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Large hazel aphid, Hazelnut aphidOn this page: Identification & Distribution Biology & Ecology: Plant defences Cryptic coloration Natural enemies Other aphids on the same host Damage & Control
Identification & Distribution:
Corylobium avellanae apterae are usually yellowish-green (see first picture below) but in one form the aphid is mottled with red (see second picture below). The antennae are pale with dark tips to the segments and black apices. The dorsum is granulate with numerous small cuticular structures (best seen in picture in alcohol below) and 6-8 low conical tubercles per segment, each with one or two long thick capitate hairs. The siphunculi are long, thin and tapering, 4.2-5.5 times the length of the cauda. The body length of Corylobium avellanae apterae is 1.7-2.9 mm.
The alate (see first picture below) has the head and thorax brown, and the abdomen green or dirty red, with marginal sclerites, and intersegmental pleural sclerites or crossbars. The dorsum is not granulate. The body hairs are short and thin and not capitate. The siphunculi of the alate are 0.27-0.32 times the body length. The micrograph (see second picture below) is a dorsal view of an apterous adult in alcohol showing the granulate dorsum.
The clarified slide mounts below are of adult viviparous female Corylobium avellanae : wingless, and winged.
The large hazel aphid does not host alternate but spends its entire life cycle on hazel (Corylus avellana). Sexual forms occur in autumn and overwintering eggs are laid on hazel. It mainly feeds on the fast growing shoots and only rarely on the leaves, which are utilized by another species - Myzocallis coryli. Corylobium avellanae is distributed over most of Europe east to Ukraine and Russia and west Asia. It has recently been found in Canada.
Biology & Ecology:
The leaves and stems of hazel (Corylus avellana) are densely covered in hairs, the primary purpose of which is presumably to discourage herbivores. Kasapligil (1964) (in Burgess , 1983) described several kinds of hairs (trichomes) which may be found on Corylus avellana. The largest trichomes have a long (0.4-3 mm) stalk topped with an enlarged, reddish, glandular head 50-195 μm in diameter which exudes a resinous secretion (see picture below). Less common are the smaller (36-76 μm) club-shaped or capitate glandular trichomes with a head of secretory cells. Simple, unicellular (40-143 μm long) or septate trichomes (160-610 μm) have a bulbous base which tapers to a pointed tip, with thickening of the secondary wall.
The long legs of the adult Corylobium avellanae apterae usually enable them to keep well clear of most of the capitate glandular trichomes, as can be seen in the picture above. The immatures are usually found feeding on the leaves in between the leaf veins, where the density of short simple trichomes is lowest (see first picture below). However, even the smallest nymphs can sometimes be found amongst the glandular trichomes (see second picture below).
There is evidence that the number of hairs affect the susceptibility of different hazel cultivars to feeding by aphids, at least by Myzocallis coryli. Weryszko-Chmielewskai et al. (2006) compared the anatomical traits of four large-fruited hazelnut cultivars (known as filberts) showing different susceptibility to feeding by aphids.
On leaves of 'White Filbert', the cultivar most resistant to 'filbert aphid' feeding, the number of mechanical hairs was the highest, while the lowest number of them was noted on the most susceptible cultivar 'Wonder from Bollwiller' - but it is unclear whether these 'mechanical hairs' included the glandular hairs.
Adult Corylobium avellanae seem well adapted to their hairy habitat and sometimes walk around on top of the hairs.
Given the apparent vulnerability of aphids to predators, one would expect cryptic coloration to be a widely employed strategy. And indeed, as Dixon (1997) points out, most of the aphids that live on leaves are green, whilst those that live on the woody parts of the plant are brownish. This form of colour matching is termed homochromy and is very common with aphids.
With hazel it is a bit more complicated because, although the leaves and stems are green for most of the year, when the new growth is produced in spring and early summer the stems are often reddish - sometimes very much so. At this time the normal green aphid forms contrast wonderfully with the red stems (see picture below), and must pose a tempting target for bird predators.
