Green industry professionals should understand exactly how Asian longhorned beetles affect trees; it is critical to being able to detect ALB

Transcription

1 Green industry professionals should understand exactly how Asian longhorned beetles affect trees; it is critical to being able to detect ALB infestations! 1

2 It's important to review the basic structure of tree trunks before demonstrating how ALB kills trees. 2

3 This simplified graphic shows the general structural layout for a deciduous hardwood tree such as ash, birch, oak, or maple. Simply stated, a deciduous tree consists of a series of concentric rings. Bark: protects the tree. Phloem: "food flows through phloem." This ring enables trees to transport carbohydrates (sugars) produced in the leaves through photosynthesis down the tree to the roots. It also allows the tree to transport stored carbohydrates up the tree; this is made evident in the spring when maples are tapped to produce maple syrup. The phloem within a tree's trunk is actually much thinner than depicted in this graph. "Debarking" a tree removes that phloem ring as well as the bark. The phloem is very sticky during the growing season. Cambium: the cells in this thin rings are called "meristematic" because they can differentiate during cell division to become other types of plant tissue. It's only 3 cells thick. The cells can become new phloem cells to the outside and new xylem cells to the inside. This is how trees increase their diameter (gain girth). The cambial layer is very slimy. Xylem: is what we know as the wood of the tree. The xylem consists of the Pith, Heartwood, and Sapwood The sapwood transports water and nutrients absorbed by the roots up the tree to support leaf and stem health and growth as well as photosynthesis that produces the tree's food. The heartwood along with the sapwood and to a lesser degree the pith provides structural support for the tree, a very important function. 3

4 The Cambium is only 3 cells thick, but it is from the cambium that both the phloem and xylem develop. The meristematic cambium cells can become new phloem cells to the outside, and new xylem cells to the inside. This is how trees increase in diameter (girth) from year-to-year. 4

5 The meristematic cambial cells can also become another type of tissue that's different from phloem and xylem, but is very important in helping trees to respond to wounding. The cells can differentiate during cell division to become a type of tissue known as "Wound" or "Callus" tissue. Callus tissue is used by the tree to close a wound which is why both wound and callus tissue are sometimes referred to as "Wound Closure Tissue." The function is to close seal wounds against pests and plant pathogens that cause disease. Wound response by trees is very different from the way animals deal with tissue damage. If you are wounded, your body heals the wound by producing new cells that exactly match the cells that were damaged. Trees cannot do this; they do not heal, they seal. The final image in the slide shows a very nice ring of callus tissue produced in response to a proper pruning cut: you can see that the tissue is forming a donut-like ring that will eventually close the pruning wound. Applying "Wound Sealer" or paint to a pruning cut is not recommended: it will interfere with natural wound closer can keep trees vulnerable to pest and pathogens. 5

6 The feeding activity of Asian longhorned beetle larvae can also stimulate meristematic cambial cells to differentiate to form callus tissue. NOTE: research has shown the production of callus tissue in response to tree borer larval feeding activity may be more closely related to tree defense rather than tree wounding. Rapid production of callus tissue can actually crush larvae as the feed on the phloem. The expansion of the lip-like callus tissue can cause the overlaying bark to split or crack open. While this symptom can be caused by Asian longhorned beetle, it can also be produced by the activity of other insect borers as well as a number of other causal agents such as rapid changes in winter temperatures (e.g. freeze-thaw cracking), root damage, exposure to certain herbicides, just to name a few. 6

7 Asian longhorned beetles also wound trees when they emerge and when they lay eggs. As noted in the previous module, adult emergence holes punch through the cambium, phloem, and bark. Females chew oviposition pits through these rings to lay an egg. Wound Closure: Both adult emergence and egg laying produces tree wounds. In response, the meristematic cambial cells form callus tissue to close the wounds. Total wound closure can occur within a few weeks during the growing season on healthy trees, particularly on healthy young trees. Wound closure can obscure these ALB symptoms. Wound closure occurs more slowly on older, "over mature" trees as well as on trees that have been heavily damaged by ALB larval feeding activity. Adult emergence holes and female oviposition pits remain visible for a long period of time. 7

