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Scouting Canola Insects

August 16th 2011

By Dave Cubbon, P Ag

To get an idea as to what is going on your canola field for insects, proper scouting is essential.  Each insect has specific characteristics and scouting techniques that must be looked at to insure that the right decision is made for controlling a potential problem.  Here are some tips that I find useful when looking for problem insects in canola fields.

My scouting general starts at the time when I am checking fields for sclerotinia.  I will look for shot holes in the bottom leaves.  These holes indicate where there may have been a previous hatch of diamond back moths.  If I see these, I will take out my sweep net and look at what I catch.  The most likely insects present at this stage will be diamond back larvae and lygus bug adults.  IF I find these insects are close to heading to the next generation (with diamond back moths this means they are about to pupate or are one centimeter long and with lygus the bugs should have wings) I will make a note to come back and look at this field in another 30 days.  With warm summertime conditions, this is about the time that it takes to get to next generation.

Bertha armyworm development is driven by growing degree days and moth emergence is an indication of infestations that are about to come.  The growing degree day can be looked at on various weather sites and the bertha armyworm trap numbers can be looked at on the provincial agriculture sites.  These tools give me an idea if the larvae are going to be present as well as the time that I should start looking for them.

The next time I am in the field looking for canola pests is when the crop is done flowering.  When I go into a field, I will look for feeding damage from all three insects.  With lygus bugs, there will be a honeydew that forms where the insect has been feeding.  This appears as a droplet of liquid on the pods.  With diamond back moth, there will be an area on the pod that looks like it has been scraped.  With bertha armyworms, the damage will be pods that have been eaten.  I will also look at the perimeter of the field for feeding damage from grasshoppers.

A sweep net is an essential tool to get count on lygus bugs.  10 sweeps need to be done and counts made.  If the counts are high, it is important to look at the staging of the insect.  Adults do not feed as much as younger lygus do.  At this time, looking for the larva of both the diamond back moth and the bertha armyworm in the sweep net will indicate if this bug is present.  Lygus bugs need to be checked when the temperatures are warm and the wind is down.  A heavy rain will knock the both lygus bugs and diamond back moth larvae out of the canopy so wait a day after a heavy rain to check for these bugs.

The technique for checking for diamond back larvae is different.  A handful of plants are pulled from the field and taken carefully to an area where the plants can be beaten on a smooth surface.  I usually have to hit the plants 4 or five times on the hood or the end-gate of my truck.  Hitting the plants hard on a flat surface will dislodge the insects from the plant.  Counting the numbers of insects on the surface will give you an approximation of how many larvae there are per square meter.

To check for bertha armyworms the technique to check requires an aluminum scoop shovel.  This shovel represents about a square meter of surface area.  The shovel is gently placed under the canopy of the canola.  Tapping the canola plants above the shovel with your hand will dislodge any feeding larvae.  Note the larvae size on the shovel surface.  The bertha armyworm larva will pupate around the inch and a quarter to an inch and a half size.  The bigger ones are just about done feeding.

Monitoring insects in canola fields is essential to see if it is economical to spray.  These tips should help get this job done.