Those engaged in California almond production loathe leaffooted plant bugs (Leptoglossus species). In 2006, the worst infestation in 20 years obliterated some almond orchards in the San Joaquin Valley causing more than 50 percent crop loss.
Leaffooted bugs are despised as the mouthparts penetrate almond hulls to feed on the kernels. Damaged kernels can result in nut abortion, a sticktight without a kernel, or cause a black-stained nut at harvest. Reduced almond quality or quantity is the result.
Sticktights are nuts where the kernel and hull dry down, remain stuck on the tree, and don't usually fall off the tree even during harvest shaking.
Growers and pest control advisers (PCAs) want answers to a myriad of questions — what is a leaffooted plant bug, when does damage occur during the growing season, why are varieties affected differently, how long does the pest inflict damage, and do all damaged kernels abort or do some hang on the tree until harvest?
“A question I've heard is — why are half of the Fritz (variety) almonds on the ground while one row over the Nonpareils (variety) are all on the tree looking good?” said David Haviland, one of four researchers who conducted leaffooted bug research in the 2006 Kern County Regional Almond Trial.
Haviland is a University of California Cooperative Extension (UCCE) entomology farm advisor who along with UCCE farm advisor emeritus Mario Viveros and staff research associate Stephanie Rill, all of Kern County, teamed up with UC, Berkley UCCE specialist Kent Daane to unearth answers to the perplexing infestation.
Research was conducted on eight almond varieties in 2007 where an insect pinning needle poked through the hull and shell of individual almonds as a simulation of leaffooted plant bug feeding. One hundred nuts of each variety were injected every seven to 10 days for a two-month period in the spring. The team evaluated the fate of each nut throughout the season looking for the types of damage that occurred.
The researchers also measured for hull and shell characteristics to help explain data on varietal differences in susceptibility to naturally occurring damage.
“Research results show shell hardness didn't help explain varietal differences to damage, but that thinner-hulled varieties are more susceptible in April and May,” Haviland said. “The Fritz variety by far has the thinnest hull which is the easiest to penetrate, versus the Nonpareil that grows a thicker hull.”
Haviland shared the research results during an entomology session during the Almond Industry Conference in December 2007. Other major findings — overwintering leaffooted plant bug adults migrating into orchards in the spring cause 100 percent of the damage. Second generations either do not survive well or the mouthparts are too small to inflict kernel damage.
Leaffooted bugs can cause gummosis on hulls, but the kernels are not damaged.
The damage type depends on when feeding occurs. Of the nuts stung in April, nearly 100 percent either aborted or became sticktights. The exception was early-harvest varieties that by mid-April already had 50 percent to 70 percent of the nuts developed and stained. Staining was prevalent in 20 percent of the Monterey variety.
Damage to other varieties at the same time still produced sticktight or aborted nuts. This suggested that kernel maturity is a key factor in determining the damage type.
Growers and PCAs in 2006 reported damage from aborted nuts, nuts that became sticktights, and nuts with fully developed but damaged kernels.
In the 2006 Kern County Regional Almond Variety Trial, 15 varieties were evaluated for susceptibility to leaffooted plant bug damage. Fritz was by far the most susceptible with 33 percent nut abortion. Thirty percent of the harvested nuts contained shriveled or black-stained kernels (63 percent).
The bug impacted the Sonora variety second most with 12 percent nut damage, followed by Aldrich (8 percent), Livingston (6 percent), Monterey (4 percent), Carmel (3 percent), and 2 percent or less in the varieties 2-19E, Butte, Mission, Nonpareil, Padre, Price, Ruby, Winters, and Wood Colony.
In general, varietal harvest dates as well as hard-soft shell varieties are not the key factors related to damage susceptibility. Hull thickness is the primary factor that influences varietal susceptibility.
“Did the damaged nuts in 2006 fall on the ground or did some damaged nuts stay on the tree?” Haviland asked. “In our trials, any nut injected in April was aborted. In early May, some nuts remained on the trees, but most were shriveled. At harvest, the nuts were shaken and obliterated in the hulling process since the kernel wasn't developed.”
During the same time frame, Kent Daane, Haviland, plus three other project cooperators conducted research on how to predict leaffooted bug outbreaks to improve control.
The most common methods to predict densities include: a direct count of the bugs on plant leaves and branches, bugs caught in pheromone or sticky traps, or caught by sweeping nets or beating trays. The efforts are generally ineffective for leaffooted bug populations in the spring.
In 2007, the researchers sampled almond and pistachio orchards for overwintering populations. From the winter through spring sampling, they predicted that cold winter temperatures would significantly reduce leaffooted bug populations from the 2006 numbers. Sampling from the spring through summer proved the prediction accurate.
“We believe we can establish annual winter monitoring programs that will help to forecast spring bug abundance, thereby giving managers a warning about bad years,” said Daane.
In October and November 2006, leaffooted bugs were seen leaving a pomegranate field in Tulare County and an almond field in Fresno County, and then aggregated on eucalyptus and citrus fields in adjacent fields. Several aggregates of 15 to 40 adults were tightly clustered on often-exposed terminal branches.
Following the cold weather in January 2007, about 90 percent of the leaffooted bugs died. Bugs hiding under the bark of palm trees that offered better protection showed at least a 50 percent survival rate. In March, the aggregations started to break out and adults moved back into almonds.
The winter sampling program confirmed that exposed leaffooted bugs have a high mortality rate while protected shelters offer overwintering survival. The goal is to develop inexpensive, joint surveys to predict possible bad years based on high wintering populations.
According to Daane, winter studies conducted in 2006 and 2007 suggest a number of factors determine overwintering population size:
Nymph numbers that develop to the adult stage in the fall and early winter from eggs deposited by the second and third generation adults determine the size of overwintering populations. In years with a cold or wet fall period, a large number of nymphs fail to reach the adult stage and die during the winter;
Cold winter temperatures result in mortality for a majority of the overwintering leaffooted bugs. Cold spells with lows near 23 degrees Fahrenheit (5 degrees Celsius) kill most exposed leaffooted bugs; and
The abundance of overwintering shelters near the orchard provides protection from cold temperatures. Growers should seek out potential shelters areas near orchards and first check for leaffooted bugs to treat the overwintered populations, and consider removing or reducing the shelters.
Daane also provided his research findings during the 2007 almond conference. The others involved in the research included: Jocelyn Millar, entomology professor, UC Riverside, Riverside, Calif.; plus Glenn Yokota, cooperative Extension specialist, and DeAnna Romero, laboratory assistant, both UC, Berkeley staff based at the Kearney Agricultural Center, Parlier, Calif.
The Almond Board of California and the California pistachio industry sponsored the research.