Alfalfa Weevil Alert for Texas High Plains Alfalfa

Alfalfa Weevil Alert for Texas High Plains Alfalfa

              The alfalfa weevil pressure for our Plains Pest Management (PPM) scouting program alfalfa is usually hit or miss.  Some years I see them around economic levels and some years that are just absent.  Even in the seasons I do see them, we don’t always need to treat, and it is usually only for the first cutting.  Many alfalfa fields in the High Plains region are not so lucky with the weevil being an annual and season long issue.  The reasons for this are debatable and hypothetical but I usually attribute it to the few number of fields in my territory that have dwindled with the diminishing water resources.  I suspect that the more serious alfalfa areas in the region have the weevil bad enough in some seasons that we get the overflow., but without serious study, there is no way to make certain the reasons for this usually minor issue.

Alfalfa weevil feeding near growing point, Lubbock.  Photo by Dr. Pat Porter

Light alfalfa weevil damage in Lubbock this week.  Photo by Dr. Pat Porter

              What is certainly known is that the few alfalfa fields in Hale and Swisher counties are very important to the producer investing huge amounts of inputs to raise this alfalfa here and the consuming livestock in the area depending upon the resulting hay.  Without it, feeding costs would skyrocket with high shipping costs.  So, why the alarm today?

Alfalfa weevil larva.  Photo by Dr. Pat Porter

              Last week I made my usual visit to our PPM alfalfa field and found newly hatched young alfalfa weevil larva at a level I had not seen locally before.  This population averaged 12.8 weevils larva per 20 sweeps with the economic threshold (ET) being about 4 per 20 sweeps.  Alongside these weevils were a notable population of aphids.  Plainly these weevils were over ET and needed to be treated.  I recommended prompt chemical treatment with a labeled product that would control the weevils and leave the beneficials to control the aphids.  This week, I returned to our PPM field to find good control, but still more freshly hatched weevil larva, the majority of which were freshly dead or seemed sick with residual control still having good impact.  With this very high level of infestation, I opted to spot check a few other fields in the area I am aware of.  These fields today exhibited 20.25 and 16.5 weevils per 20 sweeps in spot checks.  This season’s alfalfa weevil issue seems much larger compared to a ‘normal’ year’s pressure.  So much so, I can imagine more traditional alfalfa growing areas could also be seeing higher than usual pressure and a regional-wide alert should go out, alongside a few tips on how to scout for this key alfalfa pest.  Aid to point, Dr. Pat Porter found ample weevils at the Lubbock Research Farm on a sample alfalfa plot this weekend.

Photo from PPM alfalfa field this week near Tulia.  Note old feeding damage with new growth showing little to no new feeding damage following treatment.  Photo by Blayne Reed.

              Visual scouting with kneeling and inspecting sprigs and growing stems for damage can find some of the pest, but to truly find the level your field is at, you need a bit more knowledge, and possibly some simple tools.

Alfalfa Scouting

              There is one main tool you almost must have for scouting alfalfa correctly and accurately.  That is a sweep net.  No, entomologists do not use them to chase butterflies across pastures or parks as cartoon caricatures show.  At least not often.  They are much more useful to gleam insects from vegetation.  If done correctly, the operator will find insects in higher numbers than any visual inspection would ever find.

Common sweep net used in checking near Tulia this week.  Note strong wire frame, durable net, and long handle.  Photo by Blayne Reed

              The idea is to vigorously ‘sweep’ the net through the upper 1-3 inches of vegetation reversing rapidly as stepping forward for a backhanded reverse sweep to gleam the adjacent vegetation.  This ‘backhanded’ sweep should be made after twisting the net over so that the open net is ready to receive the next sweep.  Ideally, this process should be repeated, sweep, twist, step, sweep, until you have completed 20 sweeps.  For most other crop pest scouting where sweep nets are essential, this 20 sweep per data set is the standard. 

              On the last ‘sweep,’ pop the captured insects in the bottom of the net and harvested vegetation over with the last twisting momentum to prevent escapes.  This process sweeping and capturing motion must be maintained vigorously without pause making use of inertia and other forces so that ‘swept’ insects have not chance of escape.  Once completed, place the net handle between the knees so both hands are free to carefully draw up the bottom of the net little by little.  Along the way, slowly identify all insects caught.  Remember, not all insects caught are pests.  Many are inconsequential and better yet are beneficial.  It should be good to know exactly how many good insects you have in the field to combat the pests.  Count all pests and beneficias alike.  Once the net is clear, I like to kneel and visually inspect two alfalfa plants.  Record all relative insects, plant damage, and make note of any weed populations in the area.  This completes one data set.  There should be 4-8 data sets taken per field, size depending, to form a proper representation of the insect population present.  These data sets should be made evenly across the field and results averaged for each species. 

Once the data set is complete, pop the contents over to trap insects in the bottom of the net.  Photo by Blayne Reed.

Alfalfa weevils near Tulia this week.  Note both dead, sick, and live larva among sub ET aphids captured by sweep net.  Photo by Blayne Reed

For alfalfa weevil a population of 4-5 per 20 sweeps should be ET, beneficial population depending.  For aphids, there will need to be visual stickiness on about 40% of the plants with more infested before it is economic. 

Photo of the distance traveled across the field near Tulia this week from the pickup to get good data and eliminate any edge or spot effects.  Photo by Blayne Reed.

Good Luck out there folks!

Blayne Reed