Mild winter temperatures and sufficient rainfall to grow a good crop of weeds over the winter are two factors that point to a healthy population of large bugs—insect pests that threaten almond and pistachio crops.
True bugs, including leaffooted bug, flat, green stinkbug and red shouldered stinkbug use piercing sucking mouthparts to feed on nuts. In pistachio orchards, early feeding by large bugs causes damaged nuts to drop from the cluster. If bug densities are high, they will cause crop loss exceeding the normal drop. Large bug feeding midseason, prior to shell hardening, causes damage to the kernels and can lead to spread of fungal pathogens like Botryosphaeria into the tree if the pathogen is already present in the orchard.
Kent Daane, UCCE IPM specialist, said monitoring for the presence of large bugs in pistachio orchards early in the season is important. Populations of large bugs do not just build up in the orchard; bugs are migrating into the orchard from overwintering sites, he said.
“You might see a few green stink bugs early in the season and then in April see a lot and find fruit drop. Leaffooted bugs are coming in from all kinds of possible refuges such as ditch banks or under the frons of palm trees,” Daane said.
There is also concern that brown marmorated stink bug, a pest that has been found in almond orchards in the Merced area, could adapt to warmer temperatures and move into pistachios.
The critical period for large bug damage is midseason as the shells are hardening and nuts are set. Large bug species can use mouthparts to penetrate the shell and feed on the kernels. Nuts remain in the cluster and the damage is found after harvest when nuts are processed.
Daane said the most effective response to large bug damage has been application of pyrethroid insecticides as they are effective and less expensive than other types of insecticides. The concern, he said, is that navel orangeworm, a primary pistachio pest, will develop resistance to the pyrethroids and become more difficult to control.
Use of trap crops planted in the orchard middles to provide a more attractive feed source for large bugs is being studied, but establishment of the crop has proved difficult without usual fall rain. Daane said during the last two years, research trials changed to fall seeding for the trap crop. The trap crop trial did reduce bug numbers in tree canopies, he said, but there would have to be a significant reduction to determine if planting a trap crop was worth the investment as a pest control tool.