The holly and the Ivy.
Wildlife to search out in the winter months.
Flower arrangers often seek out the evergreen leaves of holly and ivy for their mid-winter displays. Yet, I suggest they look especially closely at the holly leaves they select … for many of them are less than perfect. The leaves can show blemishes, swirly white marks and even ugly tears to their surface … and every imperfection is of real interest to the winter wildlife enthusiast.
Organisms need a supply of energy to maintain their lives. The green plants capture that energy in the process of photosynthesis. The fungi often obtain theirs through the release of energy in the decay process (although others can be parasitic or disease causing). Animals glean their food supplies by eating or parasitizing other organisms – plants, animals, microorganisms and fungi. The snag with being an animal is that you could become food for another organism. Being wary, having camouflage or sneaking out when no one is watching are used. Others, such as leaf-miners, hide away inside leaves, stems or leaf stalks (petioles). Boring beetle larvae eat into tree trunks and so try to avoid being food themselves. Probably some organisms hide away in roots. These are all useful niches … except their prey species do their very best to find and eat them anyway. We’ve all spotted greater spotted-woodpeckers drilling a semi-rotten tree branch for grubs or a blackbird quartering the lawn and consuming a tasty worm that had hidden too poorly.
Life cycle of the holly leaf-miner.
The larvae of the holly leaf-miner fly (Phytomyza ilicis) burrow into leaves after hatching from eggs laid at the underside base of the leaf stalk in June. Initially they eat within the petiole, but by September they move into the leaf itself, eating the green photosynthesising cells between the upper and lower surfaces (cuticles). The chlorenchyma cells of the leaf’s mesophyll layer. By leaving the outside surfaces uneaten the minute white larvae are less likely to be spotted or to dry out in hot weather.
This feeding produces meandering mines which lack green colouration. As the larvae increase in size so their channels become bigger. By late spring the immature flies are at their maximum size, they pupate within the leaf (previously preparing a minute (1mm approx.), hinged triangular area in the upper leaf surface). The new adult flies hatch from the pupa, push on their ‘emergence plate’ and fly off to find a friend of the opposite sex in May or June.
Clearly, the holly bush is capturing the sunlight energy – it is called a ‘producer’. The larvae eat the holly cells, obtaining their nutrition – a primary consumer. Anything that can find and eat the holly leaf-miner will be a carnivore or secondary consumer. And, there are a range of organisms that do consume larval holly leaf-miners including blue-tits, parasitic wasps and bacteria. (Yes, even larvae get ‘ill’!)
Blue-tits rip open the upper surface of the holly leaf to gain access to the holly leaf-minors – leaving a ‘V’ shaped tear.
Female parasitic wasps use their long ovipositors to lay eggs within the larvae. These hatch and turn the turgid, bright, shiny, whitish-lemon of a health larvae into non-moving dirty yellow organism that has ‘taken over’ by the wasp’s larva. Eventually, the holly leaf-minor is killed, the wasp larva forms a shiny jet-black pupa and this eventually emerges through a pinprick-sized circular hole.
In reality things are even more complicated than this, for other parasitic wasps can lay their eggs in the larvae or pupae of the first wasp! But, let’s ignore that!
Search out some local holly bushes. Collect as many leaves as possible and observe them to find out the percentage that have been colonised by leaf-miners. Some will have been consumed by blue-tits and this will be clear as the tunnel will end, and the leaf is ripped open. Others will have been parasitized and there will be a minute round hole in the upper or lower leaf surface. Bacterial death will show as the mine ends and there is no small triangular exit hole. You may wish to open some mines to search out living larvae.
Often found near the holly bushes will be brambles (blackberry plants). These to have their own miners.
Other locally-found plants that have their own miners include: ferns, sloe (blackthorn), beech, dogwood, hawthorn, apple, hazel, ash, birch … and many more.
The Hart’s Tongue Fern is readily found along the length of the Test Valley preferring damp and shady sites such as banks, hedgerows and woodland, often being found growing out of the brickwork of old bridges along the now defunct ‘Sprat and Winkle’ railway line that forms the base of the Test Way south of Andover. Unlike most ferns it is not formed of feathery fingers of fronds but of a rosette of long narrow leaves that reach out, 30-75 centimetres, from the plant’s base.
It, too, plays host to a fly with a small leaf-mining larva, Chromatomyia scolopendri. The adults lay their eggs in May and the larvae, upon hatching, proceed to tunnel their way through leaves leaving a highly visible pale track. The tunnel shows where the egg hatched and the larva began to eat. The track will widen as the grub grows larger. If the larva survives, a pupa will form and, eventually, an adult emerges the following April or May.