What happened to the holly leaf miner?
Holly trees are common in both the town and countryside. In this investigation, students collect evidence of the food chain of the holly leaf-miner, studying energy flows through population numbers.
Animals are usually so transitory in feeding that it is impossible to assess the ecological impact of each species on its food source. However, the holly leaf-miner food chain provides an opportunity to study living organisms in the field and gain experience of ecological interactions in greater detail.
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Learning outcomes
- investigate energy flows through living organisms and population numbers
- consider different sampling methods
- evaluate the collection technique used to collect the primary data
Background information for teachers
(a) Life cycle of the holly leaf-miner
The larvae of the Agromyzid leaf mining fly (Phytomyza ilicis) burrow in the mesophyll of holly leaves (Ilex aquifolium) and produce characteristic white patches known as mines beneath the surface. The larvae (miners) are parasitized by a number of parasitic wasps and fed on by birds, such as blue tits (Parus caeruleus).
The tiny adult leaf-miner flies lay their eggs at the base of the midrib on the underside of the leaf in June. On hatching, the larvae enter the midrib, slowly eating their way forward until the months of September through to November when they enter the mesophyll. Further feeding produces the meandering mines which reach maximum size in March. Larvae pupate within the cuticle of the last larval stage from March to May. The cuticle is retained as a protective covering (puparium) over the pupa. Before it pupates the larva prepares a thin triangular area on the leaf cuticle against which will fit the hinged ‘emergence plate’ of the puparium. The adult fly escapes by pressing against the ‘emergence plate’ in late May – June, leaving a large emergence hole (> 1mm). The cycle continues as the adult fly lays her eggs.
(b) The food chain of the holly tree
The basic food material of the food chain is synthesized by the holly leaf (producer), which is consumed by the larvae of the holly leaf-miner (primary consumer), which itself in turn is predated by the blue tit (secondary consumer). A v-shaped tear, with no remains of a miner inside the mine, indicates that the leaf has been opened by a beak.
Competing with the blue tits are a number of parasites. The most important of these is the parasitic wasp Chrysocharis gemma, which attacks leaf-miner larvae. The adult wasp has a long ovipositor which she uses to insert a single egg through the leaf cuticle into the body cavity of the larva. Attacked larvae appear flaccid and a pale, dirty yellow in colour compared with the turgid, bright, shiny, whitish-lemon appearance of healthy larvae. The parasite larva feeds within the fly larva, and eventually kills it. It then forms a shiny jet-black pupa which lies free inside the mine.
Evidence that the adult parasite has emerged is a very small, neat, round hole which may be found on either side of the mined leaf. Also leaving evidence of a small emergence hole is another parasitic wasp Sphegigaster flavicornis which attacks the pupa of the leaf-miner. The adult parasite bores through the leaf cuticle and the tough skin of the puparium with its ovipositor. The parasite larva bores into the leaf-miner’s pupa, feeds and then pupates. The pupa of the parasite is black with a bluish tinge. Another less common parasite is Pleurotropis amyntas, which is unique as it may either live as a primary parasite in the pupa of the leaf miner or feed as a secondary parasite on the miner’s pupal parasite S. flavicornis. Thus P.amyntas forms the fourth link in a parasite food chain.
In practice it is difficult to distinguish easily between the different parasites that feed on the miner and to find out whether P.amyntas is working at the fourth trophic level. It does however provide opportunities to discuss the contrasting feeding strategies of organisms (herbivores, carnivores, parasites and hyper-parasites).
Before fieldwork
Locating a holly tree infested with mines prior to the lesson is crucial. First, this will assist you to collect worthwhile data (i.e. a tree with mines), and second it will prevent ‘over plucking’ of a tree. Introducing the investigation to the pupils
Key questions
- What is the holly leaf miner dependent upon for its survival to mature into a fly?
- Do all holly leaf miners become flies? (Discussion of energy flow and what might happen to the pupa and the fly).
This discussion should lead into the practical as evidence is now necessary to confirm possible routes the leaf miners life may take and the numbers involved at each stage.
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