Dispersion, contagion, and population stability of red scale, Aonidiella aurantii, in citrus orchards with low or zero insecticide use

Abstract

Red scale, Aonidiella aurantii Maskell (Hemiptera: Diaspididae), has a contagious distribution among fruit and among trees largely due to the fact that most young settle on the same fruit as their mothers. The pattern of distribution and degree of dispersion of red scale within various sets of contiguous citrus trees in several orchards were recorded each autumn for 4 years. Changes in density and site occupancy within trees and within contiguous groups of trees were made to assess the strength of any link between populations and their dynamics on different trees. Aonidiella aurantii was found to have a very clumped distribution whereby trees with a level of scale infestation well above average for their block could be adjacent to trees with few or no detectable scale. However, patches of high density on a given tree usually did not recur for more than 2 years but were replaced with above average infestations on other trees. The stability of the red scale populations varied from block to block. Some blocks had few or no infested trees over the study period whereas others had widely fluctuating levels of infestation. Dispersion coefficients at 'within tree' and 'within block' levels were similar. Theoretical and statistical models of the relation of average scale levels to the percentage of fruit or trees infested were less precise than they are for other pests because of high variation of clumping intensity of scale within any block.

Mortality on fruit in citrus orchards caused by naturally occurring enemies of red scale Aonidiella aurantii: relation to spatial distribution of patch density and season

“…In the geographical region of the present observations, densities of A. aurantii can vary widely in time and space so that fluctuations over several years in different orchards (or even in adjacent trees within an orchard) can follow different patterns (Meats & Wheeler, ). Thus, one cannot claim low equilibrium as a sign of biological control.…”

“…Hence, sampling on fruit represents a significant and distinct portion of the population. Moreover, sampling of the peak population of red scale is more precisely done on the fruit with each of the latter being able to support a potential sampling unit or a defined subpopulation (Song et al., ; Meats & Wheeler, ).…”

“…Red scale is very contagiously distributed, so that between 20 and 80% of trees in a block could have no scales on fruit and many infested trees would also have no scales on some of the fruit (Song et al., ; Meats & Wheeler, ). Therefore, random sampling of fruit would have yielded quite a low percentage of infested ones and obviously fruit with no scales cannot yield information on parasitism of scales.…”

“…Accordingly, we present a study of naturally occurring populations of red scale with naturally occurring natural enemies. The aim was to obtain quantitative measures of the relative and total magnitude of mortality rates imposed by the most common of the natural enemies of the red scale on fruit by date, age, sex, and host stage at four sites, with a view to explaining biological control at the relatively low pest levels that are observed in the region (Liang et al., ; Meats & Wheeler, ). The data gathered also enable the examination of competition and intraguild predation between natural enemies which could possibly affect their efficacy as control (mortality) agents.…”

Dispersal of Aphytis melinus (Hymenoptera: Aphelinidae) after augmentative releases in citrus orchards

“…The ability of A. melinus to disperse in the field was poorly studied in the past (Samways, 1986) or only over a period of several generations (Campbell, 1976). A good knowledge of its dispersal ability is particularly relevant considering the extremely patchy distribution of the host species (Meats & Wheeler, 2010) as well as A. melinus poor ability to disperse and short adult life (Rosen & DeBach, 1979;Samways, 1986;Heimpel et al, 1997). The results obtained are discussed in the context of biological control with specific reference to the ability of A. melinus to disperse after augmentative releases in citrus orchards.…”