Elm - Trap Trees for DED

GardenLine | Trees and Shrubs | Elm - Trap Trees for DED

Russell Friesen

The use of trap trees as a part of a Dutch Elm Disease control program can greatly improve success in saving elms. This is especially true if a prompt removal system is not possible, whether due to a lack of funding or increased workloads.

Theory behind Trap Trees

An elm tree which is unwanted or which has been diagnosed to have Dutch Elm Disease is killed using herbicides applied to wounds made in the trunk. This renders the elm attractive to the European and Native elm bark beetles (Scolytus multistriatus and Hylurgopinus rufipes, respectively). The bark beetles subsequently attack and colonize the dead/dying tree and set up brood colonies. However, the herbicides result in drying of the bark causing brood to fail. Therefore, each year that trap trees are used, a compounded decrease in bark beetle populations results.

For example, a study conducted in New York by professor Gerald Lanier found that each beetle which infests an elm tree normally produces 8.5 new beetles. However, if an elm which is less than 50% wilted is killed by herbicides, each beetle infesting the tree will produce only 2% of its potential brood. This illustrates the important point that early detection of Dutch elm disease is integral to the success of the trap tree method. Only 52% of potential brood was eliminated if an elm was treated with herbicide after it was already 90% dead from Dutch elm disease.

Knowledge of flight times of bark beetles is necessary for optimum "catch" of the insects. Elms treated just prior to flight periods will attract more bark beetles. Trials in Minnesota, Vermont, Massachusetts and New York revealed that native beetles attacked trees treated in September, April and May. Attacks by the European beetles occurred on trees treated 1-2 weeks after foliation and 8-10 weeks later. Therefore, knowledge of flight activity of bark beetles in Saskatchewan will increase the success of the trap tree method here. However, the earlier a diseased tree is treated the less viable brood is produced. Therefore, treating trees early (less than 50% wilted) is more important than timing treatment with beetle flights.

It is also very important to treat "wild" elm stands that are within 800 m of the elms which are to be protected. This is a conservative measurement because European elm bark beetles are capable of flights longer than 8 km although initial flights may only be around 400 m. The benefit of the trap tree method in wild stands is that there is no need to remove the trees after treatment because the trees become useless to the beetles as breeding sites.

Methodology

The herbicide is applied undiluted into the water-conducting outer sapwood rings of the elm. Application can be made with a plastic squeeze bottle or pressurized spray tank.

Preparation of the tree can be undertaken in a number of way. A frill can be made with an axe to girdle the trunk (Figure 1). The herbicide would then be sprinkled into the frill. The tip of a chainsaw can also be used to make plunge cuts into which the herbicide is applied (Figure 2). Low pressure injection of herbicide into the root collar gives the best results (Figure 3); however, the chainsaw method is the fastest.

Root Grafts

Turning a slightly diseased elm tree into a trap tree may seem like a drastic measure when the tree could possibly be saved. However, in situations where two or more elms are so close that root grafts are likely, one diseased tree lost is preferable to the loss of all. The Dutch elm disease fungus can infect nearby healthy elms by transmission through root grafts. Upon diagnosis, girdling the trunk of the infected tree will halt transmission of the fungus through root grafts. Girdling also kills the tree and may seem like a drastic measure. However, in sanitation practices, immediate girdling and removal of the tree within 20 days saved more elms at a lower cost than if diseased elms were just removed promptly or left standing until off-season.

A combination of girdling diseased elm trees and turning them into trap trees by applying herbicides via plunge cuts or injection is important for street trees, or other areas where elms are close together. However, care must be taken to avoid root graft transmission of herbicides. The girdling process and following plunge cuts/injections is described below.

Girdling a Tree:

Two parallel cuts are made around the diameter of the tree with a chainsaw. The sapwood is then removed from the area between the two cuts. A third cut is made further into the wood to ensure that the vessels are severed. (Figures 4-6)

If no streaking of the trunk wood is apparent, plunge cuts should be made above the girdle only, so that herbicides are not transferred to nearby elms through the roots (Figure 7). Streaking of the trunk wood is indicative of Dutch elm disease. Any portion of the roots which lead up to this streaking should be treated with herbicide to ensure death of the roots. Separate healthy and streaked sections with vertical chainsaw incisions 1/2" into the sapwood at least 3" on either side of the visible streak. Make plunge cuts only in the streaked areas and apply herbicide (Figure 8). Minimal damage will be sustained by adjacent elms.

