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Late Season Insects and Pre-harvest Intervals

By: Danielle Stephens, M.Sc. PAg, Integrated Pest Management Agronomist

Late-season insect pests bring a lot of factors into play. There can still be risk for yield loss as crops mature, though many crops are now post their most vulnerable stages. When scouting for insects and considering whether management is required, it is important weigh the risk to the crop based on economic thresholds. It is also important to know when the window to act has passed and it is no longer possible or economical to spray. To help with this decision, it is important to refer the pre-harvest interval (PHI for the control product you are considering using.

A PHI is the number of days that must pass between the application of the chemical and the swathing, desiccation or otherwise cutting/termination of the crop’s growth. As we get closer to harvest it is critical to consider the PHI for all chemical applications. All pesticides, herbicides, fungicides and insecticides will have a PHI that is set to ensure that the harvested grain does not exceed the maximum residue limit (MRL) of that chemical. If the crop is harvested prior to the indicated (PHI), there could be unsafe or unacceptable residues of the pesticide in or on the harvested grain.

The PHI information for specific control products (organized by crop type) can be found in the tables at the beginning of the insect section of the Guide to Crop Protection, on each product page of the insect section and on the pesticide label itself.

Bertha armyworm caterpillar on canola
Bertha armyworm caterpillar on canola
For many crops and their pests, there will also be a most vulnerable stage for the pest to be present in economic numbers. For many crops, the risk for economic damage by insect pests decreases as the plant matures. For example, aphids are a potential issue before cereal plants are at the soft dough stage (or the equivalent stage for pulse crops). Spraying past these stages may not have an economic benefit.

There was an increased risk of bertha armyworm in some areas of the province this year based on moth monitoring traps around the province. It is important to scout individual fields, as the map can only give an indication of regional risk. Even if the trap was in your field, moths within the trap does not guarantee that a female moth laid eggs there. As a result, it is still important to scout fields and look for the presence of bertha armyworm larvae. If control is needed, it is important to note that the PHIs on control products range from one to 30 days, so you need pick your product appropriately to when you will be terminating your crop.

Diamondback cocoon and potential feeding  on canola pod
Diamondback cocoon and potential feeding
on canola pod. Also cabbage seedpod weevil
exit hole on the canola pod
Diamondback moths were also seen in canola fields throughout many areas of the province this year. For late-season scouting, consider the stages of diamondback larvae and the stage of your canola crop. If you are no longer finding diamondback larvae in the field, but rather just the cocoons, and your canola crop is close to termination (cutting, swathing or desiccation), then it is unlikely the next generation will be able to affect your crop. It is also important to note that control will have no bearing on next year’s population, as few diamondback overwinter on the prairies. PHIs for diamondback range from one to 21 days, with most options needing at least seven days between application and when the crop is terminated.

One of our later maturing crops in the province is soybean, which means scouting for pests should continue for this crop. This year has seen a larger-than-normal population of a potential defoliator, the thistle caterpillar (painted lady butterfly). As with any defoliator in soybean, remember that soybeans can tolerate a large amount of defoliation and can tolerate more as the crop moves into pod fill (R6) and harvest (R7).

Mite from soybean field
Mite from soybean field, presumed
to be the two spotted spider mite
with pen tip for size comparison
Although not specifically reported in Saskatchewan, soybean aphids have been reported for some fields in Manitoba. There have also been reports of spider mites in soybean. These are tiny not-insects that favour dry conditions. They are very small (approximately 0.4 mm), so are best found by taping the leaves over something you can easily see the mites on and having a hand lens ready to identify them. Premature yellowing around field edges is a good indicator to scout for mites. The mite population can change with a heavy rain, so make sure to check fields again after a rain. Determining if it is economical to control spider mites under Saskatchewan conditions is difficult. As the crops move in to R6, the feeding from spider mites and/or soybean aphid will have less of an impact. Products for both pests have 21-day to 30-day PHIs. Note that miticide products are not listed in the Guide to Crop Protection.

There are also many insect pests that are on the move, preparing for overwintering. Noting what you see at or before harvest can be important for your records:

Cabbage seedpod weevil adults have pupated and can be found in ripening canola fields. Their exit holes from pods (see diamondback photo) and the insect feces in the pod from their larval stage are also a sign they were in your canola (or brown or oriental mustard) this year.

Soybean leaf with pea leaf weevil notches and adult pea leaf weevil
Soybean leaf with pea leaf weevil notches
and the adult pea leaf weevil that is
feeding on pulses before overwintering
Pea leaf weevil adults are also on the move and could be noticed in harvested peas. The adults will feed on various pulse crops before overwintering. Notching from pea leaf weevils can be seen in faba bean fields, soybean fields and alfalfa stands.

Flea beetles adults can also be out at this time. The stripped flea beetle will emerge earlier in the spring than the crucifer flea beetle and are also less affected by neonic seed treatments as compared to the crucifer flea beetle.

Grasshoppers are in or reaching adult stage throughout the province. When the majority of grasshoppers in the area are adults (winged), counts can be done to estimate egg-laying population and thus next year’s risk. The yearly forecast map is made from surveyed sites across the province. To monitor for your own records, check out the grasshopper protocol.

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