The Effects of Macrocyclic Lactones on Dung Insects – Myths and Realities

 In 1998, a detailed review of the effects of macrocyclic lactones (ML’s) on dung beetles and other dung-eating Insects has been  carried out by the then National Registration Authority for Agricultural and Veterinary Chemicals (the NRA). The NRA is now called  the Australian Pesticides and Veterinary Medicines Authority (the APVMA). This special review1 of most of the information available  at the time is available on line (http://www.apvma.gov.au/chemrev/maclac.pdf) or can be obtained by contacting the APVMA or your  ANCARE representative. The document is 94 pages long and rests on the examination of no less than 96 bibliographic references  and studies. The present ANCARE document is essentially a summary of the main findings of the review.

 From this review, the NRA (APVMA) has concluded that there is no clear evidence to indicate that any of the macrocyclic lactone  products have a long term detrimental effect on dung beetle populations or dung disappearance rates in the field in Australian  conditions, although different products may have differing short term effects, and differing toxicities to dung beetle larvae.

 The principal focus of this review is the dung-eating insect fauna of cattle dung in Australia, which by comparison with Europe  contains an impoverished fauna and is used primarily by several species of abundant flies and 20 or so purposely introduced dung  beetles together with a few native dung beetle species which have adapted successfully to utilising the dung of introduced livestock.
 Commencing in 1968, the CSIRO Dung Beetle Program imported 52 species of dung  beetles from southern Africa and Europe of which a total of 42 species was  released and 22 of these had established breeding populations in the field by 1991  (See Tables on page 2).
 A dung beetle (Onthophagus gazella)2

Numbers of introduced and established dung beetle species
in mainland Australia and Tasmania

Species of introduced dung beetles established
in the summer and winter rainfall zones of Australia

* abundant in particular rainfall zone.
Wherever the exotic species have become established in Australia there has been a significant increase in cattle dung dispersal through burial and shredding. Dung beetles only have a minor role in decomposition of sheep dung under Australian conditions although they undoubtedly use it as a feed resource.

Moxidectin residues in cattle dung are considered non-toxic to egg-laying adults and developing larvae.
No data are available on the effects of moxidectin residues in cattle dung on mortality of newly emerged adult dung beetles or their reproductive development. Slower rates of excretion in faeces presumably contribute to the lack of toxic effects on larval development but there has been no examination of possible sublethal effects on beetle populations arising from the longer-term excretion pattern of moxidectin in cattle dung.
Moxidectin residues excreted in sheep dung during the first two days after oral treatment inhibit larval development in the one species of dung beetle tested. This is consistent with the high residue levels in faeces at that time and suggests that other species may be similarly affected.

 Moxidectin residues in sheep dung 2 days after oral drenching caused a 46% reduction in  emergence of Aphodius constans larvae. The genus Aphodius is prevalent in Australia.
 This beetle genus was introduced accidentally c1850 from South America and is established  on Dividing Range from Victoria to northern New South Wales8.
 An Aphodius spp dung beetle5
 These observations, together with the protracted excretion profile for moxidectin residues in  sheep dung, indicate the need to evaluate possible sub-lethal effects on larvae and  immature  adults of the more important Australian species to assess the risk exposure to  dung beetle  populations of moxidectin drenching of sheep.

 A 1997 observation, not mentioned in the 1998 NRA review, and only recently  reported by two CSIRO scientists (Steel & Wardhaugh, 2002)6, also showed that  moxidectin administered subcutaneously to cattle reduced the fecundity of the dung  beetle species Euoniticellus fulvus feeding on the dung of treated cattle.
 An Euoniticellus spp dung beetle9

 Macrocyclic lactones are almost entirely excreted in faeces but the rate differs  considerably between the various commercial compounds […]. Following  subcutaneous or pour-on dosing, ivermectin levels in cattle faeces peak within 2-6  days and negligible or very low levels persist beyond 14 days. Doramectin is excreted at a similar rate to ivermectin. Moxidectin residues are excreted more slowly and persist in cattle faeces for more than 28 days.
In cattle by 28 days only trace amounts of ivermectin metabolites continue to be excreted in bile [and then in faeces] whereas 40 ppb of moxidectin residues are present at this time. Similar patterns are evident in sheep: removal of ivermectin residues from sheep tissues is substantially faster than for moxidectin.
Excretion of moxidectin residues in faeces continues for more than 28 days at which time the cumulative excretion totals 58% of the dose, whereas cumulative excretion of ivermectin residues is 62% of the dose at 7 days.
Pour-on administration of ivermectin at 500 mg/kg resulted in higher initial concentrations in faeces (9 ppm dry matter) but by 5 days these were similar (2.8 ppm dry matter) to those following subcutaneous treatment. By contrast, pour-on treatment of cattle with moxidectin results in substantially lower faecal residue concentrations than after subcutaneous dosing and peak levels were not attained until 11 days.

Ivermectin residues in cattle faeces following subcutaneous treatment are highly toxic to the development of the larvae of the horn fly, Haematobia irritans, with mortalities of 100% initially and impaired development being evident in faeces collected for up to 8 weeks. Faeces of moxidectin-treated cattle exhibited lower toxicity against this species for a considerably shorter period of 3 days, with maximum mortality of approximately 75% on day 1.
Against the closely related buffalo fly, Haematobia irritans exigua, whereas abamectin levels of 4 ppb or greater caused 100% mortality there were no toxic effects of moxidectin until levels exceeded 64 ppb.
When given subcutaneously to cattle, ivermectin reduces larval development of the fly Musca spp. in dung for at least 2 weeks and up to 7 weeks after treatment. Of particular interest in the Australian context is the impact on the bushfly M. vetustissima which exhibits reduced larval development for at least 4-5 weeks when eggs are laid in dung of cattle treated with ivermectin or abamectin. By contrast there was no effect on development of M. domestica [the house fly] and M. vetustissima [the bushfly] larvae on dung collected as early as 3 days after subcutaneous treatment of cattle with moxidectin.

GENESIS TM POUR-ON contains 10 g/L ivermectin
These observations are consistent with the fact that ivermectin has a >99% efficacy on bots (Gasterophilus spp) in horses, whereas moxidectin efficacy is variable7.