Biology, images, analysis, design...
|"It has long been an axiom of mine that the little things are infinitely the most important" |
Conifer aphids, Nettle aphids & Hellebore aphids
in mid-winter 2014On this page: Woolly conifer aphid nymphs, common nettle aphids, hellebore aphids
January 2014 has been characterised in the south-east of England by 'la deluge' - more than double our usual rainfall and the wettest January since records began. What effect this will have on insect populations - and from our point of view on aphid and predator parasite populations - remains to be seen. But probably more importantly for most insects, it has been relatively mild.
Woolly conifer aphids - cute young nymphs?
On January 18th we walked (sloshed, slithered and squelched) up the track to Batts Wood in East Sussex where we soon found some healthy populations of the Cooley spruce gall adelgid, (Adelges cooleyi) on Douglas fir. Adelgids are not aphids in the strict sense of the word, but they are closely related (same superfamily) and are sometimes called woolly conifer aphids. As with aphids, most adelgids have a primary host and a secondary host, although some species have lost one or other host. Unlike aphids, the primary host is always spruce and the secondary host is other conifer types. Again unlike aphids, for a species with two hosts, the complete life cycle lasts two years.
Douglas fir is the secondary host of the Cooley spruce gall aphid. Eggs laid on Douglas fir hatch in autumn, and the exposed nymphs overwinter on the underside of the needles. Providing it is not too cold, these nymphs soon begin to feed and produce their waxy protective threads. Initially, the white wax is just a fringe around the dark body. The nymphs are usually spaced out very evenly as in the first picture below, in what is known as a uniform or regular distribution, perhaps to reduce competition.
The adelgids were present in huge numbers, with many on virtually every needle. One might expect to find predators feeding on this bountiful resource. Well, many predators have been recorded later in the year, but in winter it is rare to find anything actively predating them. In January we did find a cream-spot ladybird (Calvia 14-guttata) on the fir needles, which may well eat the nymphs, but we did not observe it to do so.
Cooley spruce gall adelgid appears to have two anti-predator gambits. The first is their wax, which probably discourages predation either by hiding prey-identification cues from vision-guided predators or by containing unpalatable (or even toxic) chemicals. Their other likely anti-predator gambit is using winter as a 'refuge' from most predators. At that time of year, it is usually too cold for most predators to be active, so these adelgid get a period when they can hugely increase their numbers free of predation.
Last year we kept some spruce gall adelgids alive so we could photograph their development. By the time they reach maturity, the entire body is engulfed with wax. These are all females, and each produces a cluster of up to 100 eggs as in the picture below. These eggs developed within the female, and after death remain attached to that female. (An apterous female, which develops on its secondary hosts, is called an exulis.) Later in the year some mature instead to winged females in order to fly to their primary host - spruce (Picea).
The common nettle aphid - a born opportunist
Aphids can be remarkably flexible in how they get through the winter. The common nettle aphid, Microlophium carnosum, can be found throughout Europe and into Asia. In southern climes the common nettle aphid mostly overwinters as individuals (known as viviparae) which give birth to live young asexually. Further north males and females develop in autumn, mating occurs and the females deposit eggs at the base of the stems.
In southern England the common nettle aphid's behaviour seems to largely depend what the winter is like. In a mild year, as has so far been the case in 2014, one would expect many Microlophium carnosum to survive as nymphs or adults. And sure enough, a check on three inch high young nettle plants alongside a farm track on January 22nd revealed a nymph and an adult on the second plant we examined.
We can tell the aphid above is still a nymph because the appendage at the rear tip of the abdomen - known as the cauda - is conical rather than elongated and finger-like. Don't confuse the cauda with the siphunculi, which are the two tubes coming from the back of the aphid a little in front of its cauda. The various lumps of whitish matter sticking to the aphid are composed of the waxy defensive exudate containing triacylglycerols, which is produced through the aphid siphunculi.
This aphid, with a finger-like cauda, is an adult Microlophium carnosum. They are not always green - there is also a pink form - but the significance of the two colour forms is unclear. The common nettle aphid is always around - but is much commoner some years than others, supposedly in alternate years. Last year the aphid did very poorly in Sussex, so we'll see if 2014 is a bumper year for them.
What's in your garden - aphids on the hellebores?
Some species of hellebores are found in the wild in Britain, but you are most likely to encounter the hellebore aphid (Macrosiphum hellebori ) on one of the cultivated varieties in your garden. These were found in January on potted plants along the high street of a Sussex village.
The hellebore aphid is considered by gardeners to be one of the most important pests of hellebores. Apart from the direct effects of feeding damage, the honeydew they excrete encourages the growth of sooty moulds. This mould, in turn, reduces photosynthetic activity of the leaves and is unsightly, especially on the flowers.
To control Macrosiphum hellebori most horticulturists recommend use of either systemic insecticides or synthetic pyrethroids. However, effective control can almost certainly be achieved simply by spraying the plants 3-4 times a week with a jet of water. This has the advantage that it is much less likely to eliminate any natural predators, parasitoids and pathogens - and helps wash off honeydew and avoid the mould.
If Macrosiphum hellebori populations are not suppressed at all, we have found they usually die out quite quickly as a result of infection with Entomophthora fungus. This fungus is quite different from the sooty mould, which grows on the excreted honeydew. Entomophthora fungus is an aphid pathogen that grows on the aphid and kills it, covering the aphid in a dense mass of fungal hyphae as shown below.