Brown Marmorated Stink Bug Officially Deemed Pest of California Almonds

Brown marmorated stink bug (BMSB) is certainly not new to California growers, but in the wake of some more troubling finds, it is now officially a pest of almonds.

Almonds are now listed as a preferred host on the Stop BMSB website, which was created by a team of researchers from all over the country dedicated to finding a way to stop the pest from damaging a wide range of crops.

One of those researchers is Jhalendra Rijal, University of California Cooperative Extension Area IPM Advisor for San Joaquin, Stanislaus, and Merced counties, who said the first such damage was just two years ago, when they were found in peaches. The actual first finding in the state was three years before that, in Sacramento, but despite being a very large find, they never appeared to spread, adding to the mystery of BMSB movement.


But just recently Rijal has made additional findings in almonds, and his experience should concern growers. He located a huge BMSB population in an almond orchard in northern Merced County, and the grower sprayed a pyrethroid, the standard treatment. Rijal returned to check on the orchard a few times, and seeing a lot of dead BMSB, it seemed the treatment was working just fine.

However, last week he sent the technician he is currently supervising out to check on the orchard. She collected 20 to 25 adult BMSB — all very much alive — right out of the trees.

“It’s very, very unusual, we usually spray one time in June, and that’s it,” he says. “Now they might need to spray again. We don’t know exactly how much damage brown marmorated stink bug does after the shell hardening in almonds.”

Scouting is Crucial

It’s been so recent that no official recommendations for growers have been developed, says Rijal, who first found BMSB feeding in an almond orchard one year ago, when he also found them in peach orchards.

But it’s clear that growers will have to be extra careful in scouting their orchards this year, as BMSB is really making its presence felt in northern San Joaquin Valley orchards. The pests overwinter in dwellings, then as the weather warms in the spring, they move out to orchards.

Brown Marmorated Stink Bug Officially Deemed Pest of California Almonds

BMSB, which enjoys a wide range of hosts, now count almonds as one of its “preferred hosts.” (Photo: Jhalendra Rijal, UCCE)

Last year the region had a warm spring, and Rijal says BMSB moved out into orchards in mid-March. It was so much cooler this spring that they didn’t move out until April, but then they were seemingly all over.

Rijal says they are seeing more damage in several orchards in Stanislaus and Merced counties. It’s not just that they were found in more orchards, in some cases, the numbers were overwhelming.

“At one site in an almond orchard, we sloughed BMSB off the tree limbs into an insect sweep net. We got 50 in just 10 minutes, and that’s not easy,” he says. “They were flying everywhere — it was severely damaged orchard.”

Being so early in the season, the almonds were at an early stage, and can be easily damaged, Rijal says.

“It will cause nut abortion – those nuts will all drop,” he says. “A little later, when the nuts are larger, the feeding will cause gumming of nuts. Later, the fruit might not gum, but there will be yellow/brown discoloration. Slice the fruit surface, look carefully, and you will see a little pinhole where the straw-like mouthpart of the BMSB entered.”

Don’t Underestimate BMSB

Rijal says it looks like it’s potentially a huge problem in part because he’s seeing things out in the almond orchards he’s never seen before. He’s questioned growers and colleagues, and gotten a lot of head scratching.

“In one square foot under a tree, I found 20 to 25 nuts,” he says. “That’s a lot of drop for late April. This kind of drop you never see even with leaffooted bug.”

Rijal is seeing most damage on the border rows, and advises growers to pay them special attention. The orchard where the BMSB rebounded so quickly had Tree of Heaven, one of its favorite hosts, along the border. There is plenty of Tree of Heaven in California, he says.

Growers need to pay extra attention to their orchards this month because other stink bugs — and there are a lot of different types — do most of their damage in June.

“Very often, symptoms of damage might be another stink bug,” he says. “If you suspect BMSB, put out a BMSB trap. Once they get established, they can do a lot of damage — much more than native stink bugs could do.”

Several companies manufacture the traps. Put them on the ground along border rows which are more likely to get infested. Monitor them closely, because here again, BMSB is tricky. Rijal says the trap contents won’t necessarily represent the size of the population, as it will for peach twig borer or other moths.

It’s important to stay alert because BMSB may be spreading to other parts of the state. Rijal has seen several reports from PCAs in counties in the southern San Joaquin Valley about possible BMSB damage, but cautions they must be checked out.

“That’s how they spread, they first just show up. But it takes a while for it to get established. They slowly move to ag areas,” he says. “They come in on vehicles, usually from more urban areas. But once it gets to an ag area, it will get established.”