Millions of Mites

By the time that summer arrives the foliage of most trees shows signs of insect attack, but these little eruptions on the surface of an alder leaf are caused by eriophyid mites, which are not insects but are related to spiders. I think the mite species that has produced these is Eriophyes laevis inangularis.

Each of these little domes is a chamber that's formed when the mites feed on cells on the undersurface of the leaf, leading to uneven growth that results in the formation of  a pouch where the mites can feed and breed.

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. Eriophyid mites also commonly infest sycamore and field maple leaves, producing large numbers of red pouches on the leaf surface.

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.

In addition to infesting sycamore, field maple and alder leaves eriophyid mites also attack many other plants, including goosegrass (aka cleavers) Galium aparine, whose growth is distorted by Eriophyes galii.

Typically, infested leaves curve inwards at the edges and become spoon-shaped, like the bottom, second-from-the-left leaf in this picture.

Here's the goosegrass eriophyid - the dark, globular structure top left is an air bubble on the microscope slide.

In this view you can see some of the surface patterning and an internal structure - perhaps an egg?- 

... and in this plane of focus the surface pattern of the cuticle is apparent.

Aphids in a Savage Landscape

When aphids infest plants they tend to find a good spot to feed and then stay in one place, where they'll insert their stylets into the plant's phloem, tap its sugary sap and then settle down to reproduce. 

When you take a close look at plant surfaces you can sometimes see why these pests are more or less sedentary. Many plants, like this goosegrass Galium aparine, are covered with epidermal hairs (trichomes) that make it difficult to tiny aphids to move around.

In the case of goosegrass the hooked hairs are primarily for attaching their weak stems to supports as they grow, but those curved spines are also awkward obstacles for minute aphids to negotiate.

Sticky Jack

This is a cross section of the stem of the plant commonly known as goosegrass or sticky Jack and more scientifically as Galium aparine. Sticky Jack is a very common weed that scrambles up through other vegetation using its covering of hooked hairs on the stem and leaves and which sticks to clothing with these when kids throw handfuls of the stuff at each other.

This image was produced using fluorescence microscopy, staining the cells with compounds that bind to the cell walls and fluoresce. The blue cells have walls made of cellulose and their blue fluorescence is due to the calcofluor that they've been stained with, which fluoresces blue in ultraviolet light. Calcofluor has been used as a 'blue whitener' in washing powders - it binds to the cellulose in cotton fabrics and fluoresces faintly blue in the UV component of sunlight. The yellow staining is due to another fluorescent dye (fluorochrome) called auramine O, which binds to cutin in the outer cuticle of the plant, and to dead, lignified cell walls that give the stem its strength - and it fluoresces yellow. The cuticle in this cross section is the thin yellow line covering the outer surface of the section. The yellow circle in the centre is composed of dead, lignified cells - not particularly well developed in goosegrass because it scrambles over surrounding vegetation rather then investing resources in producing a stout lignified stem of its own.

Sticky Jack

Came in from weeding in the garden with bits of goosegrass Galium aparine stuck to my clothes. We used to call it 'sticky Jack' when we were kids, and lobbed handfuls of the stuff at each other on the way home from school. It's the covering of tiny hooked hairs all over the plant that act like Velco, fixing it to fabrics. The hairs on the stem are shortish and curved, but those on the leaves are like fish hooks - when you look at them under the microscope.