Biology, images, analysis, design...
|"It has long been an axiom of mine that the little things are infinitely the most important" |
Woolly beech aphidOn this page: Identification & Distribution Biology & Ecology Damage & Control
Identification & Distribution:The wingless viviparae of Phyllaphis fagi are elongate oval, pale yellowish green, covered with wax wool. The antennae are slightly shorter than the body and have a terminal process that is 0.11-0.12 times the length of the base of the last antennal segment. The body length of Phyllaphis fagi apterae is usually 2.0-3.2 mm, but summer dwarfs may be down to 1.1 mm.
Winged viviparae (see second picture above) have the abdomen wax-covered, which conceals variably-developed dark dorsal cross-bars. The pictures below show micrographs of an adult aptera (first dorsal, second ventral) in alcohol.
The woolly beech aphid is found feeding on the undersides of young leaves of beech (Fagus spp.). This causes the leaves to curl downwards on both sides of the mid-rib, and often to wither and die prematurely. Phyllaphis fagi is distributed throughout Europe, east to Turkey and Caucasus. More recently it has been reported from China and Korea, and introduced to Australia, New Zealand and North America.
Biology & Ecology:
Wax coating and honeydew
The most notable feature of the woolly beech aphid is the dense wax covering of the aphids. This is secreted afresh after each moult, so newly moulted individuals have little or no wax, whilst adults often have long tendrils of accumulated wax (see picture below).
Smith (1999) looked at the structural details of wax glands and the physical form of the secreted wax for the apterae of woolly apple aphid. The glands are composed of greatly enlarged epidermal cells underlying a modified cuticle that forms distinctive wax gland plates, clearly visible in the picture below.
Secreted wax in the form of threads passes out of the cuticle as filaments, the arrangement of these filaments in the cuticle above each epidermal cell gives rise to the distinctive wax skein found in each species hollow, solid or honeycombed. Smith suggests that the primary role of the secreted wax is to prevent the aphids becoming contaminated by their own secreted honeydew (see pictures below) and that of other members of the colony, a view supported by Pike et al. (2002).
Other secondary roles of wax may include individual microclimate isolation, protection from fungi, parasites and predators plus waterproofing and frost protection. Unlike most wax coated aphids, Phyllaphis fagi may, on occasions, be attended by ants feeding on the secreted honeydew (see second picture above).
Iversen & Harding (2007a) carried out a detailed study of the basic biological parameters affecting the population development of Phyllaphis fagi under field and laboratory conditions in order to obtain information about factors responsible for outbreak situations in forest nurseries. In the nurseries, the aphid eggs were found to hatch before budburst. The newly hatched nymphs (see picture below) were highly active in searching for feeding places, which resulted in higher nymph mortality in the first generation than in the following generations.
Ten aphid generations were recorded in the nursery during one growing season. Both temperature and aphid generation were found to affect life table parameters - although the first generation was less susceptible to changing temperatures than the following generations. Overall Phyllaphis fagi reproductive effort was more dependent on temperature than on aphid generation. No significant difference was found in reproductive effort between apterous and alate females. The formation of winged morphs (see picture below) was not restricted to a few generations but rather continued for several months, with activity peaking in mid-June. The potential for outbreak situations on new seedlings therefore exists throughout the summer.
The highest value for the intrinsic rate of natural increase was obtained at 20 °C. Nymphs born at higher temperatures were sometimes born deformed and unable to survive.
Kot & Kmiec (2012) looked at the population dynamics of Phyllaphis fagi in Poland. The highest density of overwintering eggs was observed in the bark crevices of forking shoots. The shortest pre-reproduction and reproduction periods and the highest fertility were exhibited in the second generation of aphids. In later generations, pre-reproductive and reproductive periods of successive aphids' generations were extended, while the female's fertility was reduced. In summer, only single, dwarf individuals were observed on the leaves. Apterous oviparae and alate males (see pictures below - the male is a fourth instar immature) were noted on leaves in late September and October.
