Every leaf is a mite metropolis

Insects begin to attack tree leaves almost as soon as they emerge from buds in spring. Some eat foliage but others turn leaves into secure homes.......... 

......... like these little red galls on sycamore leaves produced by a microscopic mite called Eriophyes macrorhynchus aceribus.

These little eruptions on the surface of an alder leaf are caused by another eriophyid mite, Eriophyes laevis inangularis.

Eriophyid mites are not insects but are related to spiders. 

This is the underside of the leaf, with the little yellow, sausage-shaped mites crawling around the entrances to the chambers, which are lined with nutritive cells that provide sustenance for the mites.

Here they are at higher magnification .........

............ and at still higher magnification, when the elongated body with four legs at the head end is visible in the mite in the top, left-hand corner. Each chamber is home to a brood of mites and a tree with a severe infestation could be covered with hundreds of thousands of them. 

These are three of the mites, each being about one fifth of a millimetre long, with only four legs.

The outer cuticle of the animal has a distinct pattern that differs between species, although the easiest way to identify species is via the symptoms that they cause on the host plant.

Here is the head, legs and cuticle patterning at higher magnification.

Hardheads

In summer tiny picture-winged flies, similar to this one although not the same species, lay their eggs amongst the florets of knapweed Centaurea nigra, like the one in the picture above. The usual species that parasitises knapweed is Urophora jaceana - click here for a picture.

After the larvae hatch they crawl down into the base of the inflorescence and produce a hard woody gall, where they feed on the seeds and are well protected through the winter.

Parasitised seed heads tend to have thicker stems but the easy way to detect the presence of the developing larvae in winter is just to squeeze the knapweed seed heads between finger and thumb. If they are galled you can feel the hard lump inside.

Opening up the gall reveals the developing larvae down at the base. You can see how they've chewed their way down through the gall, growing fatter as they progress and finally resting in a chamber where they'll pupate and hatch as picture-winged flies in spring.

Knapweed seed heads are carried on stiff stems that often protrude through the snow in winter. Then they tend to be shredded by hungry  birds - probably blue tits and great tits. It's hard to be sure whether the birds are after some of the smaller insect larvae that sometimes colonise these seed heads or whether they can winkle out the picture-winged fly larvae from inside their woody gall - I suspect that they can.

For a closer look at the larvae, click here.

Oak apple and Currant galls

This oak, just coming into leaf beside the river Tyne at Wylam yesterday, was carrying both oak apple galls and currant galls.

The big, spongy oak apple is caused by the tiny wasp Biorhiza pallida, which lays its eggs as the base of an oak bud and causes it to swell into this large gall that's home to up to 30 of the gall wasp's grubs. After it matures in July the hatching adults chew their way out, leaving distinctive exit holes. 



Oak apples contain either all male or all female wasps but when they emerge they mate and the fertilised females then lay their eggs on the oak's rootlets, when they form a much smaller woody gall. Only females emerge from these and in late winter climb the oak's trunk and lay eggs to form the next generation oak apples.

The small red, spherical galls dangling under the oak apple are currant galls formed in the catkin of the oak, when the gall wasp Neuroterus quercus-baccarum lays its eggs in the male flowers. Each currant gall contains a single grub  and either males or females hatch in June. After mating the female lays her eggs on the underside of the oak leaves, where they form the familiar spangle galls that cover the lower surface of the leaf in autumn.



Spangle galls drop off shortly before the leaves fall in autumn, then in spring females emerge and lay eggs in the oak's catkins, producing currant galls again.

For more pictures of plant galls click here

Nettle and meadowsweet galls

These strange swellings on leaves and leaf petioles of stinging nettle are the work of a gall midge called Dasyneura urticae, whose larvae develop inside the pouches. The midge lays its eggs on the leaf upper surface and when the larvae hatch they irritate the plant tissue on either side of the the groove in the leaf ribs, which grows over them forming a protective pouch lined with nutritive tissue. When the larvae are fully developed they escape through a slit in the gall wall and fall to the ground, pupating in the soil. 

You can find some excellent photos of the midge and its larvae at this web site

These are gall midge galls of another species of the same genus, Dasyneura ulmaria, on the leaf of meadowsweet Filipendula ulmaria. The galls and life cycle of the insect are broadly similar but in this case the midge lays its eggs on the underside of the leaf and there is only one generation each year.

All photos taken at Romaldkirk in Teesdale on 25th. September.

More photos of plant galls here.

Plant galls

This is probably the best time of year for anyone interested in plant galls - those strange growths on plants caused by insects, mites and sometimes fungi. These are a few that I've seen locally over the last couple of weeks.

Leaf petiole of common lime Tilia x europaea with a gall chamber containing larvae of the gall midge Continaria tiliarum.

Ash flowers galled by the ash gall mite Eriophyes fraxinivorus.

An old oak apple gall caused by the gall wasp Biorhiza pallida, with exit holes where the adults have hatched.

Silk button galls on the underside of oak leaves, containing larvae of the gall wasp Neuroterus numismalis

Spangle galls on the underside of an oak leaf, containing larvae of the gall wasp Neuroterus quercus-baccarum.

Galls on leaf surface of sycamore caused by the eriophyid mite Eriophyes macrorhynchus aceribus. Click here for pictures of eriophyid mites.

