Varroa does not feed on blood

Varroa is known to suck the haemolymph of bees. Well known, but apparently (largely) incorrect.

Researchers in the United States were intrigued by the magnitude of varroa’s impact on bee health in relation to the relatively small amount of haemolymph it extracts. Since insect haemolymph is comparatively poorer in nutrients than mammalian blood, they questioned how the parasite could develop on such a resource.

The study of the anatomy of the mite’s mouthparts and the structure of its digestive tract suggests that it feeds rather on semi-solid tissues than on fluids, and that it pre-digests them before ingestion. By injecting its saliva into the bee’s tissues, the enzymes contained in the saliva would allow pre-digestion of the tissues before absorption. The composition of varroa excreta shows that its diet is particularly rich in proteins and low in water content. All these observations do not appear compatible with a diet based on haemolymph.

The researchers’ hypothesis therefore focused on a semi-solid tissue, rich in fat and proteins and easily accessible to the mite: the fat body. In larvae, fat bodies are diffusely distributed throughout the body, whereas in pupae that have reached the final nymphal stage and in adults, they are mainly located on the ventral and dorsal sides of the abdomen. These findings explain why the mite shows no preference when feeding on larvae and young nymphs, but preferentially positions itself between the abdominal segments on adult bees.

A detailed microscopic study of varroa mites feeding on bees revealed degradation of the fat body at the bite site, with traces of external digestion. To confirm their observations, the researchers marked bees with two fluorescent dyes: Nile red is a lipophilic dye (attracted to fatty substances) that preferentially labels fat bodies; uranine specifically labels aqueous substances and preferentially diffuses in the haemolymph. After infesting these bees with varroa mites, they waited for the mites to feed and then produced images of the mites’ internal fluorescence. These appeared red.

Images of bee tissues and a varroa mite without staining (first column), and with uranine staining (column 2), Nile red (column 3) and both dyes (column 4). The tissues shown are the bee digestive tract, a blood sample, a fat body sample, and finally a varroa mite that has just fed on an unstained bee or on one impregnated with one of the dyes. The varroa appears hardly stained with uranine (N), whereas haemolymph clearly takes up this dye (F, H); in contrast, it is clearly stained by Nile red (O, P), which also stains the fat bodies (K, N).

The researchers then subjected mites to six different diets: complete fasting, or diets composed of haemolymph and fat bodies in proportions of 100/0, 75/25, 50/50, 25/75 and 0/100. Varroa mites fed exclusively on haemolymph showed performance levels (lifespan and fertility) that were barely higher than those of fasting mites. These performance levels increased with the proportion of fat bodies in the diet.

It therefore seems clearly demonstrated that varroa feeds on the bee’s fat bodies rather than on haemolymph.

This notably explains the observation made in another study (Xie et al., 2016) that phoretic varroa mites show a marked preference for nurse bees, and that their offspring are better ensured if their intermediate host is indeed a nurse.

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Varroa does not feed on blood

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