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. Nurse bees, which digest pollen far better than other bees, have much more developed fat bodies than newly emerged bees or foragers. Above all, this discovery explains the effects of infestation on the bee: the collapse of protein content in haemolymph (blood proteins are released from fat bodies), impacts on immune and detoxification systems (which are functions essentially linked to fat bodies), reduced longevity (linked to vitellogenin stored in fat bodies), disruption of metabolic functions (fat bodies play the role of a “chemical factory” analogous to the liver in humans), etc. The removal of haemolymph alone was insufficient to explain all these effects.

The significance of this study is multiple: it shows that varroa mites can be reared in the laboratory—an endeavour that had previously failed due to inadequate mite nutrition; it opens the way to new treatment methods via substances that target fat bodies in bees without harming them, but affect the parasite through feeding (the researchers are considering interfering RNAs—see box). It should also help to better understand synergies between varroa, pathogens (many of which are transmitted by the mite) and pesticides. Finally, it definitively demonstrates that phoretic varroa mites do not merely hitch a ride on adult bees: they feed on them, and this feeding plays a role in their reproductive capacity. These mites are rarely found on foragers, but rather on nurse bees, and preferentially position themselves between the ventral segments; it is therefore pointless to look for them “with the naked eye” at the hive entrance or on the frames …

 

Study reference: Ramsey SD, Ochoa R, Bauchan G et al., 2019: Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph, Proceedings of the National Academy of Sciences 116(5): 1792–1801.

Reference of the other cited study: Xie X, Huang ZY and Zeng Z, 2016: Why do Varroa mites prefer nurse bees? Nature, Sci Rep 6: 28228.

 

Interfering RNA: what is it?

RNA (ribonucleic acid) is a molecule whose main and best-known function is to “copy” DNA in order to enable the assembly of proteins, which are essential molecules of all living organisms. Gene transcription occurs through RNA, that is, the mechanism by which the molecules encoded by a gene (made of DNA) are assembled.

However, not all RNA in living cells serves this purpose. There are small RNAs that interfere with the transcription mechanism to prevent the expression of certain genes. This is an essential mechanism of gene expression regulation. Discovered in the 1990s, this mechanism has generated great interest among researchers, particularly because of its potential applications in medicine (for example, silencing cancer cells without damaging other cells in the body). It is therefore conceivable to envisage supplementary feeding containing interfering RNAs that would bind in bee tissues and target genes specific to varroa (but absent in bees, hence no effect on them). However, the development of such treatments is far from simple. Some are in clinical trial phases in humans, but application in bees remains a distant prospect … (Source: Wikipedia articles “RNA interference” and “Interference by RNA”.)

 

Article published in La santé de l’Abeille no. 291, May–June 2019.

 

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