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California Root Borer management in small U-pick orchard #879234

Asked July 29, 2024, 12:38 PM EDT

In 2022 we purchased an acreage in Fort Collins that had been irrigated Kentucky bluegrass lawn for twenty years prior to being left for pasture for the ten years prior to our purchase. There were no trees present in the area we planted with 50 trees in spring of 2023 with the intent of becoming a U-pick orchard. 95% of trees were 2-3 year old bare root trees sourced from a local nursery in Elizabeth, CO. In the fall, 6 of 6 peach trees experienced significant canopy die-back, and over winter and the next spring, all of the cherries died. Upon examination, the roots and crown of the trees had extensive borer damage. On July 29, I found an adult female California root borer emerging from the soil where one of the cherry trees had died and where a new bare-root cherry tree was now planted. Four peach trees have evidence of active infection as they had canopy die-back this spring and have oozing bore holes at the crown. Given the extended life cycle of this pest and the absence of trees in the orchard prior to 2023, I believe the bare root tress came infested with the root borer (none of the trees purchased from other nurseries have exhibited any issues; unfortunately the source nursery does not have a survival warranty). I am wondering how likely it is that the trees came infested and how I might manage this problem. Will the whole orchard be infested and a loss now that we have such a significant infestation? Other than supportive care, is there anything I can do to save the infested trees?

Larimer County Colorado

Expert Response

Hi Danielle,

While it could be the California root borer, this also might be peachtree borer (Synanthedon exitiosa), which is a very common and destructive pest. 

We do have a publication on this insect (peachtree borer) that also recommends controls and preventative sprays: https://extension.colostate.edu/topic-areas/insects/peach-tree-borer-5-566/

It is also very important that you help prevent stress to the trees with regular irrigation (avoiding any drought), proper pruning, mulching, etc. 

You didn't mention how you purchased the trees - were they bare root? If so, it's unlikely the borer came with them. And if they were containerized, all nurseries have to do preventative drenches to ship nursery stock across state lines.
Alison O'Connor, PhD Replied July 29, 2024, 3:27 PM EDT
Thanks so much. These trees were all bare root purchased within the state so not subject to interstate regulations. I have the adult specimen in hand so am certain this is a california root borer, although it is possible the peach tree borer is also here. 

Are there any specific recommendations for California root borer? It was a female so I created a pitfall trap with her in it to hopefully attract any others in the area since pheromones are used for mating of the adult beetle.

On Mon, Jul 29, 2024 at 1:28 PM Ask Extension <<personal data hidden>> wrote:
The Question Asker Replied July 29, 2024, 4:34 PM EDT
Hi again,

The peachtree borer is a confirmed insect in Colorado; I don't know if the California root borer has been confirmed, or if it would survive, and will have to ask our entomology and pomology specialists. 

Can you send me photos of the adult beetle? My email is: <personal data hidden>
Alison O'Connor, PhD Replied July 29, 2024, 5:07 PM EDT
Hi again,

Apologies - there is confirmation that Prionus californicus has been confirmed in Colorado (see the end of the publication). https://webdoc.agsci.colostate.edu/bspm/InsectInformation/FactSheets/PondoBorer.pdf

And this: https://wci.extension.colostate.edu/wp-content/uploads/sites/14/2017/03/PrionusLonghorns.pdf

We don't have recommendations for control, but Washington State does: https://treefruit.wsu.edu/crop-protection/opm/california-prionus/

Essentially, management is difficult and focusing on eliminating tree stress is important. The section on management from the link above is pasted here:

Management

Management options in bearing fruit orchards are limited, so avoidance and prevention are the best strategies. Tree stress should be avoided. Symptoms to watch for include canopy dieback or sudden loss of tree vigor. Avoid planting trees into a site known to be infested with prionus. Fallow an infested field for two or more years before planting an orchard crop. Plant annual (non-host) crops as ground cover during the land resting period and till under the crops each year to stimulate microbial activity and prionus larval population decline. An alternative is to fumigate the soil before planting, but the effectiveness is unproven and it is costly.

Insecticides registered for stone and pome fruits may provide incidental suppression of adults. Neurotoxic insecticides (organophosphates, carbamates, and pyrethroids) may kill adults and inhibit egg-laying when fresh residues are present on the lower trunk and soil, but will not suppress larval populations on the roots. Systemic insecticides applied through chemigation to the root zone may suppress younger larvae, but efficacy against larger larvae within a season was not confirmed in a research trial in Utah. Several years of use of a systemic insecticide may be necessary to suppress a local population.

Alison O'Connor, PhD Replied July 29, 2024, 5:20 PM EDT

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