University of California specialists are gathering information on the behavior of the tenlined June beetle to learn how to manage it on almonds and other vulnerable tree crops.
According to Mark Freeman and Richard Coviello, Fresno County farm advisors, the beetle, measuring over an inch in length, is a native species, but economic damage by its larval form, especially on sandy soils, has been increasing in recent years.
The pest, named for the 10 lines on the wing covers of the adult form, ranges from San Joaquin County to Fresno County and is suspected of being in the Sacramento Valley. How great a threat it is to Tulare and Kern counties is not known.
Freeman received a grant from the California Almond Board this year for the work. He anticipates continuing industry input for research by UC entomologists, plant pathologists, and agricultural engineers as well as their counterparts out of state.
“UC specialists saw root damage from it 10 to 15 years ago in almonds in the Ripon and Manteca area, and we've also seen its damage in Fresno County on roots of cherry, apple, stone fruit, walnuts, and grapes. It has a wide host range,” Freeman said. Almond trees up to four or five years old seem to tolerate the beetle.
Did not recognize
“We probably have had the problem all along, but we didn't recognize the extent of it. It may be that with more almond acreage, there were more sandy soils for it to show up on. Or it may just have taken more time for it to build up in an almond monoculture.”
In the last couple of years, Freeman has received more calls from almond growers in his county about established trees declining with typical symptoms of salt damage: poor foliar growth and leaf burn. Analyses of leaf samples, however, revealed no toxic levels of salt.
“Then when growers pulled out the trees, they saw the root damage and many of the beetle's larvae in the soil,” he said. The C-shaped larvae or grubs are nearly two inches long, and their two-year cycle at depths of as much as five feet makes them hard to reach.
Earlier, he said, growers used diazanon granules for control, but the material is no longer registered for use on almonds.
“That used to be the control, but since it was removed growers have just had to live with losses. Our job now is to discover the beetle's Achilles Heel — its movement and its food sources — and use that to control it,” he added.
Some things, but not enough, are already known about the beetle, a member of the scarab family that also includes the Japanese beetle, a serious pest of turf.
The larvae are also found in heavier soils, but infested roots in them show less decline. Male adults are nocturnal flyers during the summer and are attracted to ultra-violet light, while females, the real target for meaningful control, usually emerge from their protected, underground burrows only during mating.
The larvae are mobile and seem to follow soil moisture. By some conjecture, they occur more on the “wet and dry” pattern of micro-sprinkled orchards, although they also are found in almonds under flood irrigation.
There is no hard evidence that they go to particular varieties of almond. Grower reports suggest they seem to favor the Nemagard peach-almond hybrid rootstock used commonly on sandy soils. But the above-ground symptoms on those trees is less pronounced, said Freeman, who is polling growers to determine the acreage and varieties lost to the beetle.
“After talking with growers, it looks to me as if several hundred acres have been lost in Fresno County. Certainly the sandy soils around Ripon and Manteca have had considerable losses.”
Research in the mid-1980s by UC, Berkeley entomologist Robert Van Steenwyk indicated that known infestations of the beetle in commercial orchards were not numerous at the time and confined mainly to sandy soils in San Joaquin and Stanislaus counties. He also warned, however, that the beetle could appear elsewhere at any time.
Freeman is assembling UC specialists, other researchers, growers, and PCAs to examine potential applications of existing and new technology. He is looking for funding resources while considering techniques used to apply fertilizers or nematicides. He also plans trials soon with current techniques and registered materials. He declined to mention any product names.
In developing a control strategy, Freeman said the specialists are concentrating on methods of delivering chemicals to the underground larvae. One method might be to starve larvae and stop their spread by treating roots of infested sites where trees have been pulled out.
“We know we have available insecticides that will kill this pest, but the problem is how to deliver them. It would be like using a nematicide and could cost hundred of dollars per acre.
“We know the larvae are on roots down to five feet. We need to know more about the beetle's basic biology, so we'd know if we could kill enough of them for control at, say, two feet.”
One potential answer, taken from landscape practices in the eastern U.S. and outside the country, is broadcasting a freeze-dried formulation of a milky spore bacillus product on the soil surface. Rainfall carries it into the soil. When eaten by the larvae, the bacteria germinate in the their gut and destroy them.
“It's been used for 20 years against Japanese beetle. Once applied to the soil, the spores persist for a long time. They are heat- and cold-tolerant and can live in wet or dry conditions,” Freeman said.
Delivery to the deeper pest, again, is the question, and that's where UC agricultural engineers have taken up the challenge. One possibility is getting the bacillus, in a liquid form, deep enough to reach the larvae.
Coviello said another approach might be biological control with a large Campsomeris wasp enemy of the tenlined beetle. It parasitizes the larval stage, but the larvae have to be in sufficient numbers for the wasp to become effective.
To inspect problem orchards for tenlined beetle activity, Coviello said growers can first evaluate tree symptoms against water management and fertilizer programs. If no answer is there, they can then dig down two to three feet to inspect roots for damage and larvae.
Holes as indicators
Another sign is the beetle's half-inch diameter emergence holes around trees. Two or more holes per square foot suggest an infestation.
“We are asking help from anyone who sees the symptoms and beetles so we can get better estimates of where it is occurring in Fresno County and surrounding areas,” Coviello said.