Of all the arthropods living in the United States, only a tiny percentage would be considered true structural pests; most live freely in soil, waterways, forests, meadows, fields, lawns and ornamental shrubs. Outside the true structural pests, some species invade buildings on an occasional basis, and a significant number of species may do so on rare occasions.
The pest control industry has used the term occasional invader for as long as I can remember. To learn more, I went into my library of original entomology and pest control books to determine the derivation of this term in the pest control industry.
Arnold Mallis, in the first edition of the Handbook of Pest Control published in 1945, does not use the term occasional invader, instead devoting separate chapters to pests historically categorized as occasional invaders, such as silverfish, springtails, crickets and earwigs, and completes his book with a chapter titled, Miscellaneous Pests.
Published originally in 1962 by Dr. Lee Truman, the Scientific Guide to Pest Control Operations has a chapter dedicated to Occasional Invaders and Miscellaneous Pests. As defined by Truman, “…‘occasional invaders’ includes those pests which occur in buildings at some stage of their life cycle but which do not usually complete the entire life cycle within the building.” Truman included such pests as crickets, earwigs, silverfish, springtails, isopods, clover mites, box elder bugs, lady beetles, etc., in this chapter.
I did not know then to ask Dr. Truman the origin of the term when I worked with him for two years fresh out of Purdue University. Possibly “occasional invaders” was first used in an NPCA (National Pest Control Association) publication, but I cannot find it in my limited publication collection. (I would be interested in its origin if anyone might know.)
When Urban Entomology was published in 1975, Dr. Walter Ebeling did not include any mention of occasional invaders, but instead followed Mallis’ lead with a chapter dedicated to Miscellaneous Pests. Many of Truman’s “occasional invaders” were featured in that chapter while he assigned others, such as springtails and millipedes, to a chapter titled Pests in Excessively Damp Locations.
It was not until the 7th Edition of the Handbook of Pest Control, edited by Keith Story in 1990, that the chapter Miscellaneous Pests was retitled Occasional Invaders. This edition, however, still maintained separate chapters for silverfish, earwigs, crickets, etc.
When I assumed the duty as editorial director for the 8th Edition of the Handbook of Pest Control, I kept all chapters as they appeared in the 7th Edition. But when it was time to plan the 9th Edition revision, I made the decision that all the pests historically considered to be “occasional invaders” should be united into a single chapter of that title. I took the responsibility for completing this task, incorporating the silverfish, springtails, crickets and earwigs chapters into the Occasional Invaders chapter. I also moved individual pests that fit the occasional invader model, such as box elder bugs, aphids and leafhoppers, from the Bed Bugs and Other Bugs chapter. I divided the chapter into two sections in order to separate those insects that seek out buildings to spend the winter into the section, “Overwintering Pests.”
In the 10 years since the 9th Edition was published, I’ve come to realize that the category of occasional invaders is in further need of clarification and redefinition as it has become a “catch-all” for every pest that doesn’t neatly fit into one of the other pest-type categories. This amalgamation of pests thus created some confusion in some customers’ minds in regards to their pest coverages. Commercial customers, for example, whose contracts with pest management companies include “occasional invaders,” might request free additional services for pests like hackberry psyllids or thrips that are ornamental pests but, on rare occasions, may enter buildings. Such pests fit the current definition of occasional invader but are typically outside the bounds of standard service agreement coverage by pest management providers.
For these reasons, I propose this group of pests be subdivided. I also think it’s time to introduce a new term: incidental invaders.
Occasional Invaders. This grouping includes those pests that are occasional but fairly regular pests that invade structures. These pests may be able to complete all or part of their life cycle within a building, such as silverfish, house crickets or camel crickets, or just appear on a frequent enough basis inside to qualify as “occasional.” Members include silverfish and firebrats, springtails, ground beetles, black vine and strawberry root weevils, centipedes, millipedes, sowbugs and pillbugs, amphipods, crickets, earwigs, clover mites and certain psocids.
Overwintering Pests. This group is easy to define and contains a limited, but growing, club whose members find the voids of structures to be ideal sites to wait out the cold days of winter. Included are lady beetles, elm leaf beetles, box elder bugs, cluster flies, stink bugs, kudzu bugs, paper wasp queens and the Western conifer seed bug.
