Almond damage caused by elusive plant bug prompts additional scouting and insecticide treatments

By Vicky Boyd

Editor




A pest typically found in pistachios has moved into almonds this season and is causing significant crop damage in some south San Joaquin Valley, Calif., orchards.

Whether the leaffooted plant bug will continue to be an almond problem in 2007 is unknown, says David Haviland, a University of California Cooperative Extension farm advisor in Kern County.

“In 1990 and 2000, it shot up and caused enough damage that research was started,” Haviland says. “Then it disappeared from 2001 to 2005. All of a sudden this year, there are huge numbers from Kern County through Fresno County.”

In fact, Kern County farm advisor Mario Viveros estimates that 60,000 to 70,000 of Kern County's 86,000 acres of almonds are infested with the 1-inch-long brown bug this season.

Gerry Guthrie, a field supervisor for Blue Diamond Growers in southern Tulare and Kern counties, says he’s found the bug in most of the orchards he serves.

“The appearance of the bug is fairly widespread in the southern San Joaquin Valley,” Guthrie says. “The incidence of a high percentage of crop loss is not widespread.

“I don’t believe the problem, in terms of crop loss, is as bad as some people are saying. But I don’t want to minimize the severity of it, either.”

Haviland says growers as far north as Butte County report finding the leaffooted plant bug in almond orchards, although the pest is in much lower numbers and isn’t causing nearly the damage.

Norman Kline, a pest control advisor for Mid-Valley Agricultural Services in Escalon, Calif., says he’s found the leaffooted plant bug in some of the almond orchards he scouts. In a few cases, the pest was causing economic damage and he recommended the grower treat. With the others, he plans to continue monitoring them.

“I’ve seen it for the last two or three years, but this year we are more aware of it because people are talking about it,” says Kline, who also grows almonds and peaches in Riverbank, Calif.

A handful of registered insecticides, such as Lorsban and Imidan, control the pest.



Adult bugs are troublemakers

Leaffooted plant bugs have two—possibly three—generations annually, says Walt Bentley, UC integrated pest management specialist at the Kearney Agricultural Center near Parlier. Adults may overwinter on the fringes of orchards or in nearby sheltered areas, such as woodpiles. They migrate into the trees and begin causing problems in May.

Females lay eggs from May through June. Young nymphs don’t have the mouthparts to deeply penetrate hulls. As the bugs near adulthood, their mouthparts are long enough to reach to the kernel.



As season progresses, damage changes

Early in the season, leaffooted bugs feed by inserting their half-inch-long proboscis into hulls, leaving kernels unharmed.

A gummy drop of clear liquid typically marks the feeding site on the hull exterior. But Haviland cautions against blaming the leaffooted plant bug for every nut with gumosis. Shothole, for example, can cause similar damage.

To determine the cause, he recommends slicing through the gumming site to examine the penetration depth. If there’s no poke mark through the hull, then something other than the leaffooted plant bug was responsible.

Later in the season but before the shell hardens, leaffooted bugs may penetrate through to the kernels, causing wrinkling and shriveling. In severe cases, the kernels abort and the nuts drop.

Leaffooted plant bugs use an enzyme in their saliva to penetrate hardened shells to feed, leaving black inklike spots on the nuts. Processors will reject nuts with spots larger than one-eighth-inch diameter.



Varietal susceptibility varies

Depending on the variety and time of year, leaffooted plant bugs can cause significant damage or crop loss, Haviland says.

UC Extension IPM specialist Kent Daane conducted a cage study involving female plant bugs on almonds. During seven days, one female confined with 10 to 12 nuts caused 20 percent nut drop and 20 percent nut damage at harvest.

Susceptibility to damage varies among almond varieties. Because of their hard shells, Mission and Padre appear to be the least affected, Haviland says.

Nonpareil seems to be fairly resistant to nut drops, Haviland says. Butte, on the other hand, may drop after only one feeding probe.

Guthrie says Fritz probably has sustained the most damage in the orchards he serves.

“It’s slow-growing and one of the last for the shell to harden off,” Guthrie says. “Once the meat solidifies and goes from jelly to solid, the bug doesn’t seem to like it much.”



Begin scouting in spring

Haviland recommends scouting almond orchards for leaffooted plant bugs, eggs and gummosis, beginning in late April. But growers and PCAslikely will see the telltale damage or eggs rather than the insect itself.

“They are a very large bug [such] that if you see them flying or if they are standing right in front of you, they are very easy to see,” Haviland says. “However in reality in the field, they are very difficult to find. They run around the back sides of limbs and hide, or they are inside of trees or in the tops of trees.”

So far, nobody has developed a monitoring program or established a treatment threshold, Haviland says.

“What I typically tell growers [is] if they have adults, they need to treat, and most growers already have made at least one application down here,” he says.

Guthrie says nearly all of the growers he serves have either tankmixed a preventative leaffooted plant bug spray with their seasonal miticide treatment or applied an insecticide when they first found the pest.

A small wasp—Gryon pennsylvanicum—parasitizes leaffooted plant bug eggs, but Haviland says it may take several weeks or months before the beneficial insect builds to the levels to control the pest.