It may happen in a few days or several weeks, but sometime before May the adult leaffooted plant bug (and stink bug, too) can be expected to show up in some San Joaquin Valley almond and pistachio fields.
They overwinter in or near protected areas, such as in woodpiles, barns and other wooden outbuildings, under the bark of eucalyptus, cypress, or juniper trees. They can also overwinter in the orchard in plant debris or cracks along the tree trunk.
These plant bugs will feed on the young nuts before the shell hardens. This can cause internal gumming, resulting in a bump or strings of gum on the shell. The feeding can also cause embryo abortion or nut drop. After the shell hardens, leaffooted bug feeding can still cause black spots on the kernel or wrinkled, misshapen nutmeats when their stylets – or mouthparts – pierce the shell.
New nuts begin to form on pistachio trees later in the season than almonds, and the leaffooted plant bugs will move into those orchards, too. There, they will feed on enlarging nuts, which can drop, turn black, become sunken and appear almost water-soaked. After shell hardening in June, feeding by the bugs may cause kernel necrosis, as evidenced by a brown pinpoint mark on the shell. The damage may include darkened meats, which may have an off-flavor, and, in some cases, slimy nuts. There are many forms of bug damage, depending on the insect stage, nut development, and where the bug has fed on the nut.
The leaffooted plant bug damage can appear similar to stink bug damage, which also feeds in almond and pistachio orchards. The stink bugs can reside in or near the orchards, moving from weeds or field crops into the orchard. One stink bug, Acrosternum or green stink bug, commonly overwinters in almond orchards:
Stink bug damage to almonds and pistachios is similar to that caused by the leaffooted plant bug. However, the critical period in pistachios is a bit later in the season than in almonds. Typically, it is after pistachio fruit have set and there is no more plant compensation for dropped nuts. Also, when feeding on hardened nuts, the bug leaves a characteristic black spot where its mouth punctures the shell. Both leaffooted plant bugs and stink bugs can also facilitate some pistachio diseases, such as stigmatomycosis and Botryosphaeria, or panicle and shoot blight.
Once leaffooted bugs start dispersing from their overwintering sites, these strong fliers can move with astonishing speed. They typically gather or aggregate into a single large mass to overwinter, says Kent Daane, University of California Cooperative Extension entomologist. Such aggregations can number anywhere from about 50 to more than 1,000 adults in any one spot. “We’ve counted aggregations of 500 or more adults overwintering in an olive tree and gone back a week later and all but one or two were gone,” he says.
Such concentrated numbers also explain the extensive damage they can cause in just a short time. “You could go out in an orchard in March or early April and find no evidence of leaffooted plant bugs,” he says. “But, you could go back to that same orchard five days later and see the trees covered with blackened nuts (in pistachio) or the ground littered with dropped nuts (in almonds) from the bugs’ feeding activities.”
Just how many leaffooted plant bugs show up in almond and pistachio orchards this year may depend on the weather this past winter. For large numbers of the bugs to survive the winter requires a lot of bugs in the previous fall, Daane notes. He’s seen populations plummet by 60 percent to 80 percent with nighttime temperatures in the 25- to 28-degree range for three or four nights in a row. In fact, he wonders if leaffooted plant bugs form aggregations as a way for them to keep warm.
The adults are vulnerable to rain storms, too. In the Daane laboratory, Glenn Yokota was monitoring the insect as it overwintered in an olive orchard. He’d look at as many as 50 trees before finding any of the bugs. But, when he did find them, he might see a thousand or more leaffooted plant bugs in one tree. Following a huge rainstorm a few days later, he went back into the orchard and all the bugs had been knocked off the trees by the rain. However, by the time he went back the following week, the bugs had crawled back up into the trees and had re-aggregated.
Despite the nut damage leaffooted plant bugs can cause early in the season, pistachio trees and, to a lesser extent, almond trees, usually produce enough nuts to compensate for the loss. One exception is the perfect storm – or lack of it – when a large fall population overwinters and mild winter temperatures reduce mortality. “A tremendous number of bugs can then move into nearby orchards, causing some trees to drop more nuts than the tree can compensate for,” Daane says. “The trees can’t make up for the loss and those growers can end up with small crops that season.”