Small Plant Bugs

A variety of small plant bugs in the family Miridae attack pistachio. The particular species varies depending upon location and natural vegetation. All of these bugs have a small, triangular-shaped marking on the back.

California buckeye bug adults are straw colored, slightly hunched, and about 1/3 inch (8.5 mm) in length. They often overwinter as eggs at the base of buds or leaf-petiole scars on 1-year-old pistachio wood. The nymphs are greenish with brown markings on the back. Both immatures and adults are easily identified by the relatively long, hairy first antennal segment and brown and white bands on legs and antennae. California buckeye bugs have been found only in orchards near their native plant hosts: California buckeye and Rhamnus species. However, once established in a pistachio orchard, buckeye bug will overwinter there.

Lygus bug adults are about 1/4 inch (6.4 mm) in length, vary from brownish to green, and it has a yellowish, triangular-shaped area on the back between the wings. They are most commonly found near alfalfa and beans, or plants such as clovers, Russian thistle, tarweed, London rocket, and lupine. Lygus bugs usually migrate into a pistachio orchard from nearby weeds. When rainfall and spring temperatures are ideal for the growth of broadleaf weeds, lygus bug numbers can be very high. When they move into the orchard, they tend to stay in the cover crop and move into the trees when the cover crop becomes unsuitable.

Calocoris bug adults are about 1/4 inch (6.4 mm) in length with a green body. The wings have a reddish brown tint and are black where they overlap. There are also two black dots on the thorax. Calocoris bug is usually found on mustard, wild radish, and vetch hosts and is most common in the northern San Joaquin and Sacramento valleys. Calocoris bug does not overwinter in pistachio trees but migrates into the orchards as native weed hosts dry or are cultivated in spring.

Phytocoris bugs overwinter in the egg stage on one-year-old fruit wood on pistachios as well as on other deciduous trees. Adults are about 1/4 inch (6.4 mm) long and are predominantly gray with flecks of black and white; they have long antennae and legs and can move rapidly when disturbed. The nymphs are also gray with white bands on the legs and antennae. Moderate numbers of phytocoris bugs are not considered damaging and seldom require treatment. They are predators of other insects, especially immature soft scale in March and April and second-instar scales that are migrating from leaves to woody shoots in fall; they also feed on navel orangeworm eggs in spring.

Psallus vaccinicola (there is no common name) is the least common of these small plant bugs. The adults are about 1/8 inch (3.3 mm) long and are brownish red in color. They have been found predominantly near oaks in the Sacramento Valley.

These bugs only cause damage for a short period of time, from bloom through shell hardening (early April–late May). However, during cool springs shell hardening is delayed and injury from plant bugs may be extended until June.

Small plant bugs insert their mouthparts into the nut, causing damage known as epicarp lesions, which on the inside of the nut appear as white netting. This damage is done before shell hardening and, except for damage caused by California buckeye bug, is more random in occurrence than damage caused by leaffooted plant bugs and stink bugs. Damage to small nuts results in blackening and nut drop. As the nuts enlarge, the hull tissue turns brown and necrotic, and the outside will often become sunken. On the inside of the nut there will be a small black spot or irregular-shaped pit in the area where the bugs fed on soft shell tissues.

Careful attention to vegetation in and around orchards is the key to effective management of small plant bugs. The general pattern of bug appearance and distribution in an orchard is that phytocoris bugs and California buckeye bug overwinters in the trees. Calocoris and lygus bugs overwinter on preferred weed hosts in the ground cover. As vegetation in the pistachio orchard or surrounding areas dries, these bugs can move into the orchard canopy where they damage the developing crop.

Options to consider are

• Elimination of all herbaceous vegetation
• Maintenance of monitoring strips
• Use of trap crops with insecticides
• Use of cover crops that are not attractive to pest insects

Success or failure of each option will likely depend on the specific site and the species of bugs in the orchard.

Careful cover crop management is an organically acceptable management strategy. Mowing ground cover before bloom reduces small plant bug numbers.

Monitor from early April until late May when shells harden and plant bugs no longer cause damage.

1. Sample the trees with a beating tray.
   1. Hold the tray under nut clusters while striking the limb sharply three times with a lightweight club.
   2. Examine bugs that drop onto the tray.
   3. The best time to take beat samples is in the morning when bugs are less active and easier to examine.
2. Sample the cover crop and surrounding vegetation with a sweep net for lygus bug and calocoris bug.
3. Phytocoris bugs can also be monitored with pheromone traps, but research has not been done to correlate trap catches with economic injury.
4. Look for small damaged or blackened nuts. Cut them open to confirm bug damage (black lesions inside the hull).

There are currently no treatment guidelines based on the number of small plant bugs found.

• If populations are found uniformly throughout the orchard after bloom, begin treatment.
• If insect numbers are low in the trees (three or less per ten beats), and lygus bug and calocoris bugs are present in the ground cover, consider treating just the ground cover.
• If lygus bugs and calocoris bugs move into the trees from drying vegetation, they (as well as phytocoris bugs) can cause damage into early May. California buckeye bug can also be a problem in May if it is established in the orchard or migrates in. By early June or after the shell has hardened, these bugs are no longer damaging.

Due to the variety of small bug species and range of dates they are present, more than one insecticide application may be justified. Be sure to rotate insecticides based on modes of action. Prior to using pyrethroids (Mode-of-action Group 3) for small bugs, also consider whether pyrethroid use is planned for leaffooted bug, stink bugs, or navel orangeworm and plan a resistance management program accordingly.