For North Carolina blueberry growers, pest management is a regular part of their work, and for the most part they know what to expect from year to year. However, in recent years, an unfamiliar visitor has been turning up beneath the soil. Now researchers are working to understand it and figure out how to deal with it.
When Dr. Lorena Lopez joined North Carolina State University in early 2024 as assistant extension professor and leader of the Small Fruits and Tobacco Integrated Pest Management Lab, one of her first assignments came directly from blueberry growers.
“At our blueberry meeting in February 2024, a grower brought larvae of these Prionus beetles and was asking how to deal with it because he had quite a few of his fields infested with it,” Lopez said. “Since then, I’ve been visiting the farms that have found it on their own and hearing about the reports. I’ve been dedicating my time to figuring out what the phenology is on blueberries because this is not really a pest that’s supposed to be feeding on blueberries.”
The culprit is Prionus imbricornis, a species of longhorn beetle whose larvae feed underground on plant roots. While the insect has long been associated with hardwood trees, North Carolina became the first state to document the beetle actively feeding on blueberry bushes.
Reports of unidentified Prionus larvae damaging blueberry roots date back to 2010, but very little research had been done to understand exactly what growers were facing.
“There was not much information about it in blueberries,” Lopez said. “Even in hardwood trees and other crops, there’s some information, but it’s still limited. This is a very hard insect to study because it has a very long life cycle.”
What Lopez has seen is that adult Prionus beetles come out of the ground, mate and lay eggs on the soil near blueberry plants. A few weeks later, the larvae hatch and dig into the roots of the blueberry bushes and start feeding on the roots – basically hollowing the plant root.
Those larvae can remain underground for three to five years, feeding on roots as they grow larger and tunneling deeper into the plant. The deeper into the root they go, the worse the damage, and some even spread into the stems of the plants. Over three to five years, the larvae pass through the pupal stage and emerge from the soil as adults to repeat the cycle.
Life cycle of Prionus beetles: (A) Prionus laticollis eggs [photo: Lynn Curtis –https://www.inaturalist.org/observations/225296175], (B) Prionus imbricornis larva extracted from a blueberry bush [photo: Matt Bertone] (C) pupa collected in the soil at the base of a blueberry bush [photo: Lorena Lopez] and (D) adult female (left) and male (right) [photo: Kenneth Geisert] (Caption adapted from Journal of Integrated Pest Management)
A major problem is that the symptoms aren’t usually seen until it they’re really severe. By the time growers notice declining plants, the damage has often already been done.
“I’ve seen machine harvest where the plants don’t have enough strong roots because they’re hollow, and they just fall to the ground,” Lopez said. “They’re still producing a few branches with fruit, so it’s not until it’s very advanced that the canopy starts dying off completely. We don’t know yet at what stage of the larvae the plant starts to show symptoms of die-off and such. That’s the hard part.”
(A) A collection of P. imbricornis and P. pocularis that were mass-lured with panel traps and (B) blueberry bushes at a commercial farm showing a sparse canopy, branch die-off and exposed roots caused by hollow root mass due to infestation by P. imbricornis, which made it easy for machinery to knock down the bushes (Photos: Lorena Lopez) (Caption adapted from Journal of Integrated Pest Management)
Following the beetle’s life cycle
One of Lopez’s first goals was simply to understand when the beetles are active in blueberry fields. Researchers began monitoring infested farms and discovered that adult beetles typically emerge from late April through August, with populations peaking during May.
“We’re having triple-digit numbers per trap sometimes in a couple of farms during the peak time,” Lopez said.
Understanding that seasonal activity is critical because it helps researchers identify the best opportunities to manage the pest before young larvae burrow deep into blueberry roots.
Looking beyond insecticides
Lopez is currently conducting insecticide trials to determine whether newly hatched larvae can be controlled before they become established inside the root system.
“They’re so deep into the roots and these are very woody, older plants, so it’s hard to reach them with chemicals, but hopefully we can reach the newly emerging larvae from those eggs,” she said.
Still, insecticides are only one piece of the long-term strategy. Lopez is hoping to figure out other tools to fight the problem. Her research plans also include evaluating mass trapping and mating disruption using pheromone lures that attract male beetles.
Mass trapping removes large numbers of males from the field before they can reproduce. Mating disruption takes a different approach by flooding a field with synthetic versions of the female beetle’s natural sex pheromone.
“What we’re doing is overwhelming the entire field with that sex pheromone,” Lopez explained. “The males get lost. They can’t locate where that lure is coming from, so they don’t find the female. They don’t mate, and then we don’t have those eggs laid. Over time, the population shrinks.”
The technique has already been successfully used against several other agricultural or forest pests such as diamondback months (in cabbage, broccoli and collards) and spongy moths (in trees and shrubs). That success gives researchers hope it could eventually become another tool for blueberry growers.
Still answering fundamental questions
Although Lopez says progress is being made on management options, many basic questions remain. So although she thinks researchers are about halfway to finding tools for growers to use, she says they’re still in the early stages of understanding the pest.
For example, researchers still don’t know why North Carolina appears to be the only blueberry-producing state where Prionus imbricornis has emerged as a significant pest.
“There are a lot of questions about the biology and ecology of the pest that I’d like to answer,” Lopez said. “We’re still a long way away from answers on that, but thankfully for the growers, I think we’re a little bit closer to giving them something they can use to fight them.”
That work depends heavily on partnerships with growers and Extension personnel because the insect is nearly impossible to study under typical research station conditions. Lopez said the pest’s lengthy life cycle means normal research station trials wouldn’t work well, so the research needs to be on farms.
Protecting an important North Carolina crop
North Carolina consistently ranks among the nation’s leading blueberry-producing states, making the research especially important for growers. Once plants become heavily infested, they often cannot be saved, forcing growers to replace mature bushes after years of investment.
“We’re one of the top producers of blueberries, and we want to continue being that,” Lopez said. “Unfortunately, infested plants are likely to be dead in a few years if unmanaged. That causes a lot of economic pressure for growers because it’s a very large investment to replace those plants.”
Because growers may not realize they have a problem until significant damage has already occurred, Lopez is also working on some ways to notice and diagnose an infestation before it’s too late. That would help growers identify if they have the problem and help them avoid the big cost of replacing plants.
While many questions remain, Lopez is encouraged by the progress being made through collaborations among NC State researchers, Extension agents and blueberry growers.
“We are making progress – thankfully,” she said.