Scouting for Black Cutworm

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Black Cutworms favour fields with pre-plant weeds, cover crops or crop residues, as well as no-till fields or late spring plowed fields with annual weeds.  Fields with a history of black cutworm infestation are also more susceptible.  They primarily target corn but on rare occasion, will infest soybeans.

Larvae range from 3.5 to 50 mm in length and are a brown-black to grayish-black with a lighter underbelly and two pairs of black spots on each body segment (the outside pair is twice as large as the inside one).  The larvae curl up when they are disturbed.

Adult black cutworms are a greyish-brown moth with a small black triangle running through a kidney shaped spot on the forward wing.  They have a 40 to 55 mm wingspan.

Damage

You should be able to notice damage on individual leaves, plants and an entire field.

·         Leaves

o    Small pinholes or notches from young larvae

o    Holes with irregular edges or chewed leaf margins looking as if they were spreading (larger larvae)

·         Plants

o    Sudden wilting, cut or hollowed out stems at ground level or just below soil surface (final larvae stages)

·         Field

o    Spotty plant populations and uneven growth

Scouting

·         Adults

o    April to end of June

o    Pheromone traps are a great tool for evaluating Black Cutworm pressure

·         Larvae

o    Early May to mid-June in areas of the field that show favourable conditions for feeding

o    Inspect 20 plants in five areas of the field once or twice per week from emergence to the 5-leaf stage

o    Look for pinholes, wilted or cut plants

o    Dig up the ground surrounding a damaged plant to a depth of 5 cm to find larvae

Economic threshold is reached when 10% of corn plants in the 1-4 leaf stage exhibit damaged leaves or pinholes; or, 3% of plants are cut and larvae found are smaller than 25 mm.

Control

The preferred control method for fields with a history of black cutworm infestation is to use Bt (Bacillus thuringiensis) hybrids.  Insecticide seed treatment and foliar insecticide applied in problem areas also help to control Black Cutowrm.

 

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Sources: Grain Farmers of Ontario. Guide to Early Season Field Crop Pests. (verified May/15). Ian Kimber, Bugwood.org.