European chafer larvae starts in April and is completed by mid- to late May. 

Figure 5 - White Grub Larva
Figure 5 – White Grub Larva

Risk Factors: Sandier knolls where past injury was evident.

Scouting: Scout for chafers in the fall in standing soybean stubble fields. Scout for grubs on the sandier knolls and areas with past injury. Using a shovel, dig up 30 cm2 (1 ft2) of soil, 7.5-10 cm (3-4 in.) deep, in at least five areas of the field. Count how many grubs are found in each sample. The presence of two or more larvae per 1 ft2 (30 cm2) indicates the need for control.

Management: No rescue treatments are available for this pest. Cultural options include disturbing the soil by tillage or disking, which brings the grubs to the surface where they are exposed to the elements and natural enemies. For this strategy to be effective, plow in the fall before the grubs migrate below the plow depth. For corn, use an insecticide seed treatment that is registered for European chafer. If grub populations are high (i.e., four or more grubs per square foot), use the higher rate of an insecticide seed treatment. If the chafer population is extreme, avoid planting corn and plant soybeans instead.