Soybean
Emergence = Risk of Bean
Leaf Beetle Injury
Soybean planting is almost finished in some areas and the beans will be popping
out of the ground soon. If your field is one of the first to emerge, it could
get hit with bean leaf beetles. The overwintered adults have been active for a
few weeks now. I've been finding them in alfalfa fields. I've even been catching
some in my sweep net while scouting canola! So as soon as the soybeans start to
come out of the ground, these beetles will be flocking to those plants.
Usually
most at risk are the fields in the more southern counties, including Essex, Chatham-Kent
and south Lambton but this does not exclude others. The beetles that will be entering
the fields are the very same beetles that were around late in last summer. They
found a nice place to sleep off the winter in the woodlots and ditchbanks and
are now out and hungry for breakfast! Many regions last year had higher than normal
beetle populations late in the season which could put them at risk this spring.
Though the winter may have seemed cold to us, it was not nearly cold enough to
kill all of the adults, especially the further south you go. Scout as soon as
the crop emerges. Look for adults that may be feeding on the plants. They sometimes
drop and hide if they see you approaching so try to slowly approach the plants
to sneak up on them. Tip toe through the soybeans sort of speak. Signs of feeding
include circular holes in the leaves or cotyledons or leaves that have been clipped
off.
Figure
1. Early season bean leaf beetle injury
in soybean field in Essex County in 2006. Right side of the field was treated
with CruiserMaxx seed treatment, while the area to the left was only treated with
fungicide seed treatment. (Photo Credit: Wayne Bennett, Syngenta Crop Protection)
Plants
are most vulnerable before they reach the V3 stage. Scout 5 areas of the field,
looking at 10 plants in each area. Determine the average number of beetles per
foot of row. 16 beetles per foot of row or if there is significant clipping taking
place indicates that control is necessary, especially for seed, IP or food grade
soybeans. Once the soybeans are in the V3 stage, the concern then turns to the
percent of defoliation taking place. It takes a lot of defoliation before a spray
is necessary, especially before the plants are in the reproductive stages. Growers
who have a history of early season bean leaf beetle seedling injury should consider
using Cruiser seed treatment which is quite effective on these early season adults
(Figure 1). However, Cruiser seed treatment will not work on the later
generations of adults that make the holes in the leaves and pods later in June,
July and August.
