Southern Spotlight: Pests in our Alfalfa Fields

James Banahan

James Banahan

I’ve been looking at a lot of alfalfa fields this week and they look, well, a little lackluster.  Of course, as cattle continuing to sell at solid prices, not one of us is too happy to see their food supply looking so shabby.

So what to do?  The main culprits of our less-than-stellar alfalfa fields right now are pests, and four in particular: pea aphids, clover worms, alfalfa weevils, and potato leaf hoppers.  I’m including some identifying information for each below as well as some info on their usual M.O. to help you diagnose any infestations in your fields.

With our dry weather, we might only get 2-3 cuttings this year, so we need to take extra care to protect the growth we will get.  Infestations of pests such as the four that I’ve named here can be detrimental to fields, easily causing a ¾ ton reduction in our first cutting alone.  Conservatively, that’s $75 per acre.  With alfalfa selling in excess of $100 per ton in most areas, just 2-3 weevil larva and/or 50 aphids trigger the economic threshold for spraying.  Fortunately, these pests are relatively easy to control with an application or two of insecticide, the cost of which hover around $19 per acre (as sprayed by your friendly co-op).

  • Pea Aphids are pretty big monsters: adults are a quarter inch in length.  The mark that best distinguishes them from other aphids is the dark bands on their antennae.  Don’t count on color to help you distinguish a pea aphid, as adults might be many shades of green, yellow, or even pale pink. These pests hang in your alfalfa all summer, feeding on stems and growing leaves.  They can turn leaves yellow and in moderate numbers (like 50-100 per stem), stunt plant growth.  Heat is an enemy of your enemy with the pea aphid: temperatures over 90 degrees put the brakes on their reproduction.  Even pea aphids don’t like to cuddle in the heat.
  • Clover Worms aren’t too tough to identify: larva is pale green and usually has two white racing stripes on each side.  They thrash violently when disturbed.
  • Alfalfa Weevils make artwork with your leaves.  If your leaves look like lace, it’s probably the work of alfalfa weevils.  Their larvae eat the soft parts of terminal leaves but leave the veins, creating the lacy look that’s cool, but not so great for us and our cattle since their handiwork reduces forage quality.  Further, adult alfalfa weevils damage regrowth by crunching on new buds.  Alfalfa weevil larvae are initially yellow or light green—they turn dark green as they get older.  All have a black head and a white stripe down their backs. Adults are monsters: a quarter inch long with a dark brown stripe down their backs and a long, ugly snout on their faces.

    Screen Shot 2015-04-22 at 10.53.42 AM

    Alfalfa Weevil Larvae

  • Potato Leaf Hoppers are smaller—an eighth of an inch long—yellowish-green and shaped like a wedge.  They often move (jump, fly) sideways when bothered and wreak their havoc on your alfalfa fields mostly midsummer.  Affected plants may have v-shaped yellowing on their leaf tips (this is “hopper burn”).  Some years, hoppers can do incredible damage to alfalfa fields.  Chances are pretty slim right now that hoppers are your problem, but you should still be open to the possibility and on the lookout.  If you find yourself with an infestation, experts recommend harvesting to remove eggs before putting down an insecticide treatment.