Whats killing my trees?
Oak Roof Fungus Or June Beetle?
Dave Dias plucks a fat, creamy-white grub with a dark-brown head from the exposed root of a downed almond tree, holds it up to the light and points a tanned finger at the curled insect, "We didn't know what was killing our trees until we pulled one and found these chewing on the roots," he says. "At first, we thought the problem might be oak root fungus, but we couldn't find any signs of that." The problem, it turns out, was an invasion of tenlined June beetle.
Dias, farm lab manager at San Joaquin Delta College, discards the struggling grub and gestures in a sweeping motion at the sick trees surrounding him. "We've lost 85 trees to June beetle since 1986 and it's too late to stop it. The orchard is 17 years old, so we'll continue to farm it as long as it is economically viable. Then we'll pull the trees and fumigate the ground."
If identified early in its infestation, the beetle can be stopped, or its invasion severely impeded, says Dias. "Had we known what the problem was when the first trees began to decline, we could have done something about it, but too much time went by before we knew for sure what we had." Now he knows what to look for.
"The problem began in one corner of the orchard about three years after it was planted," he explains. "The trees were stunted. We thought there may have been some kind of contamination in the soil that was affecting those trees. A few years later, the trees began to die. That's when we thought we had oak root fun fungus, but we could never find it."
A few years later the problem began to spread at a more rapid pace. "About that time, the drought set in and the problem began snowballing on us," Dias recalls.
"When it appeared in another spot in the orchard, we got real aggressive about finding out what it was," he says. "At first, when we pushed a dead tree over to examine amine the roots, we couldn't find anything suspicious. Later, we noticed some grubs among the roots and identified them as ten-lined June beetle larvae. By that time, our orchard was heavily infected."
What Dias found were larvae with a brownish head with six tiny feet up close to the head, and a long, cream-colored abdomen. The grubs, which are an inch and a half long and about 3/8-inch wide, curl into a "C" when taken from the soil. The adult beetles are chocolate late brown with ten creamy-white stripes down the wing covers.
The ten-lined June beetle infestation at the Delta College farm is not unique. According to San Joaquin County Farm Advisor Paul Verdegaal, the pest is known to be gnawing away at roots in other almond blocks in sandy areas of the county, as well as in some walnut, apple and cherry orchards in other areas of the state. He suspects the beetle is chewing its way across a steadily-widening swath of California farmland, largely undetected, because its symptoms are general enough to mask as oak root fungus or other stresses.
Dr. Robert Van Steenwyck of U.C. Berkeley conducted a baseline study of the beetle's biology in the mid- 1980s. His study was written up in the July-August 1990 edition of California Agriculture. Van Steenwyck says that known infestations of June beetle in commercial orchards are not numerous at this time and are largely confined to the sandy areas of San Joaquin and Stanislaus counties, but the pest could appear elsewhere at any time.
According to Van Steenwyck, the slow, outward spread of June beetle infestations occurs for two reasons: the beetle's two-year larval stage, and the stay-at-home habit of the female beetle. She is little inclined to travel from her burrow to establish a new site unless her food supply is running low. She emerges only to mate and then at the entrance of her burrow. Afterwards, she dives back in to lay her eggs. If food is running short, she will travel as far as the next tree and dig in there.
The larvae do the damage to almonds, feeding on roots for two years while developing underground. Most are found in the top 14 inches of soil, but have been found feeding five feet below the surface.
Controlling the pest isn't easy. As Dias points out, "You don't know they are there until the tree starts to decline. By then it is usually too late to save the tree, but it serves to let you know you have a major problem."
He says, "Affected trees probably won't die the first year you see them in decline, but by the second year it is usually so far gone it will die or you will want to pull it out. You usually see the symptoms early in the year. Infested trees leaf out earlier than normal. Then, as the weather warms up, usually in April, and the trees begin to push, they won't have the necessary root capacity. So they collapse and within a few weeks are dead."
So what can a grower do to fend off a suspected infestation of ten-lined June beetle? According to Verdegaal, the best control is to fumigate before planting. "After that, they are very difficult to control," he says, "because the larvae are too deep to be effectively controlled with soil-applied materials."
"If a grower discovers he has June beetle, it is not panic time," says Van Steenwyck, "but he should take it seriously and develop a plan for controlling it. If he ignores it for even a few years, it could get out of control."
If trees are dying, tough measures are called for, says Van Steenwyck. He recommends taking out the damaged tree(s)) plus two trees in every direction outside the infested area. "Then, fumigate the area with methyl bromide to kill all the larvae and beetles living in the soil," he says. "Pulling the trees bordering the infested area is important. While it may seem a radical measure, it helps as sure a complete kill, because some beetles may have migrated or larvae could have been carried outside the infested area by farm equipment."
If tree pulling and fumigating are out, it is worth trying soil applied materials to keep the infestation from spreading, although most currently-available materials are not likely to penetrate the soil profile far enough for a major kill. The most suitable times for this type of control are during the larval stage in September and again in May.
Unfortunately, there is no chemical guideline for June Beetle at this time, but this is being pursued, says Verdegaal. Research is underway in both chemical and biological control.