Leafminer, shuckworm pests target New Mexico pecans

New Mexico pecan growers and consultants are on alert after a new leafminer species was found last year in the state along with the reappearance of the hickory shuckworm.

Carol Sutherland, New Mexico State Entomologist, late last summer tentatively identified the critters as the pecan serpentine leafminer, Stigmella juglandifoliella, and hickory shuckworm, Cydia caryana.

“The pecan serpentine leafminer creates winding tunnels inside pecan leaflets which can cause tattered foliage when the mines dry and break apart in the wind,” Sutherland told a 600-plus crowd at the 45th annual Western Pecan Growers Conference in Las Cruces, N.M., in March.

“The pecan serpentine leafminer will likely be a very minor pest in New Mexico pecans – a spotty critter found today, perhaps not again for several years, or found on another farm down the road,” said Sutherland who also serves as New Mexico State University Extension entomologist.

The insect was found in two New Mexico commercial pecan production areas; first in the Roswell area in Chaves County on a late-season flush of new leaves. The second find was in sparse foliage in small caliper trees transplanted in early 2010 in the Mesilla Valley in Doña Ana County.

Healthy, mature trees can lose a few leaves but are easily replaced by vast reserves. The pecan leafminer is an occasional but sometimes serious pest in the southeastern U.S. pecan belt.

At least five species of leafmining caterpillars from three families in the Micro-Lepidoptera family attack pecan. The wingspan on the tiny moths are 10 millimeters or less; smaller in width than a thumbnail.

“Leafminers are likely controlled by weather, climate events, plus natural enemies especially in ‘wild’ stands or backyard trees,” says Sutherland. “In commercial orchards, cultural practices and insecticide treatments for other key pecan pests can reduce leafminer numbers or keep the insect at low levels.”

Many chemicals with numerous active ingredients registered in New Mexico are available for pecan serpentine leafminer control. Unknown is whether the insect has been in New Mexico for awhile or if the bug was transplanted from elsewhere. If the leafminer has been around, Sutherland says insecticide treatments for aphids and the pecan nut casebearer may have suppressed pecan leafminer numbers.

More than one generation of pecan leafminers are likely in New Mexico annually. Leafminer moths have a complete metamorphosis: egg, larva, pupa, and adult stages.

“I encourage producers and consultants to spend more time looking for mines and foliage irregularities,” Sutherland said. “If mines are found, determine how common and widespread the mines are and whether economic damage is occurring before making a decision to treat.”

Mines cannot heal and insecticides cannot repair damaged foliage. Pheromone traps do not exist for the tiny leafminer. Unknown is when insect flight activity peaks.

The other pest now on the New Mexico pecan radar screen is the hickory shuckworm (HSW). The pest was found late last summer in the Roswell area after a nearly 20-year hiatus. Alert scouts found black spots, blemishes, and bruises on the outside shucks and numerous small, white caterpillars tunneling inside the shucks.

“The nuts looked like they’d been through a hailstorm,” Sutherland noted.

The pest was found in 100 to 150 acres of commercial pecans. Inside a single shuck, Sutherland found 30 to 35 crawling caterpillars and narrow diameter, winding trails through the dried, blackened shucks plus the remains of two pupa cases typical of the caterpillars.

Caterpillar chewing jaws tunnel into the shuck disrupting the flow of moisture and nutrients to the developing nut. Very immature nuts are lost while more mature nuts may remain on the tree. HSW damage to older nuts can produce sticktights and nuts that never produce a mature or dry kernel.

HSW was found in the early 1990s in backyard pecan trees in downtown El Paso, Texas and then in the Mesilla Valley. HSW was also found in commercial and backyard pecan nuts in Chavez, Eddy, and Lea counties, but populations dwindled for unknown reasons by the mid 1990s.

HSW is a native insect found mostly in the eastern U.S. pecan-growing region from central Texas, Oklahoma, and Kansas to the Gulf Coast, Georgia, and South Carolina. HSW is a potential serious pest of pecan nuts, second to the pecan weevil, in those areas.

Sutherland said, “Hickory shuckworm has the potential to decrease pecan nut quantity and quality although its predictability and economic impact seem sporadic now in New Mexico.”

As with the pecan leafminer, HSW has a complete metamorphosis. Pheromone traps do not work well for shuckworm and many other species of small, look-alike moths.

More than 200 pesticides including 50-plus active ingredients are registered in New Mexico. Producers should check product labels already used for pecan casebearer control to determine if HSW is also included.

The adult HSW is dark gray to smoky black in color with a half-inch wing span. At rest, the insect is about three-eighths inches long with the wings held tightly against the abdomen.

Suggested HSW management includes looking for shuck abnormalities during scouting. Female HSWs leave small chalky markings on shuck surfaces when laying the eggs. The marks may resemble bird droppings.

“Mark the nuts and come back in a week to see if anything has hatched,” Sutherland suggested. “Open the husk and check for caterpillars or maggots with a hand lens.”

HSW caterpillars are white with well-defined, tan-head capsules and have three pairs of short thoracic legs and five pairs of short, fleshy prolegs on the abdomen. In contrast, the walnut husk fly, an occasional pecan pest, does not leave chalky markings when laying eggs. Immature husk fly larvae are legless, conical shaped, dirty-white maggots with tiny mouth hooks.

Mike Hall, Louisiana State University research and extension entomologist in Shreveport, La., also discussed HSW at the conference. Hall says HSW has one distinguishing characteristic that separates the insect from the similar pecan nut casebearer.

“The hickory shuckworm moth has a series of stripes running along the outside edge of the front wings,” Hall said. “The pecan nut casebearer is silvery gray with a ridge of scales across the back.”

Hall has conducted insecticide trails for HSW in commercial pecans in Louisiana. He says insecticides are the best method to control the insect. All tested products provided good control.

Hall offers these suggestions before applying insecticide sprays for HSW.

First, make sure the insect is present – there is not a good way to monitor its activity. Pheromone traps do not catch HSW during the hot active months of July through September. Black light traps are the most practical way to monitor insect activity.

Second, look for damage. Orchard sanitation helps control the insect. For a small pecan planting with HSW, destroy the old shucks and dropped nuts to reduce the infestation.

Third, select the appropriate insecticide keeping the pest complex in mind. If the stink bug is also found, use a product mixture or a pyrethroid effective again the stink bug and HSW. Make sure the spray equipment is working and calibrated properly.

And last, use water with a 5.5 to 6.5 pH level in the spray mix for maximum product efficacy. Insecticides are ineffective with water at 8 to 9 pH, Hall says.

Pecan serpentine leafminer and HSW are found throughout the Texas pecan belt, but not in Arizona and California pecan orchards, according to university Extension specialists in the respective states.