Herbicides

Literature on the trap tree method cited the use of two herbicides, cacodylic acid (sodium dimethylarsenate) and MSMA (monosodium methylarsonate). Both are pentavalent organic arsenicals; neither causes significant eye or skin irritation nor are they carcinogenic. The elm wood itself contains very low levels of arsenic after treatment, although the wood at the points of treatment has higher levels. Therefore, all the wood except that at the points of treatment can be safely used. Wood that is burned would release arsenic at a rate comparable to that of burning coal.

Cacodylic acid has no registration in Canada. MSMA is commercially registered for Forestry as CLEAN CROP GLOWON LIQUID TREE KILLER, manufactured by United Agri Products in Dorchester, Ontario. Although the label specifically states that GLOWON MSMA is not effective on most hardwoods, MSMA was a herbicide used in the United States for elm trap trees. However, recommended doses described by Lanier are 12.5 mL per 2.5 cm diameter (27% AI) versus 1 mL per 2.5 cm diameter recommended dose for softwoods on the GLOWON label (25% AI).

Two injection herbicides are commercially registered for forests and noncrop land in Canada. EZJECT HERBICIDE CAPSULES are manufactured by Monsanto Canada Inc. in Mississauga, Ontario. These are administered into the trunk with a gun-like apparatus. GEL CAP-G HERBICIDE CAPSULES are manufactured by Pace Chemicals Ltd. in Burnaby, British Columbia, and can be administered by drilling holes into the trunk and inserting the capsules. Both are glyphosate herbicides with Gel Caps containing slightly more glyphosate (0.18 gm/capsule vs. 0.15 gm/capsule). These herbicides result in a much slower kill than MSMA. The labels state that symptoms may take 2-4 weeks to appear and 1-2 years must be allowed for complete control. This slower kill may postpone the development of saprophytic fungi (mentioned below); however, desiccation of the bark may not be fast enough to inhibit bark beetle brood development. More information on the action of these herbicides is required.

Complementary Activities

Saprophytic fungi which colonize dead elm wood can reduce the success of trap trees because the wood becomes less suitable to the bark beetles as breeding sites. This further emphasizes the need for beetle flight periods and herbicide treatments to coincide. However, beetles which land on the tree and find it unsuitable for breeding may be killed by the use of insecticides. An insecticide such as chlorpyrifos (DURSBAN) sprayed on the bottom 4-5 cm of the trunk can result in high beetle mortality for more than 10 weeks. Other insecticides which could be effective are pyrethroids (Esfenvalerate, cypermethrin, permethrin, and Fluvalinate).

Benefits of the Trap Tree Method

If treated early enough (less than 50% wilted) these trees are rendered incapable of producing live brood, therefore eliminating the need to immediately remove and destroy the wood. In fact, trees can be left standing for removal during off-season, and the wood does not have to be destroyed. In fact, trees treated in shelter belts, etc. would not have to be removed at all, yet would no longer serve as breeding sites for the bark beetle.

Since the herbicides can be applied into cuts made in the trunk, the danger and damage often sustained by herbicide drift is nearly eliminated.

No special equipment is required to carry out this technique. Equipment required would be axe, chainsaw, corrosion-resistant pressure tank, and herbicide. Time to treat each tree ranges from 5 to 45 minutes, with an average of 15 minutes. Lanier states that one or two people working full time during the growing season could administer treatment to all diseased trees within a population of 100,000 elms.

RECOMMENDATIONS

The city of Winnipeg has enjoyed tremendous success with their Dutch elm disease control program, keeping annual losses of elms below 0.01%. Their program consists of immediate detection and removal of infected trees. Trees diagnosed before signs of flagging will be pruned to try to save the tree; however, upon any signs of flagging the tree is immediately removed. Tree diagnosis and removal combined with regular city upkeep and construction during the summer months must be a very costly and labour-intensive undertaking. Therefore, the trap tree technique could be a useful tool where cost and labour are to be kept at a minimum. Survey crews could carry chainsaws and herbicide with them and upon diagnosis, the elm could be immediately killed. Prompt removal of the tree may be beneficial if it is past the 50% wilted point because some live brood may be produced.

The trap tree technique is most important in areas where prompt removal is impossible. This technique is fast and cheap and will prevent the bark beetle population from growing at an alarming rate. If all infected trees are treated, the beetle population will actually decline.

BIBLIOGRAPHY

1. O'Callaghan, D.P., P.M. Atkins and C.P. Fairhurst. 1984. Behavioral responses of elm bark beetles to baited and unbaited elms killed by cacodylic acid. J. Cham. Ecol. 10:1623-1634.

2. Lanier, G.N. 1989. Trap Trees for Control of Dutch Elm Disease. J. Aboric. 15:105-111.

3. Cannon, Jr., W.N., J.H. Barger and D.P. Worley. 1982. To Improve Sanitation Performance. J. Aboric. 8:129-135.

Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.