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Essays 5

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Some insects are sedentary and some are habitually "migratory". Discuss the relative advantages and disadvantages of these two types of behaviour.

Migratory insects move from one spatial unit to another, so the house fly can be considered as migratory as it disperses to find food sources after its pupal - adult moult. In some species all individuals are migratory, e.g. the Monarch butterfly, and in others, e.g. the small tortoiseshell butterfly only some of the individuals are migratory. Defining which insects are sedentary is rather difficult as most insects do move from food source to food source to mating place and to oviposition site, however scale insects are more sedentary than most. Really there is just a continuum from sedentary to migratory.

Changeable environment. The environmental change may be periodic, e.g. seasonal, or irregular. A sedentary insect must somehow adapt to the changing environment; it may go into diapause, pupate, or it may just lay eggs and die. The migratory insect can leave to try to find a more suitable environment, and so can continue its lifestyle. To migrate insects usually fly or balloon. Flying uses up a lot of energy, and the insect is usually flying over unknown territory which is a disadvantage when trying to avoid predators. The time spent flying means that less time is available for searching for food and larval migrations are rare or short, e.g. the processionary Lepidoptera. Many migratory insects fly en masse. This may be a predator overload strategy. Ballooning is found mainly in Arachnids, but some insects also balloon to escape from an unsuitable environment, e.g. the winter moth balloons if it hatches before budburst. The ballooning insect has no control over direction and almost no control over where it lands. In the winter moth ballooning insects suffer high mortality. The sedentary insect can get to know its environment very well, and need not invest the energy in flying to new places.

Overcrowding. To escape competition migration is a good strategy. It is seen in aphids and combined with the ability to reproduce parthenogenically must have contributes much to their success. However they have little control over where they land, if it is an unsuitable place they must fly again. Locusts change from solitary to gregarious forms as population density rises and food availability and quality falls, then they migrate en masse away from an area depleted of food resources and travel to an area with food. Some wood boring beetles, e.g. Ips typographus, have an aggregating pheromone when population density is low and a dispersal pheromone when population density is high. This serves to regulate numbers feeding in a tree and avoiding competition. Sedentary insects have their populations regulated by inter - and intra- specific competition.

Genetic exchange. Insects that migrate to mating areas, or away from feeding areas to find a mate will have greater chance of not mating with a relative, so promoting genetic exchange. However as in all journeys they are liable to predation, and at the mating site there may be competition for mates. Sedentary insects may mate with their siblings, so enhancing the chances of inbreeding depression caused deformities, but predation chances may be lessened as the environment is known, and competition may also be lower.

Migratory movement is found more in species associated with temporary habitats than those associated with more permanent ones. Migration usually takes place before reproduction, and often after the main eating stage, i.e., when reproduction values and colonizing ability is at maximum. When the change in the habitat is reversible and periodic diapause might be the favoured strategy, but if the change is irregular then migration must be the favoured strategy.

Insect hormones in moulting and reproduction

Moulting is controlled by Juvenile Hormone (JH) from the Corpora Allata (CA). JH is high in larva to larva moults, lower in larva to pupal moults, and absent in pupa to adult moults.

There is a critical period before ecdysis when prothoraciotropic hormone (PTTH) is secreted from one of two cells in the brain. PTTH activates the prothoracic gland which secretes alpha ecdysone, which is converted to beta ecdysone in the tissues. There are two peaks of ecdysone during the moulting period, the first peak smaller than the second.

The first peak in a caterpillar in the 5th instar causes a cessation of feeding the start of wandering to pupal site. At this stage the insect is committed to moult.
1) The epidermal cells divide.
2) Apolysis - the new cuticle separates from the old cuticle.
3) The epidermis folds - it is larger than the old cuticle.
4) Cuticulin is deposited to protect the new epidermis.
5) Moulting gel, (a mixture of enzymes) is secreted into the space between the old and new epidermis, and the old cuticle is digested.

The second peak of ecdysone.
1) New cuticle is deposited.
2) Ecdysis - the sloughing off of the old cuticle.
3) Time delay during body/wing expansion, water or air is swallowed to pump up the cuticle to its full size.
4) Bursicon from the Corpora Cardiacum (CC)allows permeability of blood cells to tyrosine which causes sclerotization of cuticle.

The timing also depends on an eclosion hormone from CC, in certain species this is triggered by photoperiod, with the release of the eclosion hormone causes wriggling and peristaltic movements enabling the insect to slough off the old cuticle.

The CA is reactivated by the cerebral neurosecretory cells which are triggered by an appropriate environmental cue, feeding or photoperiod, which releases JH into the fat body to start the process of vitellogenisis. Initiation of vitellogenisis leads to 90% of the protein from the fat body being directed to yolk protein production. JH induces formation of spaces between follicle cells allowing vitellogenins access to the surface of the oocyte.

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.

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