From mid-October, aphids went through the branches and stems, where females deposited numerous, fertilized eggs. No significant differences were noted in the numbers of aphids on two varieties of beech.
The syrphid Melangyna cincta is usually thought of as a specialist on woolly beech aphids (Gilbert (2005) ) and appears to be the species we have twice recorded attacking Phyllaphis fagi (see pictures of larvae below). Ball & Morris (2000) also say the species has a clear preference for Phyllaphis fagi on beech, but with isolated records for consuming aphids on on oak, maple and lime. Adult syrphids are commonly seen visiting flowers, especially white umbellifer flowers along wooded tracksides.
Effect of pollution & global warming
A number of studies have looked at the effect of pollutant levels, and global warming, on the dynamics of Phyllaphis fagi populations. Whittaker (1997) looked at the responses of tree-feeding aphids to elevated carbn dioixide levels. The concentration of total soluble amino acids in beech foliage was unaffected by growing saplings in elevated atmospheric carbon dioxide concentrations. Experiments on individual aphids indicated poorer performance of Phyllaphis fagi in that fewer, smaller nymphs were produced, but resultant populations did not differ from those in ambient conditions. No evidence was found that, under the conditions of these experiments, populations of aphids will change as concentrations of carbon dioxide increase. Flückiger & Braun (1999) monitored nitrogen concentration in the foliage of mature beech in permanent observation plots across Switzerland. Nitrogen concentration was found to have increased by 15% and phosphorus concentrations decreased by 12% between 1984 and 1995, leading to increased nitrogen:phosphorus ratios. Experimental application of nitrogen over four to five years caused nutrient imbalances in various afforestation plots comparable to those observed in the permanent observation plots. The changes in the trees caused by nitrogen treatment led to increased attacks by various insect pests including Phyllaphis fagi on beech.
Braun & Flückiger (1999) looked at the effect of ambient air with increased ozone concentrations and artificial acid mist on the population growth of two different aphid species, Aphis fabae on beans and Phyllaphis fagi on beech seedlings. Whereas the increased ozone concentrations inhibited growth of Aphis fabae, it stimulated population growth of Phyllaphis fagi. In beech, analysis of a phloem exudate revealed that the amino acid:sugar ratio was significantly increased in the ambient air compared to filtered air. Acid mist inhibited population development in both aphid species; the strongest effect was observed in the first weeks following artificial infestation.
Damage and control
Feeding aphids cause beech leaves to curl downwards on both sides of the mid-rib. Clear damage can be visible even there are only a few individuals on the leaf. The decorative value of trees can be reduced as early as June.
Gora et al. (2007) looked at the physiological defence reactions of young beech trees to attack by Phyllaphis fagi. Defence reactions were found at three levels. Infested plants showed a strongly decreased amino acid:monosaccharide ratio in the entire leaf and leaf phloem. In addition, secondary plant defence substances were detected in beech saplings and lamma shoot leaves of beech seedlings. A systemic reaction, dependent on the infestation density of the lachnid, was observed in the leaf phloem of the beech saplings.
Iversen & Harding (2007b) tested biological and other alternative control methods against Phyllaphis fagi. Field applications of mineral oil to the egg stage reduced initial aphid population by 75%, but only when the eggs were exposed to oil close to the time of hatching. Bioassays with the insect pathogenic fungus Lecanicillium lecanii in the commercial formulation Vertalec® were conducted. Both nymphs and adults were susceptible to fungal infection at both dosages. It was demonstrated that the existence of a dense wax-covering in adult woolly beech aphid had no protective effect against fungal infection. In semi-field trials with two Lecanicillium lecanii treatments at the recommended dosage, the aphid population was reduced. However, it was thought that adequate control would requires several treatments as opposed to the two that were tested in the present experiment. Furthermore, efficiency may depend on summer temperatures and humidity. Yazdgerdian et al. (2015) tested eleven essential oils especially from conifers against woolly beech aphid as contact and residual toxins. Several of the oils were found to be strong contact insecticides and were considered to be effective envionmentally friendly insecticides.