Biology and ecology of the garden chafer,Phyllopertha horticola(L.). IX.—Spatial distribution
Authors: Milne A. 
Published: July 2009

The garden chafer,Phyllopertha horticola(L.), has been recorded in most counties of England and Wales (43 out of 52) and is probably to be found in all. It is more abundant the warmer and wetter the region,i.e., it is more abundant in the west and south and most abundant in the south-west. It flourishes best in permanent pasture on light soil in hilly country, and occurs from sea-level to an altitude of at least 1,160 ft.

Patchiness in local distribution was investigated on the pastures of fell farms in the Lake District.

An infested fell pasture includes from one to five plant associations which are described as (1) Grass, (2) Sparse Bracken, (3) Bracken Bed, (4) Heath Rush and (5) Common Rush. Typically, all five are present in a pasture. The proportions of ground covered by grass turf (Agrostis-Festuca) in these associations are (1) 100 per cent., (2) 99 per cent., (3) 0 per cent., (4) 80 per cent, and (5) 50 per cent., the grass turf in the rush associations (4 & 5) filling the spaces between clumps of rushes. Soil stages of the garden chafer are found only in the grass turf and so the Bracken Bed association is never infested. In terms of density per unit area of ground, the other four associations exhibit the following relation between their infestations: Grass>Sparse Bracken>Heath Rush>Common Rush. This relation is only partly explained by the similar relation between proportions of ground covered by grass turf (see above). As to other factors that might be partly responsible, no correlation exists between botanical characteristics of grass turf and infestation but there are two negative correlations among physical characteristics which are suggestive: Grass and Sparse Bracken-<Heath Rush and Common Rush with respect to toughness of turf; and Grass<Sparse Bracken<Heath Rush<Common Rush with respect to wetness.

Grassland, an unbroken expanse of grass sward, is, in the Lake District, by far the most important habitat since it carries by far the greatest part of the garden chafer population.

In grassland no evidence was found of an association between density of soil stages and proximity to bracken.

The population of any soil stage, from egg to pupa, is always much greater in drier than in wetter parts of grassland.

Sampling at intervals within a generation indicated that the mortality rate is higher in the wetter parts of grassland. This is not due to any lack of food, or difficulty of utilising it, or to the presence of more enemies (competitors, predators, parasites and pathogens) in the wetter parts, but, as established by laboratory and field experiments, is attributable to a direct effect of moisture. Further experiments suggested that the egg stage is most and the feeding stages are least resistant to excessive moisture.

Field experiments also indicated that the so-called ‘effective bee-liners’, females characterised by a direct and sustained flight from their roosting sites and that lay some eggs far afield, exercise some degree of discrimination in the choice of oviposition sites, the majority tending to choose the drier parts of grassland, the kind of site that does in fact, become more highly infested. Less than a quarter of all females become effective bee-liners and as such lay only about 15 per cent, of all eggs deposited by a population.

There is a dynamic aspect to the phenomenon that population is greater in drier than wetter parts of grassland. Although population is always greater in the drier parts, the magnitude of the population ratio between drier and wetter parts can change considerably from one year to the next as a result of either excessive rain or excessive drought during the larval feeding period. The mechanism of such change is indicated.

Spatial distribution on grassland was assessed statistically by employing Fisher's Coefficient of Dispersion. Eggs, and hence newly hatched larvae, are strongly aggregated. This strong degree of aggregation is gradually lessened, however, by the subsequent wanderings of the larvae in the course of feeding. Nevertheless, even in the final instar, larvae are still appreciably aggregated and hence so also are pupae. By the end of the feeding period, average dispersal from the hatching point is probably not more than 12 inches.

Apart from proper food, which is provided by grassland, the most important living condition for the garden chafer in the soil appears to be proper soil moisture. Although actual limits have not been specified, survival clearly depends on the soil containing neither too much nor too little water for too long.

As regards geographical distribution in England and Wales, the finding that the garden chafer is more abundant the warmer and wetter the climate is explained as follows. Females grow larger, and hence produce more eggs, where the larval feeding period is warmer, and more larvae survive where the climate is wetter, because the greater rainfall means less chance of too little water for an animal that has to live in freely draining soil to avoid too much water.

Fundamentally, the patchy local distribution of the garden chafer is mainly a matter of the patchiness of soil-moisture conditions. The patchiness determined by these conditions is maintained and intensified by the reproductive behaviour of the adults.

Accounts of other species in this and other countries suggest that, for pasture scarabs in general, soil moisture is the chief factor concerned in determining spatial distribution.