There’s a thief hiding out in some central Arizona alfalfa fields killing healthy plants and leaving frustrated growers with slimmer wallets. In some cases, an entire crop cutting is stolen.
The unexpected thief is two types of cutworm: granulate cutworm (Feltia subterranean), formerly known as Agrotis subterranean; and variegated cutworm (Peridroma saucia).
“These cutworms have caused significant damage this summer in some alfalfa fields in Maricopa, Pinal, and Pima counties,” says Ayman Mostafa, University of Arizona (UA) Cooperative Extension field crops IPM area agent in the tri-county area.
After an alfalfa cutting, cutworms feed at the soil surface or just below it, dining on new growth with the grower picking up the tab.
“This year we’ve had heavy damage from cutworms,” Mostafa says. “In some fields, there’s heavy devastation 24 days after cutting with little vegetation despite two irrigations. In some cases, growers have abandoned entire fields.”
Plant death has been common this year from heavier pest numbers feeding more in fields from May through October. The highest pest numbers and crop losses occurred from late June into early July.
In an average year, an alfalfa field can average 15-20 percent cutworm infestation.
The cutworm has an appetite for ‘green’ – moving from nearby host plants including many nearby natural hosts in the desert, or commercial crops including alfalfa, as a new snack for hungry cutworms.
Mostafa said, “Cutworms are looking for new growth after an alfalfa cutting. It’s a good habitat for cutworm feeding.”
The last major infestation of cutworm in the state was more than a decade ago. About two-thirds of Arizona’s 300,000 alfalfa acres are located in the tri-county area mentioned earlier. Mostafa says no alfalfa varieties grown in the state are specifically cutworm tolerant.
The least common species of cutworm in alfalfa in the region is the black cutworm (Agrotis ipsilon). The No. 1 pest statewide in alfalfa is aphids.
Why the heavy cutworm numbers this summer? Mostafa said heavy rains earlier this year allowed many natural hosts to strive in the desert. These hosts likely supported higher than usual cutworm populations.
Then, the hot, dry period of April through June resulted in the desiccation of many natural hosts. Also, many crops in the regions were harvested during these periods.
Cutworms in these habitats found only alfalfa to continue their feeding adventure.
Cutworms not big problem in California
There are no reports of heavy cutworm numbers in neighboring California this year, according to a handful of University of California Cooperative Extension (UCCE) specialists contacted by Farm Press, including farm advisor Rachael Long in Yolo County at Woodland. Yet, she calls the cutworm a “sneaky” pest overall.
“You need to look under the leaf litter to find them. They mostly feed on the roots of established plants and cause general stand decline, in part due to pathogens…(Cutworm) feeding on seedling fields can spell major trouble for stand establishment.”
Long is aware of large numbers of alfalfa caterpillar butterflies this summer in the Sacramento and San Joaquin valleys which deliver their own type of damage, explained in a blog authored by Long, plus UC’s Dan Putnam and Larry Godfrey, located online at http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=21796.
Mostafa says cutworms are nocturnal insects which feed at night and hide during the day in soil cracks and under debris and loose soil. Growers, pest control advisors, and crop consultants should vigorously scout for the worm at night or early in the morning.
Light traps can be a useful tool to estimate the population based on the adults the traps attract.
Since summer lower quality hay typically fetches lower hay prices, Mostafa says growers need a cheaper way to control cutworms. Effective management tools include flood irrigation, tillage, weed control, and insecticides.
On the chemical end, Mostafa recommends a mix of pyrethroids and other groups of insecticides, plus more selective insecticides, including the product Intrepid.
Mostafa recommends flood irrigating the field during the day to attract birds to eat the larvae.
Arizona growers have the highest alfalfa yields in the nation with 8.4 tons per acre on average, compared to about 6.4 tons per acre in California. The national average is 3.4 tons per acre.
Growers in the Grand Canyon State harvest an average of eight cuttings annually with some growers netting 12 cuttings per year.
Texas root rot
Mostafa is conducting field trials on another threat to Arizona alfalfa growers in the low desert – Texas root rot (TRR). This pathogen, caused by the fungus Phymatotrichopsis omnivore, is more commonly known for its impact in cotton and alfalfa fields in the Southwest where it attacks plant roots including the tap root, leading to wilted leaves and plant death since diseased plants cannot uptake adequate amounts of water.
Following initial TRR trials conducted by Texas A&M researchers, a UA team including retired plant pathology specialist Mary Olsen, statewide cotton specialist Randy Norton, and Mostafa conducted numerous TRR trials in cotton with the herbicide active ingredient Flutriafol (Topguard) by FMC since 2012 with good success.
A Section 18 Emergency Exemption was granted for Topguard use in cotton for use in 2013, followed by a full label for the same crop in 2014. Efforts to enhance the method and timing of applications continue under the expertise of Norton and UA’s Mark Siemens.
Mostafa is testing Flutriafol for TRR control in alfalfa, now in his second year of on-farm trials. So far, the early findings suggest Flutriafol is effective.
TRR thrives in areas with high soil pH, low organic matter (most of Arizona) and old mesquite tree areas reclaimed for commercial agriculture. Mostafa estimates about 20 percent of all alfalfa acreage in the state have TRR, perhaps more acreage than Arizona cotton.
“Once a field has Texas root rot it ruins it for alfalfa production,” Mostafa said. The only other crops which can be grown in infected ground are several grains and grasses.”
In the field trial this year near Arlington (Maricopa County), Mostafa has applied the new label Topguard Terra over the top at three different application rates – two ounces, four ounces, and eight ounces.
“From the 2015 results and those obtained so far in 2016, there are significantly higher yields using Flutriafol Terra at all the rates compared to the untreated check,” Mostafa said.
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Mostafa says discussions are underway with FMC for the farm chemical company to apply for a one-year Section 18 Emergency Exemption for Topguard Terra use in alfalfa, in part with Mostafa’s alfalfa field trial findings to support it.
“If we can get a label for alfalfa, Topguard Terra could be a good option for growers to increase yields despite the disease, and possibly bring abandoned fields back into production.”
Mostafa plans to conduct trials next year again in Arlington, plus in Parker (La Paz County) and the Gila Bend area (Maricopa County).