Those of you who keep or observe blue orchard bees (BOBs a.k.a. orchard mason bees, Osmia lignaria) may have noticed a brown scunge on your bees this year. Some bees may be unable to fly, and fall to the ground to become ant food in an untimely way. These bees are infested with Krombein's hairy-footed mite, Chaetodactylus krombeini (Chaetodactylidae), a native parasite and phoretic "hitchhiker" of cavity nesting bees in the family Megachilidae (leafcutting bees, mason bees, etc.). Populations of this mite have been building for several years in managed and semi-managed populations of BOBs, but they also appear to be infesting wild bees. (This mite is entirely different from the mites now decimating honey bees.)

Osmia head; red arrow shows mite under eye © Rod Crawford

Mites between leg bases of Osmia © Rod Crawford

Cleared Chaetodactylus hypopus (0.29 mm) in phase contrast; arrows show giant claws © Rod Crawford

What we know about life history of the mite, albeit incomplete knowledge, is due in large part to its discoverer, Karl Krombein, the famous hymenopterist, and his collaborator E.W. Baker, a mite specialist. They described the mite from a Maryland population in the 1960s.

[The chaetodactylids are one of the mite families that have a special nymphal stage, called the hypopus, which is modified for phoresy (hitch-hiking on another organism without feeding on it). The hypopus has no head or mouthparts but has special structures for hanging on! With Chaetodactylus, these structures are giant claws on the first three pairs of legs. The hypopus of Chaetodactylus is 0.29 mm long.]

The female mite hitch-hikes [as a hypopus] on a bee to the nest, where it jumps off and makes itself at home in a larval cell. After it matures, it rapidly produces a small number of parthenogenic young that may feed on the bee egg or larva, usually killing it. These mites then reproduce, some sexually, and begin to feed on the bee hosts' pollen ball. The population grows explosively, and within a few months can turn the bee food into a solid writhing mass of mites. You may open a BOB nest and find such a mass any time from midsummer through the following spring. In the mass are various developmental stages and both sexes, although mostly females. The female has two pink sclerites on her dorsum, which gives groups of the adult mites a pinkish tinge. She also has two long setae on the hind tarsus, whence the common name.

As bees emerge from the infested nest, they may crawl through an infested cell. When they do, they instantly acquire an overload of hitch-hiking mites in the hypopus stage, often enough to cripple the bees from flight. The mites at this stage [since they lack mouthparts] do not feed on the bee. Among the mites remaining in the nest are encysted individuals which look like pearly eggs and are found in small cracks or other hideaways. These apparently wait until later in the season before before emerging to re-infest a new nest in the same cavity, made by either another BOB or a related species. We don't know much about the cysts. [They are actually a second, sedentary type of hypopus with reduced legs.]

The mites therefore kill host bees at two stages, egg or early larva and adult, because of completely different activities. We currently have no method of managing the mites, although keeping BOBs in paper tube inserts is helpful because it allows heat sterilization of the wooden block (to kill cysts) in the winter after the nests have been removed. Also, the tubes can be cut open to survey for the mites, which eventually may facilitate mite control. BOBs kept in bare wood blocks are basically helpless against this pest.

We have learned little since the pioneering reports of Krombein and Baker. More research is badly needed to find an effective management system for the mite. Some private indivuduals, including some Scarabs, are currently experimenting with various miticides to kill the mites at various stages in their life cycle, but trying to insert these safely into the life cycle of the host bee is difficult. The USDA Bee Biology & Systematics Lab in Logan, Utah is aware of the problem, and may soon include the hairy-footed mite in its research program. In Japan, a similar mite infests native populations of the Japanese hornfaced bee, an important fruit pollinator. The Japanese have had some success in controlling their mites with miticides and heat shock.

Addendum (2005): These mites are susceptible to dehydration in their early stages. The following combination of methods can achieve good control: 1) Use porous nesting materials, wood or, better, paper inserts in wood. 2) Take down the nest box early in the season, even before the females have made their last nests (about the last week in May in the Puget Sound area). 3) Incubate the larval nests in the nest box indoors under warm and dry conditions, ideally a mildly heated (up to about 85° F) and dehumidified room. 4) Starting about mid-August, check the nests periodically for mature adults. If you are using straw inserts, you can examine whole nests non-destructively by carefully slitting the straw lengthwise with a sharp blade. (Use of plastic straws for easier observation causes moisture problems, lowering bee survival and raising mite survival). Remove any remaining mite-infested cells. When all bees are mature, move to outside temperature for a few days, then into the refrigerator for winter storage (as nest inserts if using straws). Using these methods, mite infestation of about 50% has been reduced to 5%.

[Editor's notes: Information on the hypopus stages has been added to the original article. It is not completely certain that the mite we see in the Pacific Northwest is the east-coast C. krombeini; the European C. osmiae is recorded from Oregon.]

References

Baker, E.W. 1962. Natural History of Plummers Island, Maryland. XV. Description of the stages of Chaetodactylus krombeini, new species, a mite associated with Osmia lignaria Say. Proceedings of the Biological Society of Washington, 75: 227-236.

Fain, A. 1966. Notes sur la biologie des acariens du genre Chaetodactylus et en particulier de C. osmiae, parasite des abeilles solitaires Osmia rufa et O. cornuta en Belgique. Bulletin de la Société Royale Entomologique de Belgique, 102(16): 249-261.

Krantz, G.W. 1978. A manual of acarology. 2nd edition. Oregon State University Bookstore, Corvallis. 509 pp. P. 379, 419.

Krombein, Karl. 1962. Natural History of Plummers Island, Maryland. XVI. Biological notes on Chaetodactylus krombeini Baker, a parasitic mite of the megachilid bee Osmia (Osmia) lignaria Say (Acarina: Chaetodactylidae). Proceedings of the Biological Society of Washington, 75: 237-250.

This page last updated 26 July, 2005