Thursday, 2 February 2017

"Moths may hold the key to cutting in lab tests on mammals"

An article in The Herald [1] was headlined "Moths may hold the key to cutting in lab tests on mammals". It was one of many articles in the past weeks about the use of wax moth larvae (Galleria mellonella) in toxicity and antibiotic testing, reducing dependence on laboratory mammals such as mice and rats. This is not a new approach and the use of these moth larvae in laboratory testing has been reviewed by Desbois and Coote [2]. The lack of a standardised source of larvae has been a hindrance in testing as, without a consistent supply of known strains, there is likely to be variability in test results caused by variations in the stock used. This problem has been overcome by Biosystems Technology [3], a company that provides wax moth larvae as a standardised product under the name TruLarv.

It is understandable that the articles in the media focussed on the reduction of the need for testing on mammals, but none of those I read discussed the biology of the moths, their association with honey bees, and the possible evolution of the relationship. For bee-keepers, wax moths (above, upper) can be a problem as their larvae attack honeycombs (above, lower from [4]), while being protected by the silk that they produce. In addition to Galleria mellonella, the most important wax moth for apiarists, another species Achroia grisella is also found and the two have slightly different life cycles in temperate countries: G. mellonella overwintering as mature larvae or pupae in silk cocoons and A. grisella in the egg or larval stages [5].

The association of wax moths with hives of bees is of long standing and was known to Aristotle [6], bee-keeping having been a well-established practice for millennia. Larvae, the only feeding stage of the insect, eat wax and any material that is in close contact with it. They do not destroy nests of bees, but cause a decrease in the efficiency of raising brood and they reduce the output of honey from hives - the silk and faecal material produced by larvae also make an unsightly mess in the honeycomb. Having hives arranged in close proximity, as may be the preference of a bee-keeper, results in an increased likelihood of the spread of wax moths once they have entered one of the hives, a problem familiar to those who investigate pests in monocultures.

The natural history of wax moths is as interesting, if not more interesting, than the use of the larvae as test organisms. It is also fascinating to speculate on how the original association between wax moths and bees came about. Ancestrally, the moths colonised the nest of wild bees, but it is not known when this occurred and it presumably resulted from a sudden shift from feeding on vegetation or animal by-products, the typical diets of moth larvae. The biting mouthparts of larvae were certainly pre-adapted for feeding on honeycombs and the use of silk, common in larval moths, gave some protection from possible attack by the bees. But how did they first invade bee nests and what steps then evolved to allow the more specific association between bee moths and honeybees? This can only be speculated upon and it is interesting that it occurred in two closely-related taxa, G. mellonella and A. grisella. Perhaps speciation of the moths occurred after the evolution of the wax-feeding habit - or is this a case of adaptations evolving in parallel, way before humans came on the scene?

[2] A.P.Desbois and P.J.Coote (2012) Utility of Greater Wax Moth larva (Galleria mellonella) for evaluating the toxicity and efficacy of new antibacterial agents. Advances in Applied Microbiology 78:25-53.

[4] A.J.Cook (1880) Manual of the Apiary. Chicago, Thomas G. Newman.

[6] Warren Whitcomb Jr. (1936) The Wax Moth and its control. United States Department of Agriculture Circular no. 386.

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