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Extension Entomology

Category: Alfalfa

Leafhoppers in Alfalfa

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Alfalfa can be kind of a forgotten crop during this very busy time of year.  Most growers are busy planting sorghum, soybeans, sunflowers, and/or harvesting wheat, working wheat ground, etc.  This year, so far, has been a rough one for swathing and baling alfalfa.  Now, there are substantial infestations of potato leafhoppers, both adults and nymphs.  Potato leafhoppers can be quite damaging from now until fall.

Leafhopper adult

Leafhopper nymph

 

Both nymphs and adults feed by sucking juice from the plant and in so doing inject a toxin into the plants.  This can cause serious yellowing of the leaves, and even stems, if it continues for very long.  Generally, swathing and hay removal will disrupt this feeding and remove the leafhoppers from the fields.  They rarely re-infest fields after this physical removal or after an insecticide application, if that is justified.  If swathing is possible within the next 7-10 days, that should take care of the potato leafhopper problem.  However, be sure to keep scouting these fields as the damage caused by potato leafhoppers usually occurs much before the yellowing is noticed.  This feeding damage is cumulative – the stressed plants don’t seem to regain their original vigor and therefore don’t ever yield as much tonnage as expected.

Alfalfa – Pest Update

–by Dr. Jeff Whitworth, Dr. Holly Schwarting & J.R. Ewing

Cool, wet weather continues to dominate the state. This has allowed the alfalfa weevil adults to remain in the alfalfa fields. As they remain in fields they are feeding in/on terminals and the new foliage. However, they don’t feed anywhere near as much as the larvae, plus there aren’t near as many so, the feeding really is negligible. However, they do cause concern and their feeding on the new foliage and “barking” of the stems can slow regrowth if they remain in fields. Swathing alone will not eliminate the adults if the weather continues to be only in the 80’s for daytime highs.

 

AW adult barking

The first potato leafhoppers of the summer were detected in north central Kansas on 19 May. This is a little earlier than usual but fits right in with the unusualness of this spring.

Pea aphid populations have been steadily increasing the past couple of weeks. However, sampling some of the same alfalfa fields on 19 May indicated the aphid populations have reduced relatively dramatically. There are many larval and adult lady beetles in these fields and they can even be readily observed feeding on the aphids. So, these beneficials really seem to be helping with aphid control.

lady beetle larvae w aphid

Alfalfa Update

Wet weather has kept equipment out of the fields and has allowed the insects to flourish.  Fortunately, the plants are also doing well and tolerating these healthy populations of pests.  When these fields dry out and are able to be swathed, there will probably be relatively significant populations of alfalfa weevil larvae and adults still present.  They are accumulated into the windrows at cutting; they will feed on the stems under the windrows until the hay is picked up.  This causes the characteristic stripping in the fields where the regrowth under the windrows is chewed off while the areas beside the windrows grow back.  Once the hay is removed, the adult weevils will exit the fields and no longer be a problem.  Weevil larvae in north central Kansas on 7 May were 93% 3rd instar, and 7% 1st or 2nd instar.  There are many pupae and adults also.

weevil

weevil2

There are significant populations of aphids, mainly pea, in these central Kanas alfalfa fields as well.  Swathing will remove this problem, but there are many beneficial insects helping to regulate these aphids, and hopefully they will stay around after swathing to help manage any other aphids and/or potato leafhoppers that may show up (no leafhoppers detected yet).

aphidlacewing 2

Picture1lacewing

Some small grasshoppers were also noted.  Usually, grasshoppers do better in drier conditions, but not always.  So be aware that grasshoppers are present now and eggs will continue to hatch for the next month.

grasshoppergrasshopper2

—Dr. Jeff Whitworth and Dr. Holly Schwarting

Alfalfa – Weevils and Aphids

—Dr. Jeff Whitworth – Dr. Holly Schwarting – J.R. Ewing and Salehe Abbar

Alfalfa weevil feeding is winding down in north central Kansas, but not as fast as usual. If treatment is still considered prior to swathing do not forget to read the label and follow the PHI (post-harvest interval) required by the product you choose. The untreated control plots in our insecticide efficacy trials are still above our treatment threshold of 1 larva/2 stems, or 50% infestation, and contain many small larvae. Remember, larvae were 1st detected on 13 March so they have been actively feeding for six weeks. However, the good news is many insecticides are still providing excellent protection from one application three weeks ago. See below:

Screen Shot 2015-05-02 at 9.46.15 AM

Aphids continue to increase in alfalfa fields. They are primarily pea aphids with a few spotted aphids. Have not seen or heard of any at, or even close to, treatment thresholds, but populations are increasing. Lady beetle and parasitic wasp populations seem to be rebounding which should help slow down these aphid populations.

 

Alfalfa – Weevils and Aphids

—by Dr. Jeff Whitworth and Dr. Holly Schwarting

Alfalfa weevils continue to be very active in north central Kansas. The recent cooler weather has slowed down development a little but they are still feeding. We determined development from larvae collected on 20 and 22 April. Here is what the population breakdown looks like:

20 AprilNo. larvae   23 April No. larvae
12 1st Instars 4
25 2nd Instars 16
15 3rd Instars 30
   numerous Pupae numerous

alfalfa weevil

So what does this mean? Alfalfa weevil larval feeding will continue for another 7-10 days, depending on the weather. Egg hatch and consequent larval feeding has been going on since 13 March in north central KS. Insecticides applied since that time have provided adequate protection, for the most part.

field trial

This photo shows KSU chemical efficacy trials with many different products being tested, and the obvious untreated plots plus the border around the plots. The rest of the field was treated with Stallion® by MKC in Abilene, KS and, as illustrated here seemed to work relatively well with 1 application. Remember, feeding will continue for at least another week and therefore treatment (or re-treatment) may still be appropriate.

Alfalfa aphids, mainly pea aphids, are becoming more numerous throughout north central Kansas. Treating for alfalfa weevils probably pretty much decimated the natural enemies/beneficials and they will not repopulate as quickly as the aphids migrate in to infest fields.