June 13, 2011
September 26, 2010
Does anyone have any ideas?
Read these from article (but all are dangerous alliens!): "Which invaders were the biggest enemies of the dragonfly, and which had actually become potential allies? To sort out friend from foe, Moe Miyke and Tadashi Miyashita of the University of Tokyo turned to a technique that uses stable chemical isotopes to trace who was eating who. The pair focused on the pond’s four major alien predators: the snakehead fish (Channus argus); a bullfrog (Rana catesbeiana); the red-eared slider turtle (Trachemys scripta elegans); and the red swamp crayfish (Procambarus clarki).
... As suspected, it suggested the crayfish were eating dragonfly larvae."
> Source: Miyake, M., & Miyashita, T. (2011). Identification of alien predators that should not be removed for controlling invasive crayfish threatening endangered odonates. Aquatic Conservation: Marine and Freshwater Ecosystems DOI: 10.1002/aqc.1178
June 13, 2011
Bojan, the only even half-decent idea we've come up with is to add a couple of adult Trachemys to each pool! They're living feral in the park and the staff routinely trap the adults and remove them so we could easily obtain some, and there's no way they could escape or reproduce provided we only introduce one or two of the same sex.
Colin, yes, the fish are in the swimming pools and we have a stable population of 3-5000 in each, which means we've nowhere else to put them for more than a few hours if we drain the water out. 2 year life cycle - yes I think so.
It just doesn't seem quite right to think about using an invasive species for this hence the post...
okay - is predation really such a bad thing then? Too many times I have seen batches of fish raised in captivity where the pretty much the entire brood survives including all the runts and misfits! They are not all meant to survive so the ones you eventually release are just the ones worthy of survival????
June 13, 2011
July 11, 2012
This topic is pretty old and measures have probably been taken, or not, but I'm still interested. I spent my time as a graduate studying dragonflies, particularly their larvae. Most species in Northern Europe have a one year life cycle. They survive the winter period as larvae of eggs. A few may not grow as fast and spend another year under water. The late Philip Corbet wrote one of his earlier articles on the life history of Anax imperator and found that some individuals did indeed spend more than a year as larvae. They were bigger than their siblings. Only a very few species normally spend more than a year as larvae. Good examples are all species of the genus Cordulegaster (at least in the West-Palearctic region) and a few members of the Gomphidae. They inhabit nutrient poor, cold water, which is usually fast flowing.
Species that have a wider distribution range, like many libellulids and many aeshnids, as well as several damselflies of the family Coenagrionidae have more than a single generation each year.
Based on the above, draining ponds, particularly is Spain, won't really help.
Most species won't feed on fish unless they really have to. All Gomphidae, Cordulegastridae and Libellulinae are bottom dwellers and they feed on crustaceans, isopods, annelids and insects. Many others are too small to eat anything larger than the youngest fry. Their mandibles are too fragile and at the same time too valuable to risk. Only the larger aeshnids, when they are really plentiful, may have a real impact on your stock.
In fact, most fish will happily eat as many dragonflies as they can. They are nutritional (pretty fatty) and easy to catch.
Do you know which species you have flying around your pool? I'd be happy to identify them for you if you have pictures of adults or later instar larvae. Exuviae (the skins left on branches, reeds and the likes after de dragonfly emerges) are more than welcome too 😉
If your dragonfly problem consists of large quantities of aeshnid larvae, you could discourage breeding in your ponds by clearing any helophytes.
July 11, 2012
It's a libellulid, although from the picture I can't decisively tell you if its of the subfamily Libellulinae or Sympetrinae. It could be one of the smaller Orthetrum species or a species of Sympetrum (or one closely related to that genus). The first are bottom dwellers, the second are primarily living amongst vegetation.
I'm not afraid to guess and I'd say that it's a Sympetrum. You can verify that by telling me something about the adults you see around the ponds. Are they yellow to reddish, perching on branches and laying eggs in 'tandem' (the male holding the back of the head of the female while flying close to the water's surface)? If not, they would blue when mature, resting flat on the ground and the females will generally lay their eggs solo.
The Orthetrum would be rather harmless, the Sympetrum will encounter the occasional fry. They are not the hunting and killing machines that most aeshnids are however. I kept larvae at home in small tanks and I sometimes fed them the fry of Rutilus rutilus. The aeshnids would readily take them, but the libellulids had to be starving before attempting to catch something that fast.
If I am correct and these are indeed Sympetrum, draining the ponds would be useless. Many species of that genus can have multiple generations per annum, and they are adapted to living in ephemeral habitats. The same applies to several other species within the subfamily.
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