The Emerald Ash Borer Invades Colorado

I am usually excited in the positive sense when insects make headlines, but there are exceptions. It was devastating to learn recently that the Emerald Ash Borer, Agrilus planipennis, has been detected here in Colorado. Why is this discovery such a big deal? It has to do in part with the customary response to this exotic invader in other locations where it has shown up.

The Colorado Department of Agriculture announced on Friday, September 27, 2013 that the Emerald Ash Borer (EAB) had been discovered in Boulder County earlier in the month. While ash trees are apparently not native to Colorado, they are planted widely as part of the “urban forest” in many cities along the Front Range. Consequently, those trees are vulnerable to this invasive pest.

The reaction of many municipalities to the presence of EAB is the pre-emptive removal of the insect’s host trees. Visions of a wholesale slaughter are going through my mind right now. It is not pretty. It is costly, too, and leaves one less alternative for greenery and shade in what is a pretty bleak landscape on the fringe of the Great Plains. Shade is hard to come by, and there are few tree species that don’t have objectionable side effects. Cottonwoods shed copious amounts of cottony seeds in the spring. Locust trees grow quickly but their falling leaves get sucked into car air filters and find their way into every crack and crevice.

The thing that is really tragic is that the spread of the EAB is pretty much preventable. Various agencies and organizations have tried to get out the message about not moving firewood between states (even between counties), but the plea has either fallen on deaf ears, or not been broadcast loud enough. This is such an important message that it really warrants the Ad Council’s help in airing public service announcements on television and radio.

What is the history of this pest, you ask. According to EmeraldAshBorer.info, the Emerald Ash Borer was first discovered in the United States near Detroit, Michigan in 2002. At least, that is when entomologists first recognized it. It could have been present prior to 2002. Authorities surmise that it probably arrived in solid wood packing material originating from its native Asia. In short order the EAB was also found in Ontario, Canada, Ohio, and northern Indiana (by 2004). More were detected in northern Illinois and Maryland in 2006, then western Pennsylvania and West Virginia in 2007, followed by Missouri and Virginia in 2008. Since then it has also turned up in Minnesota, New York, Kentucky, Iowa, Tennessee, Connecticut, Kansas, Massachusetts, New Hampshire, North Carolina, Georgia, and now Colorado.

The life cycle of the beetle makes it difficult to quickly detect and almost impossible to control. The adult beetles typically emerge in mid-late May, with peak numbers in late June. Mated females begin laying eggs within two weeks of their emergence. The larvae hatch in about one to two weeks and begin boring into the inner bark and cambium layer of the host tree, disrupting the transportation of nutrients to the top of the tree. The larvae continue boring from late July to early October, overwintering in a chamber where they will pupate in the early spring. Sometimes the cycle is extended to two years.

The only symptoms of an EAB infestation outwardly visible are the gradual and subtle thinning of the tree’s canopy, and dying of branches in the uppermost reaches. Small trees can be killed by the beetle in one or two years, whereas larger trees may succumb in 3-4 years. The beetle probably selects stressed or otherwise already weakened trees as preferred hosts, but healthy trees can be attacked, too. The ongoing drought in the Front Range enhances the probability of the EAB becoming a widespread problem very quickly.

Please be on the lookout for the Emerald Ash Borer in your own state, province, or county, whether or not the species has already been detected there. Report your observations to your state department of agriculture, taking them specimens whenever possible. Thank you.

Wasp Wednesday: Pigeon Tremex Horntail

People who have minimal knowledge of wasps can’t really be faulted for having a startled or panicked reaction when confronted with one of these creatures, especially if it is a large insect. Among the more intimidating of wasps are the horntails in the family Siricidae. They do not sting, but they look like they can. Here in the Front Range of the Rockies, the most common species of Siricidae is the “Pigeon Tremex,” Tremex Columba.

So far, I have encountered only male specimens, resting quietly on foliage at about eye-level. They can be approached easily and closely if you are careful. This is a native species ranging widely from Nova Scotia, New Brunswick and Quebec, south to Florida, and west to Mexico, Arizona, Utah, and Colorado. There are even a few records from southern California.