Bean gall in the leaf blade of willow, containing the larva of the sawfly Pontania proxima

Robin's pin cushion or bedeguar gall on wild rose, caused by the gall wasp Diplolepis rosae. 

For pictures of the gall wasp click here. 

For pictures of the parasite that attacks the wasp, click here.

Survivors

Yesterday I posted some pictures of the ichneumon wasp called Orthopelma mediator that parasitises Diplolepis rosae, the gall wasp that causes bedeguar galls on wild roses. Today seven more parasites hatched out, but so did two surviving D. rosae gall wasps, pictured below. Large galls, like the one I collected, can contain up to 60 D. rosae larvae.

These dumpy little wasps, about 4mm. long, have a very distinctive profile with an enlarged red abdomen that's keeled underneath.

Almost all D.rosae individuals are female and reproduce parthenogenetically, producing fertile eggs without fertilisation by males, which are rare. The female inserts her eggs in rose leaf buds just before they begin to open and the plant cells that they are attached to proliferate to produce the spectacular mossy, crimson gall, which contains a nutritive tissue that the larvae feed on.

Below are a couple of pictures of the gall produced by this little wasp

Deadly Assassin

A few weeks ago I collected a bedeguar gall (aka Robin's pincushion gall) to see if I could hatch out the tiny gall wasps, called Diplolepis rosae, that cause these weird growths on wild roses (click here for pictures of the gall).

So far no gall wasps have appeared, but this little ichneumon wasps has.  It's called Orthopelma mediator and it's the only ichneumon that parasitises British gall wasp larvae. It's about 4mm. long and this is a male - the female has a needle-like ovipositor on the end of her tail, like an assassin's stiletto (see last photograph).

O.mediator lays its eggs in the larvae of Diplolepis rosae before the gall forms, then the parasite's larvae feed on the internal organs of the gall wasp larvae within the gall, until nothing is left but the host's skin and jaws. It may be that all the gall wasp larvae have been parasitised, but so far only two of the parasites have emerged from the gall.

I've constructed a little chamber for photographing tiny insects like this, made from two microscope slides with a spacer in between, which seems to work pretty well - although the lighting (flash) needs a bit or work. The trickiest stage is coaxing the insect into the narrow gap between the slides,

This is a female, with her lethal ovipositor extended


Sixteen insect species, including parasites, hyperparasites and inquilines live in bedeguar galls, together with the gall wasp larvae. One down, fifteen to go......

Yew Midge Gall

This year some of the yew trees in Teesdale are heavily infested with this 'artichoke' gall, caused by a tiny midge called Taxomyia taxi. This insect has a strange two-speed life cycle. Eggs are laid on the yew shoot tip foliage in late spring, inducing the formation of a swollen terminal bud where they spend the winter. Some emerge as adults in the following year but others, like his one, have slower development and spend a second winter inside an enlarged and more conspicuous gall before emerging as midges two years after the eggs were laid.

Margaret Redfern's recent New Naturalist book, Plant Galls, is a mine of information on the strange world of these plant-insect interactions.

A Tree-spotter's guide to Buds: part 3

Still bearing the fruit of last spring's blossom, this blackthorn Prunus spinosa twig with its tight little clusters of buds will be clothed in a froth of white flowers in a little over three months from now. Blackthorn flower buds are carried on short woody spurs.

The attractive buds of bird cherry Prunus padus - glossy, purple-brown and pointed. A hedgerow tree that's commonest in the northern half of Britain.

Hawthorn Crataegus monogyna with short, pointed buds and twigs that often end in the thorns that make this such an effective, stock-proof hedge.

Small greenish buds of silver birch Betula pendula, on long, slender twigs that contribute to the tree's graceful silhouette in winter

Field maple Acer campestre. The short space between the bud scale scars that girdle this twig show that it made little growth over the last two years.

The twin buds at the end of this field maple Acer campestre twig show that last year it would have carried a single flower bud that developed into a bunch of winged seeds at the end of the twig, which have since been dispersed. A bud developed on each side the the bunch of seeds, so next year this twig will branch into two.

Winter buds of oak Quercus sp. , with their small, overlapping bud scales. The bottom right bud seems to have been galled by a gall wasp, which I think may be Cynips divisa - anyone know what it is for certain? I don't think it's easy to tell from buds alone whether the tree is common oak Q. pedunculatus or sessile oak Q. petraea but a search on the ground below will sort them out: if the acorn cups have long stalks, it's the former; if the acorn cups have little or no stalk, then it's the latter.

For more posts on tree ID click here

Life's a Beech

This autumn the upper surface of many of the leaves on young beech trees around here are carrying these cylinrical, hairy galls caused by a midge called Hartigiola annulipes that laid its egg in the leaf surface back in the spring. Opening them up reveals the hollow chamber inside with the larva developing down at its base. In a week or two, when it's mature, the gall will separate and fall from the yellowing leaf, shortly to be buried under a carpet of fallen leaves. The larva will pupate there, until the adult midge emerges in spring, at just the right time to lay its eggs in the soft tissue of a newly expanded leaf.

Gall sectioned vertically.....inside, there's a large, hollow chamber

The larva, tucked in here down at the bottom of the gall chamber, still has plenty of room to grow.