.
Raport says they found 6 days after trickling small droplets in the hive. They conclude that bees does not suck actively oxalic syrub.
Just bouncing this back to the thread from a few days ago.
Post 23 From Gavin.
That is one of the least accurate posts I've seen on oxalic acid.
Larvae get fed with the secretions of workers, and so are not directly exposed to solutions poured on the workers but buffered by the internal physiology of the worker. Furthermore I've never known a larvae to reach out of its cell and groom a passing worker so it will not be over-exposed through that route either.
Workers ingest oxalic acid. Whether this is by imbibing syrup or grooming doesn't matter, it gets inside them. When these Spanish researchers dosed bees with 10% oxalic acid solution (greatly in excess of normal treatment levels) they saw various effects on internal tissues of bees, essentially the degeneration of tissues.
http://revistas.inia.es/index.php/sjar/article/viewFile/270/267
The implication of all of this is that the haemolymph is affected, and that may be a route that affects Varroa. It would certainly explain the often slightly delayed effect on Varroa.
The paper looked for and explicitly declared a lack of visible lesions on the exterior of treated bees (see Fig 2).
And post 27.
Just to make it easy, I'll post some text from the paper. Technobabble warning, but it is clear enough. None of this will stop me from using oxalic acid. The 100+100+7.5 (aka 4.5% aka 3.2%) recipe is more dilute, and the small loss of bees (or reduction in vigour) is more than made up by the effective Varroa control.
G.
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Images from the SEM did not show any change in
the external surface of treated bees although a white
deposit could be seen on test bees (Fig. 2). Gross lesions
were not observed on external structures or internal
organs. There was higher fragility while dissecting the
digestive tube and the Malpighian tubes in bees killed
after 48 and 72 h but not in corresponding control bees.
Pathological changes were present in epithelial layers
covering the ventriculus, the rectum and the Malpighian
tubules of all bees killed 24 h post dosing. Lesions were
more severe in bees killed at 48 h and even more severe
in the 72 h group. No alterations were seen in control
bees. Some variations were seen in control and test bee
hypopharyngeal glands. This was probably due to tech-
nical problems and therefore is not included in this study.
At the microscopic level, the epithelia of the ven-
triculus appeared to have a degree of hydropic degene-
ration including the presence of multiple vacuoles
around the nucleus in bees killed at 24 h after treatment
(Fig. 3B). Cytoplasmatic structures were altered after
48 h especially in the regeneration crypts (Fig. 3C). At
72 h, the mucous layer was severely injured. There were
wide denuded areas or zones a few cells with a picnotic
nucleus could be seen (Fig. 3D).
Cells from control bees had an eosinophilic granular
cytoplasm without vacuoles (Fig. 4A). Lesions in the
rectum at 24 h after treatment were similar to those in
the ventriculus but were to a lesser degree (Fig. 4B).
Mucous cells had slight hydropic degeneration with
cytoplasmatic tumefaction due to small vacuoles. At 48
h more severe cellular damage was confirmed by the
dilution of organelles. Part of them could be seen close
to the nucleus, which usually appeared in the basal area
of the cell (Fig. 4C). Seventy two hours after OA adminis-
tration cells had a polygonal morphology, the cytoplasm
was optically empty and the nucleus picnotic displaced
to the basal area («ballooning cell»), which showed the
maximum level of alteration (Fig. 4D).
Malpighian tubules were also affected from 24 h.
Initially a small supranuclear vacuole could be seen
while the cytoplasm was similar to that of control bees
(Figs. 5A and 5B). At 48 h, the cytoplasm appeared
tumefacted (Fig. 5C), but it was not until 72 h that several
small vacuoles in the cytoplasm and picnotic nuclei
were present (Fig. 5D).
http://www.beekeepingforum.co.uk/showthread.php?t=14938&page=3