Why do ants guard holly blues?
By Chelsie Vandaveer
December 25, 2002
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In Europe, one of the first butterflies to emerge in the spring is the tiny holly blue (Celastrina argiolus Linnaeus). These butterflies are reported to be the same species called the azure blues in North America. At this time, the 'blues' appear to consist of a number of similar-looking sibling species or subspecies. The blues of North America are undergoing a revision of taxonomy.
Holly blue butterflies are a woodland species. The butterfly's underwing is a pale silvery-gray. When resting, the butterfly holds the wings upright so the underwings are exposed. The color matches the reflection of light off the shiny leaves of two woodland plants, holly (Ilex aquifolium Linnaeus) and ivy (Hedera helix Linnaeus).
Holly blue butterflies are considered common, but their numbers fluctuate wildly over a period of years. It is thought when the holly blue numbers increase, the number of parasitic ichneumon also increases. These wasp relatives eventually almost wipe-out their food source and, of course, themselves.
According to Digby Wood, the light green slug-like caterpillars are defenseless. The caterpillars, vulnerable to ichneumon, enlist the aid of ants. The caterpillars attract and reward the ants for guardianship. The caterpillars secrete sweet 'honeydew' relished by ants. The ants guard their food source against predation. But the ants do not always succeed when numbers of ichneumon are high. (I Got Those Holly Blues, Digby Wood.)
Holly blues are bivoltine, having two generations per year. Holly blues overwinter in the pupal stage, emerging with the first warm days. The overwintering holly blues fed as caterpillars on ivy the previous summer. These adults will congregate around holly, preferentially female hollies, for the spring caterpillars eat both holly flowers and berries.
Guy Padfield has two great photographs of the holly blue butterfly on his European butterfly page. To view his photographs, click on the link:
http://www.guypadfield.com/hollyblue.html
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Suggested Reading:
dahoon (Ilex cassine Linnaeus) Plant of the Week - November 25, 2002
What's so funny about holly? Weird Plants - December 26, 2002
What could disappear from the earth? Renfield's Garden - January 8, 2003
Are some evergreens insectivorous by proxy? Renfield's Garden - December 26, 2001
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