Scout for Western Bean Cutworm (WBC) egg masses…

Contributed by Robert Moloney, Syngenta Seeds

WBC moth flights spiked last week so prioritize scouting of fields that are about to tassel or tasseled last week…

WHAT TO SCOUT:   WBC show a very strong preference for pre-tasseled fields, so any pre-tassel fields will have many more egg masses laid in them.  This doesn’t mean you won’t get any eggs laid in a field that is in tassel, but the majority of eggs will be laid in pre-tassel fields (if available) so start there.  Numbers seem to be higher in sandy soils or sandy areas of fields.  Make sure you scout at least 20 plants in each of 5 different areas of the field.  The more plants you check the better.

WBC egg mass

WHERE TO LOOK: egg masses are mainly laid on the upper surface of leaves in the upper part of the plant.  Most often it will be in the upper 2-3 leaves of the plant, and often on leaves that are still upright.  The moths seem to hide between the leaf and stalk during the day, so we are finding the majority of egg masses are on the leaf within 12 inches of the stalk (i.e. near the base of the leaf, NOT toward the tip).  There can be more than 1 egg mass per plant, and we are finding that when you find a plant with egg masses often neighbouring plants will also have egg masses on them.

WBC egg mass shadow - T. Baute

 WHAT TO LOOK FOR: as Tracey Baute from OMAFRA has suggested, one of the easiest way to find egg masses is to put the sun behind the plants you are scouting and look for shadows.  Most often round or irregular shadows on the leaf will be egg masses (see picture left).  If the plants are still short you can look directly at the upper leaves for egg masses.  The egg masses can range from the size of a penny to the size of a quarter so they are fairly easy to see.

The egg masses are made up of a large number of individual eggs laid together.  The eggs start out a pearly white colour, then slowly change to tan, and finally to purple as they are ready to hatch.  Stinkbug eggs can look similar but have a “crown” around the top that give them a more shiny, silvery colour.  Hatched WBC egg masses look like popped bubbles and have purple/blue colour.  The recently hatched larvae are quite small and will rapidly move into the whorl to feed on the tassel before moving down and into the ear.

WBC egg staging

CONTROL THRESHOLD: the threshold for spraying WBC is 5% of plants with egg masses on them.  Spray application should be timed to egg hatch.  Once the larvae have moved into the cob chemical control is not possible, so timing is important to catch them on the leaf or in the whorl still.  Matador™ insecticide is registered for WBC control (including for aerial applications).  Keep water volumes up to get as much coverage of the plant as possible.  Since pyrethroid type products are less effective as temperatures increase, spraying in the evening or on a cool day will help improve control.


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