Late this past winter and early spring, leaffooted plant bugs were seen by the "truckloads" which prompted University of California Cooperative Extension (UCCE) specialists to issue warnings about the potential damage the pest could inflict to developing pistachio and almond nuts.
Growers apparently heeded the warnings and treated for the bug, since leaffooted plant bug populations were about average in late August, says David Haviland, a UCCE farm adviser in Kern County.
“Absolutely they were here this year, but people knew they were coming and managed them as needed,” Haviland said. “There was an overall awareness that leaffooted plant bug was out there. I haven't heard any major horror stories.”
Whether the large, 3/4- to 1-inch bug is a pest next season is unknown. Haviland says it will depend in part on the weather this coming winter.
“If we have some really cold weather in the upper 20’s a lot of the bugs can die.”
Kris Tollerup, UCCE Integrated Pest Management adviser at the Kearney Agricultural Research and Extension Center near Parlier, agrees. He cites studies by UCCE entomology specialist Kent Daane who found that temperatures of 27 degrees F. or lower for six or more hours can produce 70-80 percent pest mortality.
Haviland says cold temperatures in December 2013 helped knock down bug numbers. 2014 included a warm summer with a larger-than-normal number of hot degree days.
This allowed the bug to have three instead of two generations so the population was higher going into the 2014-2015 winter. The winter also was fairly mild which allowed a large number of adult insects to survive into the spring which also was early.
The large numbers prompted Haviland and Tollerup to issue warnings to growers.
This summer, Haviland says degree-day accumulations have been only slightly ahead of average. He has not heard of significant damage to pistachios but suggests that growers keep their eyes open for the bug.
According to UC Integrated Pest Management guidelines, the leaffooted plant bug can cause two types of damage to pistachio nuts. The first is an epicarp lesion early in the season, similar to that caused by other true bugs which use needlelike stylet mouthparts to probe nuts to suck out plant juices.
Nuts damaged shortly after bloom turn black and drop. If the bug damages nuts during kernel enlargement, the tissue can turn brown and necrotic. The outside often becomes sunken and appears almost water soaked.
Second, even after shell hardening in June the bugs can cause damage known as kernel necrosis which is invisible on the outside shell. The only exterior sign can be a brown pinpoint mark.
Cut the nut open, and the kernel may appear darkened and often develops a sunken or distorted area. An off-flavor can occur.
If the nuts are damaged during high humidity, fungal breakdown may cause the nut to turn slimy, referred to as stigmatomycosis.
Brad Higbee, director of entomology research for Wonderful orchards in Shafter, Calif., saw high leaffooted plant bug populations in almonds, pistachios, and pomegranates this spring. The company treated all three crops and retreated the pomegranates, due to a large jump in bug numbers in August.
So far, it appears the pistachio treatment was effective.
“We have just started the pistachio harvest and I’m not aware of any large populations yet,” Higbee said. “It would not be surprising if it occurred.”
Higbee is also involved in field and laboratory assays to determine the efficacy of various insecticides. He says some entomopathogenic fungal products hold promise.
He is also working on trapping methods including the development of synthetic and natural attractants, plus learning more about the bug’s basic biology.
Higbee is part of a research project led by UCCE Merced County entomologist Andrea Joyce to clarify the two species of plant bugs, Leptoglossus clypealis and L. zonatus, and map its distribution throughout the Central Valley.
So far, orchard surveys from Chico to Bakersfield have found L. clypealis as the predominant leaffooted plant bug in almond and pistachios. L. zonatus has been the overwhelming species in pomegranates.
UCCE’s Tollerup is conducting research into large bug management in pistachios and its dispersion into neighboring pistachio and almond orchards. Part of the project will evaluate various insecticides for almonds and pistachios, and determine chemistry longevity under field conditions.
Under laboratory conditions, Tollerup will also study whether evaluated insecticides are feeding deterrents or repellents.