FACTORS INFLUENCING POPULATION DYNAMICS
The Winter Moth Story
Budburst synchronisation and 'ballooning'
The Winter Moth is a common species in most of Britain. It occurs, as its English name suggests, from late autumn through to January or February.
The larvae feed on a range of trees and shrubs, as well as moorland species such as heather (Calluna). Sometimes the larvae occur in great numbers, reaching pest status and occasionally completely defoliating small trees.
The females are virtually wingless, and crawl up tree-trunks to await the arrival of males.
In early studies in Wytham Wood near Oxford the key-factor in influencing population size was found to be "winter disappearance", this is loss of eggs and young larvae mainly caused by hatching too early (Feeny 1970), i.e. before budburst.
So to maximise its chances of survival and reproductive output, the winter moth must time its hatching to coincide with oak budburst or be able to use alternative food plants. It has been found (Hunter 1992) that budburst and tree height have a positive correlation with caterpillar density, 75% of the variation in density is accounted for by these two variables. The main influence on budburst is the weather, but even within species in the same area budburst can vary by as much as two weeks.
If the larvae hatch when host tree's buds are not open the caterpillars hang from a silk thread till they are blown to another tree, this usually occurs within 12-24 hours of hatching (Wint 1983). They tend to feed on whatever is available and can survive for up to five days without food (Wint 1983). This method of dispersal works best when trees are fairly close together, but is always a cause of high mortality which increases if the trees are widely spaced. It is called 'ballooning'.
The rounded shape of an oak must diminish the chances of the larvae escaping from the tree unless it is at the very top or at the edge, many larvae must land in another part of the same leafless tree. Holliday (1977) grease banded four trees in an apple orchard to prevent females depositing eggs, when densities of larvae and pupae were surveyed for banded and unbanded trees it was found that both had similar densities, so on apple trees in orchards larval density seems to depend on dispersal, not the amount of eggs laid.
Larval predation and parasitism
Perhaps the best known predators are great and blue tits. They time the hatching of their chicks to coincide with the early larval stages, and though each pair may take hundreds of caterpillars to feed their young each day, it has been estimated that this amounts to only 2-5% of the available winter moth caterpillars (Feeny 1970), this may be an example of predator overload. On Orkney larval predators included starling, meadow pipits and common gulls (Picozzi 1981). In Nova Scotia and British Columbia where the winter moth was accidentally introduced, two parasitiods were imported, Cyzenis albicans, a fly, and Agrypon flaveolatum, a wasp.to control the populations and protect the fruit crop. About five years after the introduction of the parasitiods the winter moth populations declined. About fifteen years later populations in some of the original orchards, now neglected, were examined by Pearsall and Walde (1994). Pupal densities varied widely from over 300 m2 to only 7 m2, and the highest mortality was no longer caused by parasitoids but predation on and in the ground. The parasitoid C. albicans was causing mortality of 0%-20%, and A. flaveolatum was not found at all. So the introduced parasites reduced the populations from epidemic levels to a level similar to that found in countries where the moth occurs naturally, since then generalist predators of the pupal stage have been the main population regulators.
Pupal predation
In Wytham Wood, in England, predation of pupae in the soil mainly by carabid and staphylinid beetles, but also by some small mammals, (shrews etc.) was found to be the main population regulating factor (East 1974). In neglected apple orchards in Nova Scotia, Pearsall and Walde (1994) also showed that the population regulating factor was the predation by carabid and staphylinids on pupae. Some beetles could eat 2.5 times their own body weight per day, but beetle numbers showed no relation to pupal density, so it was thought that the beetles just switched to eating winter moth pupae as they became abundant. This introduced population of winter moths has come to resemble the native population (Roland 1994).
Fecundity
The number of eggs laid by the female is related to pupal weight, with heavier females laying more eggs, and the pupal weight is related to leaf toughness (Wint 1983) not to the nutritional quality of the leaf. On most trees survival of the larvae is highest if they start feeding at budbust, on oak mature leaves are too tough and both lower weight and increase mortality (Feeny 1970). As the pupal weight is lowered, this decreases the number of eggs laid, and fewer adults survive from eggs laid by females with low pupal weight, so reproductive success is lowered.
OAK LEAVES AND TANNIN
Oak is the main food plant of the winter moth at present, but to pupate at a reasonably high weight the larvae have to feed on the leaves as soon as they emerge. Older and mature oak leaves have been shown to be less nutritional because of the increased presence of tannin (Feeny 1970). This increasing tannin content is the cause of spring feeding in the winter moth and possibly in other oak-feeding caterpillars as the tannins complex with the leaf proteins so limit the amount of nitrogen available to the caterpillar (Feeny 1970), like most insects the caterpillars are nitrogen limited.
TREE DAMAGE
Oaks that leaf out early, and achieve 50% budburst relatively quickly have the highest density of caterpillars and subsequently suffer the highest levels of defoliation in woodlands. Populations on oak are sometimes heavy enough to completely defoliate the trees. When this happens the larvae must find another food source by ballooning (Feeny 1970). However in parklands where the trees are more widely spaced there is no relationship between budburst and defoliation (Hunter 1992), this may be because the trees are too far apart for the caterpillars on leafless trees to balloon over. In Canadian orchards it has been found that to prevent damage to fruit crops all trees in the orchard and within dispersal distance (perhaps as much as 50 m) must be banded (Holliday 1977). The winter moth has caused defoliation and distorted growth on Sitka spruce recently, this is a new food plant, and as it is grown commercially and planted densely in single species stands there is great potential for population explosions and extensive damage.
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