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| a good year for holly blues | the interdependence of organisms in an ecosystem KS3 Science | the numbers of Holly Blue butterflies in any season depends upon many factors, some abiotic, others biotic the size of the Holly Blue population is regulated by the parasitic wasp | being and becoming summer stories Harmony principles: Adaptation Interdependence |
The black line shows a 4 year cycle (19972001, 2001-2005) and later 6 year cycle in numbers (2005-2011) , going from very high (boom) to very low (bust).
The graph shows that when the butterfly numbers are high, the numbers of parasites are low. Likewise, when the parasite numbers are high, the numbers of butterflies are low.
This controls the number of butterflies (and parasites) found. The effects of food and weather will affect the total numbers found in any one season, but the regular cycles are caused by the interactions between the butterflies and the parasite.
The data in the chart are modified from Smith (2018) Time series analysis of Holly bull records in Yorkshire – uncovering a parasitoid. The Naturalist. 143 (1099), 166-173.
Technical details:
The counts of butterflies took place in Yorkshire between 1996-2004 and 2004-2017. This suggests the presence of a parasite living alongside the holly blues. The paper announces that the parasite (L. mycthemerus). We have no idea if this parasite is present in Trendlewood park, but it is quite widespread, so it is possible. The red lines of the graph are estimates of the possible numbers of the parasite assuming it is controlling the numbers of holly blues. These are based on similar examples of parasite-host interactions in other ecosystems.
Further notes on holly blues:
by Peter Bright, an experienced field naturalist.
Holly blues are like other common blues but the silvery milky undersides of the blue wings distinguishes them easily as does, usually, the way Holly Blues fly up the shrubs and woodland edges where there is holly and ivy, its caterpillar food plants.
Common blues fly out in the open over grassland low to the ground where its caterpillar food plant, bird’s foot trefoil grows. The underwing spotting is not silvery, more brown with white circled black spots.
This year many holly blues successfully over-wintered as pupae in the ground or, perhaps more interestingly, in the nests of black ants into which they have been taken by the ants. These ants tend, and may well protect, holly blue caterpillars, collecting a sugary secretion from glands on their body.
Many of the ‘Blue butterflies’ have such relationships with ants. This spring generation was flying in April and May looking for mates and laying their eggs in or on the flower buds of female holly bushes but may use other spring flowering shrubs like dogwood and spindle. The adults of this spring generation die after laying eggs, perhaps becoming food for spiders of birds.
This means that the the holly blues, in June and beginning of July, are all developing caterpillars eating the flowers and fruits of their host plants and tended by black ants, with no holly blue butterflies to be seen.
The caterpillars will pupate in early July and, with luck will have been sheltered in an ant’s nest. The summer generation of butterflies will emerge in late July and August, and will seek out the flowers of summer flowering woody plants typically ivy but sometimes buckthorns (Brimstone butterfly foodplants), brambles or snowberries.
The numbers of of holly blues that emerge in each of these generations depends a variety of factors:
predation by blue tits and great tits feeding their young in spring,
the success or otherwise of protection by black ants
how many of its ichneumon parasitoid (Listrodomus nycthemerus) successfully find the youngest caterpillars in which they lay a single egg. This egg hatches and feeds inside the holly blue caterpillar eating the non essential parts of the caterpillar which goes on feeding as usual without being killed. In effect the caterpillar is eating to feed the ichneumon grub!
The caterpillar will pupate in due course but the contents is actually the pupa of the ichneumon wasp. This will hatch and a new generation of the parasitoid wasp will be out and about searching for the next generation of holly blue caterpillars.
As the numbers of the wasp builds up the numbers of surviving holly blues falls giving the four to six year cycles of abundance of the holly blue butterflies.
This year there are lots of holly blue butterflies flying so that they will produce a large number of young caterpillars which will be easier to find for the parasitoid wasp so their numbers will rise in the coming year and the butterfly numbers will fall. Perhaps an abundant year for the Holly blue butterfly is actually a good year for its parasitoid wasp?
(Ichneumon lays an egg in the youngest caterpillars and pupates inside the holly blue pupa from which it will emerge. Presumably the wasp overwinters inside this pupa that is produced by the second generation of butterflies.)
Most, if not all insects and probably spiders, have a parasitoid wasp, with this sort of life cycle. Large White caterpillars on domestic cabbage plants are significantly parasitised by a wasp – Apantales, but in this case a caterpillar may host up to 20 wasp grups which kill the caerpillare when they emerge and make their small cocoons around the caterpillar’s corpse.
See also:
Revels, R. (1994) The rise and fall of the Holly Blue. British Wildlife 4: 236-8.
Revels, R. (2006) More on the rise and fall of the Holly Blue. British Wildlife 17: 419-24.