8 The phloem enables trees to transport carbohydrates (sugars) produced in the leaves through photosynthesis down the tree to the roots. It also allows the tree to transport stored carbohydrates up the tree; this is made evident in the spring when maples are tapped to produce maple syrup. Remember: the tree s food flows through phloem! The xylem is what we know as the wood of the tree. It is used for transporting water and nutrients absorbed by the roots up the tree to support leaf and stem health and growth as well as photosynthesis that produces the tree's food. The xylem also provides structural support for the tree, a very important function. 8

9 All trees are not equal in the number of functional xylem rings: the number of rings that actually transport water and nutrients up the tree. Ring Porous Trees only have one functional ring each growing season. The ring that forms from the cambial cells this season will be the single functional ring next season. Thus, the ring that's used to carry water and nutrients up the tree each season is the outermost ring. Some examples of ring porous trees include ash, black locust, elm, honeylocust, oak, and redbud. This is only a partial list; there are many other ring porous trees. 9

10 Diffuse Porous Trees have multiple functional rings each growing season. As a new ring is added, the innermost ring ceases to carry water and nutrients up the tree from the roots. Some examples of ring porous trees include birch, beech, dogwood, maple, sycamore, spruces, pines, firs, and hemlock. This is only a partial list; there are many other diffuse porous trees. 10

11 This slide demonstrates that Ring Porous and Diffuse Porous are not always a black-and-white condition; all trees do not conveniently fit into one or the other category. Certain ALB hosts such as some elms are semi-ring porous while poplars and willows are semi-diffuse porous. However, ALB's most preferred host maples are diffuse porous. 11

12 NOTE: ALB's most preferred tree host maples are used to demonstrate how ALB kills trees. Asia longhorned beetle females chew through the bark, phloem, and cambium to lay a single egg on the surface of the outermost ring of xylem. The second image on this slide shows an ALB egg deposited on the surface of the xylem. The third image shows a first instar ALB larva resting on the surface of the outermost xylem ring. 12

13 When ALB eggs hatch, 1 st instar larvae may feed a short time on the phloem, or they may bore into and begin feeding on the xylem. 3 rd through 5 th instar larvae only bore through and feed on the xylem. 13

14 ALB larvae will eventually cause extensive localized damage to the phloem and the cambium. However, since ALB larvae confine most of their feeding activity to the xylem, the damage caused to the phloem does not produce the rapid tree decline caused by the larvae of borers that feed exclusively on the phloem such as emerald ash borer. The second image on this slide shows that heavy ALB larval tunneling weakens the structural function of the xylem. NOTE: despite the loss of phloem and cambium as well as heavy damage to the xylem, many xylem rings continue to remain functional on maple trees. That's because maples are Diffuse Porous trees. 14

15 Diffuse porous maple trees don't die quickly from ALB larval tunneling / feeding damage. Note that the tree pictured here is only showing very slight canopy decline; not much greater than is common for urban maples growing in a confined space. However, the close-up image of the main stems show that this tree is riddled with ALB emergence holes! 15

16 This is another maple infested with ALB: note there is no obvious dieback in the canopy. The Bottom Line: A thinning canopy is NOT a reliable diagnostic symptom for detecting newly developing ALB infestations on diffuse porous trees such as maple. 16

17 On Live Maple: No native tree borer produces the xylem tunneling damage caused by Asian longhorned beetle: look at the ends of cut branches! The structural weakening caused by ALB larval tunneling causes branches to easily break from infested maple trees: look at the ends of broken branches! New ALB infestations have been revealed by an unusually heavy amount of branch breakage on live maples. NOTE: there are many native longhorned beetles and other borers that will tunnel into and feed on the xylem of dying and dead maples. This does not mean that such symptoms should be ignored; it just means that the symptoms do not necessarily point towards ALB since we expect to see this type of damage from native borers infesting dying and dead trees. 17

18 After new Asian longhorned beetles emerge, they spend a short time feeding on the phloem tissue on twigs and on leaf veins to gather nutrients to help mature their eggs. This is called "Maturation Feeding." This feeding behavior is very unusual compared to damage caused by native insects including other borers; however, the damage is actually very difficult to find, even on heavily infested trees. It is not a dependable symptom for detecting ALB. Indeed, adults cause such little damage their feeding behavior alone causes no appreciable harm to the overall health of the tree. 18

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