The tops of Eastern White Pine (Pinus strobus) and spruces (Picea spp.) in southwest Ohio are showing the distinctive dieback and "shepherd's crook" symptoms produced by our native White Pine Weevil (WPW) (Pissodes strobi). The tops may be covered in brown to reddish-brown needles, or free of needles.
Unfortunately, small round exit holes have appeared on the main leaders signaling that a new crop of weevils have emerged. This means it’s too late to reduce localized weevil populations by removing and destroying conifer terminals infested with the weevil larvae.
The adult emergence appears to be somewhat early in southwest Ohio compared to previous years. However, adults may not yet have emerged in more northern areas of the state. Leaders should be closely examined.
Larval Development and Terminal Destruction
WPW has a wide conifer host range from the weevil’s namesake host to Scotch, jack, red, and pitch pines as well as Douglas-fir and all spruces. Indeed, the weevil's love of spruce is exemplified by its alternate common name, Engelmann Spruce Weevil, or simply Spruce Weevil. WPW is not just found in Ohio; it ranges from the east coast west into the Rockies.
Overwintered females deposit eggs in early spring in the terminals of their conifer hosts. After the eggs hatch, the resulting white, legless, slightly curved, grub-like larvae tunnel downward just beneath the bark feeding on phloem tissue until pupation. Removing the paper-thin bark from infested leaders will reveal reddish-brown frass (insect excrement) and weevil larvae.
The outward symptoms of the tunneling activity of WPW larvae are most evident on white pines. The larval removal of the underlying phloem causes the thin overlaying bark to dehydrate and collapse producing symptoms that superficially mimic cankering.
The destruction of the phloem progresses slowly throughout the spring and early summer allowing buds to break and new growth to elongate. However, the loss of the phloem eventually causes the new candles to collapse before they stiffen to produce characteristic “shepherd’s crook” symptoms associated with WPW. Symptoms on spruce are commonly less striking with the collapse of the new growth confined to tufts of new growth.
As the larvae near pupation, they excavate tub-like pupal chambers in the xylem. Prior to pupation, the larvae surround themselves in Excelsior-like wood fibers creating so-called “chip cocoons.”
New adults emerge through the bark creating small, round exit holes. The adults mate and feed on bud and twig tissue; however, their damage is largely inconsequential. The weevils then move to the duff beneath conifers to spend the winter. There is one generation per year.
Impact
In general, the native WPW does not kill its native hosts. However, the overall impact on tree health depends on the species and age of its conifer host. The economic impact depends on the host’s location.
The larval tunneling on white pine typically does not progress downward past the first whorl on eastern white pine trees measuring more than 6’ tall; however, tunneling may continue past the first whorl on smaller trees. Tunneling commonly continues past multiple whorls on spruces of all sizes. Thus, the weevil is far more destructive on recently planted trees compared to large, established trees.
However, multiple years of successive weevil damage to terminal leaders will eventually create "cabbage trees" which are short, squat trees with multiple terminal leaders. While this damage to conifers can be tolerated in landscapes, it is unacceptable in nurseries and Christmas tree plantations.
Historically, WPW has been more prevalent east of the Mississippi where eastern white pine was part of the native forests. It's long been a recognized conifer pest in northeast Ohio, particularly in nurseries and Christmas tree plantations.
WPW was considered rare in central and southern Ohio. This has changed in recent years. I’m now commonly spotting reddish-orange tops on white pines and spruces in the southwest part of the state. Indeed, this Alert stems from spotting weevil damage on white pine and spruce in a commercial landscape in my part of the state.
Unfortunately, landscape managers and tree care professionals who are unfamiliar with the depredations by WPW may misidentify the damage. Weevil damage may be misdiagnosed as being caused by other problems such as root injury or drought stress, or symptoms may go unnoticed allowing localized populations to expand.
Management
Cultural: Pruning
Localized populations may be reduced by removing the infested terminals before adult weevils emerge. Wilted terminals should be pruned from trees and the cut ends closely examined to determine if the cut top holds all of the weevil larvae.
Larval tunnels appear as round to oblong holes in the phloem ring that are filled with reddish-brown frass. If larval tunnels are found in the portion of the terminal that remains on the tree, another cut should be made further down on the stem. Infested material must be destroyed since the weevils will complete their development in cut tops left on the ground. Young conifers typically respond well to training a lateral branch to become a new terminal.
Chemical: Topical Insecticides
WPW has long been successfully managed in Christmas tree plantations and nurseries by properly timed topical applications to the tops of potential conifer hosts. Spraying the main leaders targets the overwintered adults before they lay eggs; the entire tree does not need to be sprayed.
Effective products include pyrethroid insecticides such as bifenthrin (e.g., Onyx, Talstar, etc.), permethrin (e.g., Astro), and cyfluthrin (e.g., Tempo). Of course, the timing is critical.
Overwintered female emergence begins when the accumulated Growing Degree Days (GDD) reach 84. This roughly coincides with the full bloom of Northern Lights Forsythia (Forsythia x intermedia), Speckled Alder (Alnus incana), and Cornelian Cherry Dogwood (Cornus mas). Weevils can be monitored using so-called “pyramid traps” baited with an attractant. The traps are also helpful for determining whether a second application of a topical insecticide is required.
Chemical: Systemic Insecticides
Preventative systemic insecticide applications target the overwintered adults by killing the weevils as they feed on the phloem in the spring but before they lay eggs. The systemics will also kill early instar larvae as they begin tunneling in the phloem but before they produce serious damage.
Systemics such as imidacloprid (e.g., Merit, Xytect 2F, ImidaStar 2L T&O, etc.), and dinotefuran (e.g., Safari 20 SG, Transtect 70WSP, Zylam), and may be applied using soil drench or soil injection applications in the early spring. Some online references note that imidacloprid products work best if applied in the fall to allow enough time for the insecticide to be translocated to the site of action.
NOTE: Insecticide applications should be reserved for landscapes, nurseries, or Christmas tree plantations that have a history of significant WPW activity. This is particularly true for southwest Ohio where the weevil is much less common.
Of course, as with using any pesticide, you must read and closely follow all label directions. In Ohio, both the site (e.g., landscape, nursery, Christmas tree plantation, etc.), as well as the target plant (e.g., conifers), must be on the label. For example, products labeled for use in landscapes may not be labeled for use in nurseries or Christmas tree plantations, and vice versa.
Photo: Joe Boggs