What’s bugging my soybeans?

By Robert Moloney, Syngenta

We’re seeing all kinds of things happening in soybeans right now.  Here are some of the insect issues you might run into:

SOYBEAN APHIDS: Cruiser Maxx seed treatment will provide excellent early season control of soybean aphid, but many soybean fields are now getting to a point where the Cruiser is running out of steam.  The hot dry weather we are experiencing is very conducive for aphid numbers to build very quickly.  Scout your fields at least weekly to see if you have aphids moving in, and more often once you have established aphid populations.  Often aphid populations will first appear along tree lines where winged aphids may drop out of winds blowing across the field.  If temperatures are moderate look at the newer trifoliates on the top of the plant first as aphids love fresh and succulent leaves to feed on.  Under extremely hot conditions they may move down the plant into lower parts of the canopy.  Aphids are also attracted to soybeans plants suffering from Potassium deficiency (yellowing of the margin of the older leaves) so look in yellow areas of the field.  Watch for ladybugs or ants crawling on soybean plants and you’ll often find aphid populations on these plants or plants nearby.

Soybean aphids may or may not have wings but will always have 2 cornicles (look like tailpipes) coming out of their back.  The threshold for controlling aphids is 250 aphids/plant on 80% of plants and rapidly increasing numbers.  Natural predators (including ladybugs) may be able to keep them under control, but in dry conditions the sugars in the soybean plants become more concentrated which is super-fuel for soybean aphid growth and reproduction.  Matador 120EC is a great control option if needed.

TWO SPOTTED SPIDERMITES: This hot dry weather is also perfect conditions for a spidermite outbreak.  Usually we’ll see them move onto the soybeans/edible beans when the grasses in ditches/fencerows start to dry up from drought or as wheat plants mature and dry out.  Because of this they will often show up on the edges of the field first.  They can produce silk webs to catch the wind and move greater distances and end up in patches in the middle of fields too.  Spidermites are much harder to see than soybean aphids so scout for the damage first, then look at those leaves for the mites.  The damage caused by spidermites is stippling (small, pin sized whitish spots) on the leaves.  Severely infested leaves may have a bronzed or grey appearance to them (often it will start in a patch in the field).  The mites themselves will be on the underside of the leaves and you’re best to use a magnifying glass to see them on the leaf.  The other option is to shake the leaf over a piece of white paper and look for small (pepper size) dots moving around on the paper.  You may also see silk webbing on the underside of the leaf.

If you look at them with a magnifying glass you’ll see 2 black blotches on their back (hence the name) and that they have 8 legs.  Spidermites can be very damaging to yield if ignored.   Four mites per leaflet or one severely infested plant is the threshold.  If they are moving in from headlands and you catch them early enough you may be able to get away with only treating the edges of the field.  Cygon or Lagon are the products of choice for spidermite control.  Pyrethroid insecticides (such as Matador) can cause spidermite populations to flare up.  Damp/wet weather will help encourage diseases that can kill the spidermites.

RED-HEADED FLEABEETLE: this is an oddball that we rarely see in soybeans but they are out there this year.  Damage looks very similar to other fleabeetles (small areas round circles of the upper surface of the leaf scraped off).  The fleabeetles themselves are skittish and will jump/fly when disturbed on the plant.  They are shiny black and have a dark red head.  Threshold for these insects is the same as for most of our foliage feeding insects – typically not a concern until we lose 15% of the whole plant leaf area.  It would be extremely rare to see fleabeetle get anywhere near this level of damage in soybean, but even low levels of damage are very obvious in the field.

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