Discuss the factors predisposing British forests to pest outbreaks.
1) Monoculture. Many of the forests that are grown for commercial timber are densely planted monocultures. So once a few individuals of a pest species have reached the forest they are surrounded by almost unlimited host trees and have no dispersal problems.
2) Reafforestation. In many areas the first rotation has ended (trees planted after WW1). In many cases these trees were planted on virgin soil so there were few resident pests. Now those trees have been cut, it is inevitable that debris should be left behind. In this debris pests such as Hylobius abietis (pine weevil) breed and move on to the seedlings when reafforestation takes place. Even if most of the debris is removed the stumps are left and the adults can lay eggs on the bark. The stumps will supply sufficient food for 3 - 5 years, then there will be a move to the young trees.
3)Deer. I'm not sure if they come under the heading of pests, but they do destroy a large number of young trees. They have no natural predators, and many people object to culling them, so their populations have exploded. During the day they can hide in the denser, taller plantations, emerging at dawn and dusk to browse on the young growth.
4) The move to new host species by pests. The winter moth was a pest on oaks, birches and fruit trees, but in the last 20 or 30 years it has managed to move and thrive on new species, e.g. sitka spruce and Calluna vulgaris. It may be able to move to other conifers. No-one knows how or why it suddenly made the move. It may have been a) higher nitrogen levels because of NOX depositions, b) climate change, c) a chemical previously eliciting a deterrent response not not eliciting any response or a feeding response (this change could come about genetically). All of these suggestions have been put forward and seem plausible.
5) Planting of species not suitable to soil/climate. For example sitka spruce does very well on badly drained wet soil, and so is excellent for the west coast. In drier areas it grows well too, but in a drought it cannot mount a successful defence against wood borers and aphids. It's usual defence is to produce copious amounts of resin which seals the wounds, prevents further entry, and may even drown the pest in a bath of sticky goo. But under drought conditions resin production is limited. This has led to the green spruce aphis becoming a pest in the east of Scotland, but not in the west. Drought also leads to higher nitrogen levels in leaves, and insects are often nitrogen limited so they may preferentially feed on stressed trees, e.g. as the pine looper does in Culbin.
6) Importation of 90% of timber. This opened the door to new pests, e.g. Dendroctonus micans was introduced in unbarked logs, and was in the country for ten years before it was discovered. Generally introduced pests come without their predators which means their spread is limited only by available food and competition. D. micans has so far been confined to the Welsh/English border, but it seems only a matter of time before it is found in Scotland. Ips typographus is another pest which may make its way here no matter how vigilant we are.
7) Pest control. This may seem a contradiction, but pest control will become more difficult now that forests are seen as places of recreation. The general public does not like the spraying of insecticides. They are rarely successful anyway and are indiscriminate. Traps using pheromones are good for monitoring population levels, but not for control. Bacteria such as Baccillus thuringiensis gives 80% mortality, but it won't prevent outbreaks, and it is not suitable for all species and the spores must be eaten. Also resistance has been reported in some insects. Nuclear polyhedrose viruses are species specific, and seem to be the great new hope, but few are in commercial production.
Perhaps we should plant forests as a mix of species and of ages, as it is in nature.
Contrast the lifecycles of hemimetabolous and holometabolous insects. Discuss why there are so many more holometabolous species.
Pterygote insects undergo metamorphosis between the immature phase and the winged, or adult, phase. The metamorphosis falls into two patterns of development:
1) hemimetaboly - partial or incomplete metamorphosis.
2) holometaboly - complete metamorphosis.
In hemimetabolous insects the developing wings are visible as wing buds in the nymphs, so these insects are often termed exopterygotes. While in holometabolous insects there is no sign of the wings in the larvae, also they undergo a resting or pupal instar during which time the major structural differences between the larva and the adult take place. These insects are sometimes called endopterygotes.
Hemimetabolous, e.g., Hemiptera. The adult female lays eggs on the food plant. The eggs hatch into nymphs which feed on the food plant. The nymphs look similar to the adults though they are smaller, wingless and may be a slightly different colour - but they could be identified fairly easily to family level using a key for the adult insects. The nymphs go through five or more instars increasing in size. In the later instars the wing buds increase in size too. All this time the nymph feeds on the same food plant as the adult. The moult following the final instar produces the winged adult which can still feed on the same food plant as the nymphs. So really there is just a gradual change from instar to instar. the adult feeding and behaviour changes little.
Holometabolous, e.g. Lepidoptera. The adult female lays eggs on the food plant. The egg hatches into a first instar larva which looks nothing like the adult insect, and the larva feeds on the food plant. The larva is really just an eating machine. As it grows it moults with each instar closely resembling the last with an increase in size being the main change. (The cinabar moth which sequesters cyanide from its host plant ragwort has warning colouration from the second instar onwards. The first instar has cryptic colouration, so it does not follow the above pattern quite so closely. It may be cryptically coloured during the first instar because it has not yet sequestered enough cyanide to make itself distasteful.) The larvae feed until just before the last moult from final instar to pupa. During this time they may stop feeding or feed less often as they look for a suitable place to pupate. This may or may not be on the host plant. The pupal stage in some insects enables the insect to survive unfavourable conditions as there is no feeding during this stage. A protective cell or cocoon surrounds most species. There are different types of pupae, ecarate do not have their legs, antennae etc. pressed close to their body, obtect do. The adult emerges from the pupa. And really their only purpose in life from now on is mating and reproduction. Some do not even have functional mouthparts (mayflies). If they do feed they tend not to feed on the same food as the larva, therefore there is no competition between larva nd adult. Many Lepidoptera are nectivorous so perform the useful task of pollination.
It is believed that this niche separation between adult and larval phases has enabled greater speciation of the holometabolous insects. They have divided the tasks of feeding and reproduction so that each has its own specialized time in the life cycle. They also have two resting phases when they may be less susceptible to unfavourable environmental conditions; the egg stage and the pupal stage. Another reason for the huge number of species might be that the adults of the four biggest orders - Diptera, Hymenoptera, Coleoptera and Lepidoptera have undergone speciation and radiation at the same time as the angiosperms. There may be an aspect of coevolution with some guilds of insects specialising in certain guilds of plant. Added to this are the landmass changes and climate changes which put enormous selection pressure on the host plants to change.
Finally their great number of species may quite simply be due to their relative newness. Any type of animal that turns out to be successful undergoes great speciation and radiation. The ones that aren't successful die out. So what we see are the remnants of past successes, e.g. Gymnosperms and reptiles. Present successes, humans and holometabolous insects. Future successes we cannot see or predict yet.