Insects of concern in soybeans include below-ground feeders, sap feeders, and defoliators.¹ Crops should be scouted throughout the growing season to monitor for damage and control measures taken only if economic thresholds are reached to help protect beneficial insects present in the field. Early season insects can feed on soybean seeds and seedlings causing plant injury, stunting, delayed emergence, or stand loss.

Scouting is necessary to determine the types of insects present and their population densities in order to make informed management decisions. Sampling methods include scouting, soil sampling, and using baited traps.

The main early season soybean insects that can potentially cause issues include wireworms, seedcorn maggot, bean leaf beetle, white grub, stink bug, cutworms, and grasshoppers. Of these insects, insecticide seed treatment can help provide potential control of wireworms and seedcorn maggot. However, it is important to scout for all three of these insects from May to June, as the crop is emerging and just after emergence.²

1. Wireworms

Wireworms have a host range of pulses, soybeans, and more (Figure 1). They look slender, with length of 0.5 to 1.25 inches (1 to 2.5 cm), hard-bodies, and yellowish to brownish in color. They have a long life span and can spend two to over five years in a larval stage. There is no established economic threshold for wireworms. The best time for scouting wireworms is fall or spring when soil temperatures are just above 10°C, but below 26°C. Baits are most effective at approximately 10°C, since the bait ferments and releases CO₂ to attract wireworms. Warmer soil temperatures can cause wireworms to be more attracted to other vegetation in the soil.¹²

Wireworms feed on germinating seeds and on the underground parts of stems of young seedlings, causing reduced crop emergence and thinned stands. Larvae migrate near the soil surface in early spring and move back down deeper in the soil as soil temperatures increase later in the summer.

Damage can be seen in the field by wilting plants, resulting from wireworms chewing on the stems, and may appear like cutworm damage. The distinction is that wireworm feeding often looks more like the stem is shredded, and it is usually still attached to the roots, whereas cutworm feeding cuts the stem right off. Wireworms can eventually cause the death of the plant.

Assess soybean crops from planting to V3 (May to June). It is especially important to monitor fields with a history of wireworm infestations, recent cultivation of grassland or sandy to silty soil texture. Pay special attention to areas of fields with wilted plants or thin stands. Dig into the soil of affected areas to assess damage to germinating seeds, underground parts of young seedling stems and roots. Set out at least four bait traps in early spring to assess the wireworm population.

2. Seedcorn Maggot

Adults emerge in early spring and are active at temperatures between 16°C and 29°C. Once mated, female flies search for an egg-laying site from April until the middle of June. The females are attracted to moist soils with an odour of decaying organic matter (crop residues, pre-plant tilled weeds, freshly solid manure or freshly tilled soil). The adults lay their eggs in the crevices of wet soils. The larvae then penetrate germinating seeds where they feed on newly planted soybean seeds and can reduce stands. Peak adult activity occurs in early spring and in the fall, with larvae going into a summer diapause when temperatures are above 29°C.¹² Seedcorn maggots are yellowish-white, about 1/4 inch-long (0.63 cm) and lack a defined head and legs (Figure 2).

Assess soybean crops from planting to VE (May to mid-June). Fields that were planted early, those with cold, wet soil, recent tillage or incorporation of plant material, recent manure application and sandy to silty soil texture are at higher risk of seedcorn maggot and should be targeted for scouting.

As soon as the crop emerges, carefully scout the field for crop injury. Scout 10 areas of the field with poor emergence and dig up seeds and seedlings to look for scars and tunneling. Cut open suspect seeds or seedling stems to look for seedcorn maggots.¹²

3. Cutworms

Cutworms have a wide host range. Larvae are redbacked, dingy, darksided and mostly found in Manitoba (Figure 3). Larvae bore holes in leaves, gouge edges of cotyledons or leaves and clip plants. Infestations are unpredictable and infrequent. Insecticides may be warranted when larvae are less than 3/4 inch-long (1.91 cm) and greater than 20% of plants are damaged or missing.⁴

As soon as soybean emerge, start scouting for cutworm once every five days until the V5 stage. Assess soybean fields on a weekly basis from May to June. Inspect areas of fields with wilted, yellow or missing plants. Agronomy guide suggested in Agronomy Field guide in Ontario to scout at least five locations for every 10 hectares (25 acres) of field especially where the weeds were heavy before tillage/planting. Look for leaf-feeding (pinholes) by young climbing larvae as the first sign of damage. Also look for wilting plants, foliage-feeding or for plants being cut of at the ground.¹² Dig around damaged plants to a depth of 5 cm (2 in.) and search through the soil, as cutworms like to hide in the soil during the day. Note the size of the cutworms found and the crop leaf stage.

4. Soybean Aphids

Soybean aphid is native to Asia was first discovered in North America in 2000 and in Ontario in 2001.¹² Soybean aphids survive the winter in the egg stage and can overwinter on buckthorn (Rhamnus spp.). Soybean aphids are very small (less than 1/16 inch long or 0.42 cm), soft-bodied, pear-shaped insects (Figure 4). They vary in color from pale yellow to light green with black cornicles on the back of the abdomen. Early season aphid feeding can lead to stunted plants with fewer nodes, which can result in fewer pods.⁵

Thresholds have not yet been established for infestations occurring before the late vegetative growth stages, but insecticides may be needed if 80% of the plants are infested with greater than 250 aphids per plant.^6,7

Lady beetles, lacewings, and larvae of hover flies are some of the common and easily recognizable aphid predators in soybean fields. When assessing soybean fields, consider whether the aphid populations are increasing or decreasing, and the level of natural enemies present.

5. White Grubs

European chafer and June beetle are the most common problem grubs in Ontario field crops, although Japanese beetle grubs can also cause damage. Proper identification of the species of grub present in each field is important, as their life cycles are different, which influences the management strategies implemented.¹² White grubs can damage soybean seedlings, especially when the soybean crop follows sod or another cover crop. They feed on roots, resulting in thin stands, reduced seedling vigor, and dead emerged plants. White grubs range in size from 1/4 (0.64 cm) to over 1 inch (2.5 cm) long and have white bodies with tan to brown heads (Figure 5).

White grub species can be distinguished from one another by the pattern of rasters (bristle-like hairs) on the tip of the abdomen. Unlike wireworms, bait traps do not attract grubs. Fall is the best time to scout for grubs, though spring scouting before or after planting is also possible.¹² There is no established economic threshold for these pests, however, to meet the government-mandated restriction for neonic use, one must find a minimum of 10 white grubs (or two per dig site) per 100 acres.⁸

6. Bean leaf Beetle

Defoliation injury by bean leaf beetle adults is generally not serious in Ontario. The exception is damage caused by overwintering adults to young soybean plants (V1 to V2). Cotyledons and seedling plants can be clipped of by heavier populations. Once leaves emerge, beetles make small circular holes between the major leaflet veins. Larvae feed on soybean roots and nodules but are not of economic concern. Early planted soybean fields experience overwintering adult populations, particularly in the most southern counties of Ontario. Soybean fields neighboring alfalfa and other legume crops may also be at risk. Mild winters may also increase risk.

Select at least five sampling sites from across the entire field at random. At each sampling site, slowly walk down 4.5 to 6 metres (15 to 20 ft) of row and carefully count all beetles. Beetles may quickly drop of the plants and hide in soil cracks. Calculate the average number of beetles per metre (foot) of row.

Table 1. Treatment recommendations and action thresholds for early season soybean insects.