Timing critical factor in control of three-cornered alfalfa hopper

Three cornered alfalfa hoppers have been a periodic pest of peanut farmers in the lower Southeast for several years, but have not consistently been a problem in the upper part of the Southern peanut producing belt.

However, in fields where this pest occurs in high enough numbers it can cause real problems for Carolina peanut growers.

The hopper girdles the stem of a peanut plant and builds a nutrient dam, which backs up sugars flowing from the leaves, robbing the plant of nutrients needed to fill peanut pods. Symptoms, such as girdled stems or adventitious root growth may go unnoticed, but in the most severe cases there can be yellowing of leaves and stunted plant growth. Most growers don't know they have a problem until they dig their peanuts. In the lower Southeast, where hopper damage has been more severe, Ron Weeks, an entomologist at Auburn University's Wiregrass Research and Extension Center, says the most visible symptom of three cornered alfalfa hopper feeding is the thickened callous tissue or girdle that encircles the stem or petiole of the plant.

Other plant symptoms include a purple discoloration of the stem above the feeding site and the eventual yellowing of the affected terminal. One possible reason this insect pest has been a more significant problem in the lower Southeast is an apparent preference for runner types versus Virginia type peanuts. Georgia Green, by far the most popular runner variety now grown in the Southeast, has been particularly vulnerable to three cornered leafhopper damage.

In addition to peanuts, three cornered alfalfa leaf hoppers attack soybeans and alfalfa.

Past studies indicate a higher level of hoppers in fields with cover crops. This has not been a serious problem and should not cause farmers to not use a cover crop, if that cultural practice works for their particular operation. Likewise, the insect is easy enough to control that it should not prevent a grower from planting Georgia Green or any other variety that works.

“We do see a lot of girdling from these pests in South Carolina, but we haven't had a widespread problem with stunting and yellowing of plants as is reported in Alabama and Georgia,” notes Jay Chapin, who is an entomologist and peanut specialist at Clemson University.

Three cornered alfalfa damage may be confused with the symptoms of some soil diseases of peanuts like Cylindrocladium black rot, Rhizoctonia limb rot or tomato spotted wilt virus. Multiple girdles of the vertical terminals and lateral runners may severely affect the maturation of the peanut pods, according to Weeks, writing in an Extension publication.

In addition to peanuts, three cornered alfalfa leaf hoppers attack soybeans and over-winter on a number of crops.

Chapin points out that though the wedge-shaped, green leafhopper prefers certain runner type peanuts, they do attack Virginia types, and do cause damage in some fields. To learn more about the insect pest and to help prepare Carolina peanut growers to manage it, Chapin's peanut research team at the Edisto Research Center in Blackville, S.C., conducted a two year test in 2004 and 2005.

Based on these tests, the South Carolina researchers found that the three cornered alfalfa hopper produces two generations per season in peanuts. Adult hoppers first colonized test fields in June and produced a generation of nymphs from late June to early August in peanuts planted between May 11 and May 23.

The second generation of nymphs appeared in the peanut plots from late August through September. This produced a second generation of adults, which appeared in September.

No significant damage from girdling was seen in June. Injury levels rose gradually in July, but then increased significantly during the last week of July and first week of August. A second generation of girdling appeared in the test fields in September, corresponding with the second field generation of the insect.

In South Carolina, Chapin says a mid-July treatment has been most affective in managing the pest. In June, there was little damage from this insect. Chapin points out growers in South Carolina typically treat peanuts with fungicides on about a 15-day schedule, starting at 30 to 45 post-planting. Tank-mixing a synthetic pyrethroid insecticide with the July fungicide application was the most effective timing for suppressing hopper damage.

The first generation of nymphs builds up in June and early July, but they haven't created any significant damage until mid to late July. By using a relatively inexpensive insecticide with their standard fungicide applications, South Carolina growers have managed the girdling damage from these leafhoppers with minimal cost.

In the South Carolina tests, researchers also compared girdling damage in plots with and without Temik. Peanuts treated with Temik showed more girdling damage than those not treated with the popular nematicide/insecticide. The logical reason, Chapin says, is that without Temik, the peanuts were so badly stunted by thrips that they were not attractive to hoppers.

In-furrow insecticides apparently don't have enough residual efficacy to control the hopper colonizing fields in June.

The study also found that granular Lorsban treatment timed for soil insect control (50 days after planting) reduced hopper girdling, but was less effective than the best foliar treatment timing.

“In our tests, insecticides were applied, starting at 15 days after planting, then coinciding with fungicide timing at 30, 45, 60, 75, and 90 days after planting. It was clear that using an insecticide at 15 and 30 days was too early and 75 and 90 days was a bit too late for a 23 May planting. Though the insect was in the field earlier, our treatments at 45 and 60 days after planting were most effective in minimizing hopper damage for this planting date. Based on when the hoppers caused girdling in grower fields over a series of planting dates, treatment during the first two weeks of July can prevent injury if hoppers ever become a consistent problem in South Carolina,” Chapin says. With the percentage of Virginia type peanuts already high, compared to runner types, in the Carolinas, the three-cornered alfalfa hopper is likely to remain a pest that needs to be monitored, but not one that is likely to cause widespread economic damage. If growers do have a problem with the pest, a number of insecticides are available to provide economic, effective management of the problem — with timing being the key to both.

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