Biology, images, analysis, design...
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Identification & Distribution:Adult apterae of Dysaphis pyri medium to rather large globe-shaped, brownish-red to dark brown aphids. They are thickly coated with wax meal. The antennae are pale yellow near the base, but darker towards the apex.
The first 5 abdominal tergites have a double row of small dark spots. Hemispherical marginal tubercles are usually present only on abdominal tergites 1-5. The siphunculi are black and perpendicular to the body. They are 3.4-4.1 times their diameter at midpoint, and longer than the cone-shaped cauda (cauda is visible in the first image above). The adult aptera has a body length of 2.1-3.2 mm. Immature Dysaphis pyri are a pale yellowish brown, with reddish suffusion around the bases of their siphunculi.
The primary host of Dysaphis pyri is common pear (Pyrus communis). Leaves and shoots are yellowed and distorted to form a pseudogall (see second picture above). After about three generations on pear, alatae are produced which migrate to the secondary hosts. These are bedstraws, especially hedge bedstraw (Galium mollugo) and cleavers (Galium aparine) and sometimes squincywort (Asperula cynanchica). Dysaphis pyri may form colonies on the roots and prostrate stems, where it is attended by ants. Dysaphis pyri is found throughout Europe, the Middle East, North Africa and Central and South Asia, and has been introduced into the USA.
Biology & Ecology:
Savary (1953) studied the life history and population biology of Dysaphis pyri in Switzerland.
Dysaphis pyri sometimes 'shares' its pseudogalls with other aphids. The picture above shows a mixed colony of Dysaphis pyri and Rhopalosiphum oxyacanthae - the latter are the small green aphids.
The aphid below appears to be an aberrant specimen of Dysaphis pyri which has not developed its wax coating. The smaller aphids are its nymphs.
The only predators we have so far found attacking Dysaphis pyri within the pseudogall are syrphid larvae such as the one shown below.
Rojo & Marcos-Garcia. (1997) looked at syrphid predators of pear tree aphid pests. Larvae of the pipizine syrphid Heringia heringii were more abundant than those of other syrphid species. More than 40% of Heringia heringii larvae were collected on Dysaphis pyri colonies.
Rakauskas (2004) reported the appearance of large numbers of Dysaphis pyri necessitating chemical control in Lithuania in 2002. Aphids could not be found on the summer host Galium. so it was concluded these were probably airborne summer migrants, possibly the result of global warming.
Damage and control
Dysaphis pyri is the most important aphid pest of pear causing severe direct feeding damage to both shoots and fruits. Growing shoots and leaves are distorted and yellowed, and heavy infestations check plant growth. Large quantities of honeydew are produced, further affecting growth.
Insecticide treatment is still the primary method of control. Alford (ed) (2000) recommends application of insecticides if any Dysaphis pyri are detected before flowering. From petal fall onwards, a suitable threshold is if 1% of trees is infested. Suitable insecticides include the organophosphates chlorpyrifos and dimethoate, the pyrethroid cypermethrin, the alkaloid insecticide nicotine, soap concentrate containing fatty acids and rotenone. A tar oil winter wash can be used against overwintering eggs.
Sauphanor et al. (1993) carried out a field trial to test the nontarget effects of pesticides on beneficial insects in a pear orchard near Avignon, France. The treatments were applied in April and May to affect the first generation of another pest, Dasineura pyri. Observations were carried out for 15 days after each of the treatments (amitraz, diazinon and pyrethrins + rotenone) to study the effect on Psylla pyri, Dysaphis pyri, and the main natural enemies present. It was concluded that earwigs (Forficula auricularia) play a major role in the regulation of Psylla pyri populations.
So far no resistant varieties of pear have been developed, although Evans et al. (2008) has identified a new gene for resistance to the aphid.
Cross et al. (1999) reviewed biocontrol of pests of apples and pears by parasitoids in central and northern Europe. Only Ephedrus persicae , a species specialising in leaf-curling aphids in trees, is recorded as attacking Dysaphis pyri and then only at a low level of effectiveness. It is suggested that host plant alternation may diminish the effectiveness of parasitoids for Dysaphis pyri.