Given that selection pressure, not surprisingly, Corylobium avellanae also has a reddish form which matches perfectly with the young hazel stem (see picture below).
Corylobium avellanae is parasitized by the braconid Praon dorsale. It was originally thought that Corylobium was the only host for this parasitoid (Tremblay and Pennacchio, 1985), but Bogdanovic et al. (2014) found that its host range is wider: including Macrosiphum funestum (feeding on Rubus) and Macrosiphum cholodkovskyi (feeding on Filipendula ulmaria). Tomanovic et al. (2009) reported that Corylobium avellanae is also parasitized by Ephedrus plagiator.
We have not so far found any parasitized Corylobium avellanae, but we have found cecidomyiid larvae predating the aphids (see picture above).
We have also observed the mirid bug Phylus coryli predating Corylobium avellanae (see pictures above).
Other aphids on same host:
Blackman & Eastop list 19 species of aphid as feeding on cobnuts and hazelnuts (Corylus) worldwide, and provide formal identification keys.
Blackman & Eastop list 5 aphid species that feed upon the common hazelnut or filbert (Corylus avellana) worldwide (Show World list). Of these, Baker (2015) lists 2 as occurring in Britain (Show British list).
Damage and control
The aphid Corylobium avellanae feeds on the young stems and (sometimes) leaves of hazel. This results in a loss of nutrients. Photosynthesis may be reduced due to the growth of black sooty mould on the aphids' honeydew excretions. The reduced photosynthesis may result in substantial crop losses in the long term. Large populations of aphids feeding on husks may cause premature nut drop and lower nut quality (Tuncer & Ecevit, 1997).
Gantner (2001) gives an account of hazelnut pests in southeastern Poland. The aim of this research was to characterize the species composition of the fauna of hazelnut pests in three ecosystems formed by man to various degrees: a protected hazelnut plantation, an unprotected plantation where no chemical treatment or horticultural techniques were applied, and on shrubs of common hazel in the forest. In the protected plantation, aphids (Myzocallis coryli and Corylobium avellanae), scale insects (mostly Parthenolecanium corni) and mite species (mostly Phytoptus avellanae, which causes hazel big bud gall, shown below) occurred in the greatest number. In the unprotected plantation and on common hazel growing in forest, a greater diversity of pest species was found.
Viggiani (1994) reviews the management of hazelnut pests & diseases. Given the hazelnut agroecosystem is characterised by relative stability in space and time, even in highly intensive cultivated areas, their pest and disease problems are similar. In Europe, the major pests are the gallmite Phytoptus avellanae (cause of big bud gall, shown above) and the weevil Curculio nucum (shown below).
Severe damage is also caused by a number of other species, including the aphids Corylobium avellanae and Myzocallis coryli. In the USA, the filbertworm moth (Cydia latiferreana, shown below) is the most important pest. Other major pests include the aphid Myzocallis coryli. Viggiani suggested infestations of Myzocallis coryli are at least partially induced by the excessive use of insecticides - and recommended they should be avoided, or used with more caution.
In the USA most hazelnut production is concentrated in the Willamette Valley, Oregon. Walton et al., (2001) described the status of the newly invasive hazelnut aphid (Corylobium avellanae) in Oregon. Current key insect pests in this area included the filbertworm moth (Cydia latiferreana, shown above), the 'filbert aphid' (Myzocallis coryli), and the hazelnut aphid (Corylobium avellanae). Before the mid 1980s, the filbert aphid was considered the only aphid pest on hazelnuts in Oregon. Corylobium avellanae, a newly invasive species, was however recorded on hazelnut in the northern Willamette Valley by the Oregon Invasive Species Council during October 2003. Recent collections show that this pest has spread. At the time of the study, Corylobium avellanae had low levels of parasitism compared to filbert aphid. Data show that Corylobium avellanae prefers husks to feed on. In severe cases this feeding preference may negatively impact crop levels. Levels of hazelnut aphid are therefore monitored in order to determine changes in species composition.