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
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
Figure 1. Wireworm in soil.
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).
Figure 2. Seedcorn maggot.
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
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
Figure 3. Black Cutworm. Image courtesy of Roger Schmidt, University of Wisconsin-Madison,
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
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
Figure 4. Soybean aphids on the underside of the leaf.
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
Figure 5. White grub in soil. Photo courtesy of Alton N. Sparks, Jr., University of Georgia,
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.
Figure 6. Bean leaf beetle feeding on soybean leaf
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.
|Early season soybean insect
|Treatment options and action thresholds
|Bean Leaf Beetle
Preventative treatment (seed treatments). Consider insecticides if 5 or more beetles per plant or 1 damaged
plant per foot of row; or 30% stand loss or 30% defoliation.11
Insecticides targeted at the arrival of first-generation beetles can be used to minimize the transmission of
bean pod mottle virus.6
|Use direct soil sampling or bait stations in suspected fields. One live wireworm per bait station or cubic
foot of soil is enough to cause potential problems. Preventative treatment (seed treatments labeled for
wireworm) or replant with another crop.9
|Use direct soil sampling method in suspected fields. Two or more white grubs per cubic foot are enough to
cause potential problems. Preventative treatment (seed treatments labeled for white grub) or
|Preventative treatment (seed treatments labeled for seedcorn maggot) or replanting.2
|Preventative treatment (seed treatments). 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 > 250 aphids per plant.6,10
|Cutworm and armyworm
|Infestations are unpredictable and infrequent. Insecticides may be needed when larvae are < 3/4 inch long
and> 20% of plants are damaged or missing.4
1Field Crop and Forage Pests and their Natural Enemies in Western Canada: Identification and
Management, Agriculture and Agri-Food Canada, Page 33. Soybean Insects. Alberta Pulse growers. https://albertapulse.com/soybeans-insects.
2Manitoba Pulse Soybean growers https://www.manitobapulse.ca/2019/05/scouting-for-early-season-insects/
3Krueger, S. 2018. Address these early season soybean pests. Pressures. https://emergence.fbn.com/agronomy/address-these-early-season-soybean-pests
4Soybean insects guide. 2011. Iowa State University. https://www.ent.iastate.edu/soybean-insects-guide
5Gavloski, J. 2008. Soybean Aphids: Sampling, Thresholds and Management. Manitoba Agriculture. https://www.gov.mb.ca/agriculture/crops/insects/soybean-aphids.html
6DiFonzo, C. 2009. Heavy soybean aphid infestations on early-season soybeans. Michigan State
7Koch, R., and B. Potter. 2018. Early season scouting for soybean aphid. University of Minnesota
8Get to know your grubs. Ontario Grain Farmer Magazine. https://ontariograinfarmer.ca/2016/03/01/get-to-know-your-grubs/
9Field Crops IPM. 2009. Purdue University. http://extension.entm.purdue.edu/.
10MacRae, I., et al. 2005. Early season scouting for soybean aphid. University of Minnesota Extension.
11Scout info. University of Kentucky. Kentucky IPM Pest Information Pages. https://www.uky.edu/
12Agronomy guide for field crops. Publication 811. Ministry of Agriculture, Food, and Rural Affairs. http://www.omafra.gov.on.ca/english/crops/pub811/p811toc.html
Performance may vary from location to location and from year to year, as local growing, soil and weather
conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and
should consider the impacts of these conditions on the grower’s fields.
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