Incidental Invaders. Previous publications labeled this category as “miscellaneous pests,” but I feel that incidental invader is a more apt description for the uncommon or rare pest invasions pest professionals encounter in both residential and commercial situations. An incidental invader is one where external conditions conspire to create a situation where an insect or other arthropod appears on or inside a building, thus prompting a customer to bring it to our attention. Such insects are not true pests, rather their appearance is sudden, unexpected and generally a one-time occurrence.
Light-Attracted Pests. Think, for example, of all the insects that may be attracted to bright lights on buildings. Entomologists use lighting to attract and collect insects they may not ever find or see during the daytime. Many new species of insects have been discovered when attracted to such lights. For this reason, any home or building could become a victim of an incidental invasion by one or more light-attracted insects inside. Some such insects are often typical (but seasonal) offenders in this regard, such as aquatic insects (e.g., mayflies, caddisflies and certain midge flies) and blister beetles and leafhoppers. Other light-attracted insects, however, are rare, for example, the two cases involving burrower bugs I have been consulted on in the past — one affecting a circuit board manufacturer in Nevada and the other a nuclear power plant in the Southeast.
Any insect attracted to a light, therefore, can become a pest but it is really only incidental due to its behavior bringing it into proximity to the structure. Over my career, I have received requests to identify all kinds of light-attracted insects from odd species of ground beetles to rove beetles, click beetles, stag beetles, longhorned beetles, tumbling flower beetles, sap beetles, miscellaneous wood-boring beetles, dobsonflies, adult ant lions, fishflies, all kinds of moths, many types of hemipterans (such as seed and plant bugs) and others. All of these pests were one-time occurrences related to lighting. Treatments are rarely needed for such situations. Changing the lighting and addressing exclusion issues are typically the better course of action, although each individual case requires its own plan for resolution.
Pests From Animal Nests. Another subgroup within this category are those insects associated with animal nests. Their presence inside a building is incidental to the fact that their host has taken up residence inside. Bird mites are the most common of such pests, but bird lice, bat bugs and swallow bugs are seen in structures with some regularity. Less commonly seen are bat ticks and bird ticks, and I have encountered, on a couple of occasions, bird keds (wingless species of flies found in some nests of birds). Cases of these pests will involve removal/exclusion of the host, removal of nesting materials and possibly some limited treatments.
Ornamental Pests. Under the right conditions, certain ornamental pests may be attracted to and end up inside a building. Common examples include hackberry nipple gall psyllids, thrips, certain beetles (e.g., Asiatic oak weevil, tulip tree weevil), and some winged species of psocids (book lice). Certain caterpillars also can become an issue, including larvae of gypsy moths and Eastern tent caterpillars found crawling by the dozens or hundreds on the outsides of homes. The appropriate host plant needs to be close enough to the infested building and conditions suitable for creating a large enough population that insects moving off the infested plants find and enter the building in varying numbers. The infestation is incidental because conditions need to be just right to create the situation where the invasion occurs. Typically, it is a one-time occurrence even though the host plant(s) are present in the years before and after a case where they have caused a pest issue.
Firewood Pests. One last group to include as incidental invaders are firewood pests. Dead wood is a harborage to a wide variety of arthropods and firewood piles make an excellent food source or harborage to many insects, spiders, centipedes and others, especially over the winter. The most common firewood pests are types of longhorned and buprestid beetles that have pupated inside the wood and emerge inside in numbers ranging from a few to dozens. When firewood is carried inside and stored for longer than a day or two, overwintering arthropods awake and may be found crawling or flying about, thus prompting a customer call for help. I have consulted on calls ranging from scorpions to yellowjacket queens to carpenter ants and odd species of spiders that are associated with firewood. Customers always should be advised to bring inside no more wood than they can burn in a single day.
I believe these pest categories will bring some clarification to the industry. This should help pest service professionals better understand how to explain the nature of a pest infestation to a customer. Occasional invaders are common enough to warrant regular attention to keep them out of buildings as they are often associated with structures. Overwintering pests is an easily understood term by customers as one that prefers buildings in which to harbor from the cold. Last, the new term of incidental invader clearly defines pests that are present only due to circumstance or a specific factor such as lighting, animal nests, ornamental plants or firewood.
The pest control industry is evolving in many ways and how we categorize certain pests should evolve with it. In the future, pest professionals may be asked to be more specific in defining infestations in order to “justify” treatments being performed. Such proposals as this are but a small part of that process.
The author is a board certified entomologist and president of Stoy Pest Consulting, Lakeland, Tenn. He can be reached at firstname.lastname@example.org.