Siricids in general are more closely related to sawflies than any other wasps. The abdomen is joined broadly to the thorax, giving the entire creature a cigar-shaped appearance. They are about a third the size of a stogie, too. Adult females range from 37-50 millimeters (1.5-2.0 inches). Males average smaller, from 18-37 millimeters. Note that western specimens of Tremex Columba are usually much paler in coloration than eastern specimens.

© Andrew Williams

The females are equipped with what looks like two stingers, one short one protruding from the top of the tip of the abdomen, and another longer, needle-like rod at the bottom of the abdomen. The wasps are, in fact, non-venomous. The top “horn” is called the “cornus,” and both genders possess this spur, which gives the family its common name of “horntails.” I have been unable to find an explanation of the function of the cornus, if there is one. The longer appendage is an egg-laying organ called the “ovipositor.” The female uses this complex part of her anatomy to insert eggs into dead, dying, or weakened hardwood trees. Maple, beech, elm, apple, pear, poplar, oak, hickory, sycamore, and hackberry are all known hosts of this wasp, but other deciduous trees in poor condition are probably utilized as well.

The ovipositor acts like a drill and hypodermic needle. It is housed in a two-part sheath that helps to brace the sawtoothed egg-laying organ as it works its way into bark and wood. Lumberjacks have a vulgar nickname for these wasps, owing to what the wasps appears to be doing when they oviposit into a bole or stump. I’ll leave that epithet to your imagination.

© Andrew Williams

Not only does the wasp lay her eggs in the tree, she also delivers a wood-rotting fungus, Cerrina unicolor to the site of each egg insertion. She stores the fungus in special abdominal glands called mycangia, until she is ready to deploy it. The fungus breaks down cellulose, and both fungus and decayed wood are then consumed by the larval wasp.

Larval horntails are grub-like, and easily mistaken for beetle grubs save for the cornus on the very tip of their worm-like bodies. It usually takes more than one year for the horntail to complete metamorphosis.

© Project Gutenberg (Google)

You would think that a larva tunneling inside a tree or log would be safe from its enemies, but such is not the case. Huge wasps called giant ichneumons can drill down and reach the horntail grubs. One of my most popular blog entries chronicles the life cycle of these Megarhyssa wasps. Yet another kind of wasp attacks younger larval stages of horntails. Wasps in the family Ibaliidae (and genus Ibalia) drill down to reach horntail larvae that are at a more shallow depth.

Female Ibalia sp. © Mark MacMillan/Colorado State University

Pigeon Tremex horntails that survive the perils of youth eventually pupate, emerging as adults most often in late summer or fall. Right now is the ideal time to find them, but you have to look at a lot of dead, standing trees to find them. Horntail species that use coniferous trees as hosts are often attracted to wildfires, since fire often weakens trees. Many wood-boring beetles are also active in the wake of conflagrations.

Please help spread the word that with few exceptions (introduced species like Sirex noctilio for example), horntails are not pests, but merely exploit trees that are already damaged by environmental stress, and/or diseases or other destructive insects. They are important to forest ecology and the average homeowner has no need to control them.

Sources: Cranshaw, Whitney and Boris Kondratieff. 1995. Bagging Big Bugs: How to identify, collect, and display the largest and most colorful insects of the Rocky Mountain region. Golden, CO: Fulcrum Publishing. 324 pp.
Drees, Bastiaan M. and John Jackmann. 1999. A Field Guide to Common Texas Insects. Houston: Gulf Publishing Co. 359 pp.
Evans, Arthur V. 2008. National Wildlife Federation Field Guide to Insects and Spiders of North America. New York: Andrew Stewart Publishing, Inc. 497 pp.
Schiff, Nathan M., Steven A. Valley, James R. LaBonte, and David R. Smith. 2006. Guide to the Siricid Woodwasps of North America Morgantown, WV: USDA Forest Service Forest Health Technology Enterprise Team (FHTET-2006-25). 102 pp.