Hands off, she's mine - mate-guarding in hoverflies

Mate-guarding is well known in some groups of invertebrates such as the Odonata (dragonflies and damselflies), and has a number of functions:

• Pre-copulatory guarding - males prevent rivals from mating, and sequester the female early in the day until the temperature has risen sufficiently to permit copulation and oviposition.
• Post-copulatory guarding - after mating, rival males are still excluded as they can displace existing sperm and replace it with their own. This may include guarding during oviposition (egg-laying) for similar reasons e.g. females are highly receptive after copulation and thus males are guarding against sperm competition, and ensuring theirs retains precedence during sperm storage.

The precise behaviour varies between species and is influenced by factors such as sex ratio, but whatever its precise form, mate-guarding has developed to prevent mating of females by rival males. For more about this behaviour in dragonflies, see for example Corbet & Brooks (2008). In other insect groups it is not always so well documented. Walker (2010) describes, for the first time, mate-guarding in the parasitic Hippoboscidae ('louse-flies' or 'keds'), while Dyte, (1988), Adler & Adler (1991) and McLean (1991) do the same for a few species of flies in the families Dolichopodidae, Tipulidae (crane-flies) and Ephydridae respectively. However, this behaviour is well documented in only a tiny proprtion of the many known fly species, so I was interested to see a pair of the honeybee-mimic hoverfly Eristalis interruptus (at least I think it's this species going by leg colour, overall form, and the pattern of the 2nd tergite in the male - the genus is certainly correct) engaging in mate-guarding behaviour in our back garden.

Mate-guarding in a pair of Eristalis interruptus hoverflies.

In this photo, the female is lower - on the ground and more-or-less still - while the male hovers (note the blurred wings) about 3cm above her, occasionally dropping down and briefly tapping her. I could find no references to this behaviour, though female hoverflies in general are known to search for suitable oviposition sites on the ground (Rotheray & Gilbert 2011) - this is possible here as it is a patch of damp. bare soil near our garden pond. However, such contact mate-guarding has been noted in the dragonfly Orthetrum coerulescens (Keeled Skimmer) and been suggested as a mechanism to induce oviposition (Miller & Miller 1989), presumably as this either renders the female receptive once more, or at least removes the male's need to guard further.

So, a rarely (if ever) reported behaviour and another example of what can be seen in even a moderate-sized urban garden if it is managed in a wildlife-friendly way. As ever, if you have seen this behaviour in hoverflies, do let me know!

References

Adler, P.H. & Adler, C.R.L. (1991). Mating behavior and the evolutionary significance of mate guarding in three species of crane flies (Diptera: Tipulidae). Journal of Insect Behavior 4(5): 619-632.

Corbet, P. & Brooks, S. (2008). Dragonflies. HarperCollins, London. [see especially pp. 226-7, 250-6]
Rotheray, G.E. & Gilbert, F. (2011). The Natural History of Hoverflies. Forrest, Tresaith.
Dyte, C.E. (1988). Mate guarding and sex ratio in Hydrophorus oceanus (Macquart) (Diptera: Dolichopodidae). The Entomologist 107: 122-126.
McLean, I. (1991). Mate-guarding in Ephydra riparia Fallen (Diptera: Ephydridae). British Journal of Entomology and Natural History 4(2): 65.
Miller, P.L. & Miller, A.K. (1989). Post-copulatiry "resting" in Orthetrum coerulescens (Fabricius) and some other Libellulidae: time for "sperm handling"? Odonatologica 10: 201-218.
Walker, M.D. (2010). Previously unreported mate guarding and 'clustering' by a nest ectoparasite (Diptera, Hippoboscidae). Dipterists Digest (2nd series) 17(2): 115-116.