It’s difficult to accurately identify insects without an image.  Is it possible for you to send me a digital picture?  If so, we can probably solve this mystery.

With your description of “trapezoid shape brown thorax and legs and a darker triangular head,” I wonder if you have brown marmorated stink bugs, which go by the abbreviation BMSB, the technical name Halyomorpha halys. Please compare your insect with images on this page at Oregon State University: http://horticulture.oregonstate.edu/content/biology-and-identification

If this is what you are seeing, you can be confident that it’s not damaging your cedar.  This time of year, they are considered to be only a nuisance. Unfortunately, when present in the 1000s, they’re a major nuisance.  A wet-dry shop vac is an excellent choice to collect and destroy them.  Unfortunately, pesticides aren’t effective against BMSB

Even so, use the information on this page to report the infestation at your place: http://horticulture.oregonstate.edu/content/report-brown-marmorated-stinkbug  

If your insect isn’t BMSB, please send me an image with your response. 

If you’re unable to send me an image, take the insect to the Marion County Extension Service office to have it identified.  The office is at 3180 Center Street NE, Room 1361, Salem, Oregon 97301.  Phone<personal data hidden>.  The office is open Monday through Thursday 8:30am-5:00pm; Friday 8:30am-1:00pm.  Please call ahead to make certain the office is open because the building is scheduled to be renovated.

For your cedar, I suggest you consult several Certified Arborists for onsite evaluations to determine the cause of the problem and treatments, if any, which may be helpful.  Compare their comments, and then hire one as such a person will have access to the equipment for any needed treatment.

If the tree is large, I suggest you hire a Certified Arborist. You can locate Certified Arborists in the Yellow Pages, online or in a phone book.  Tree care companies with such a person will include that information in their ads, either as a brief statement or as a small rectangular logo stating “Certified Arborist.” We suggest Certified Arborists because they are trained in tree care, must pass an exam to acquire certification from the International Society of Arboriculture, and must accumulate mandated continuing education to retain certification.  When you contact each company, avoid any unwelcome surprises by asking if they have a fee for the visit and evaluation.