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Genus Phorodon [Macrosiphini]
Phorodon are medium-sized aphids on their winter hosts (2.0 - 2.6mm long), but smaller on their summer hosts. They are generally green, and adult viviparae may be winged or wingless. Phorodon aphids have characteristic projections on the inside of the antennal tubercles and on the inner side of antennal segment I. Their siphunculi are pale, medium length, thicker at their bases and slightly curved outwards at their tips. The cauda is short, pale and blunt. Winged forms have a black patch of fused cross bars on the upper surface of the abdomen.
Phorodon is a small genus of only 4 species in Europe, north Africa, and south-west Asia, and has been introduced to North America and New Zealand. They have a sexual stage in the life cycle. Some alternate between the primary host of blackthorn and plum (Prunaceae) and the secondary host, hops (Cannabidaceae). Other Phorodon species feed only on the secondary hosts. They are not attended by ants. One species (Phorodon humuli) is the main pest on hops, causing damage directly and through virus transmission.
Phorodon cannabis (Hemp aphid, Cannabis aphid)
Adult apterae of Phorodon cannabis are pale yellowish green without stripes (see first picture below), or yellowish green with one or three longitudinal darker green stripes, or pink with or without stripes (see second picture below). Phorodon cannabis has capitate hairs on the head, thorax, and basal antennal segments (cf. Phorodon humuli which does not have capitate hairs). Each antennal tubercle has a finger shaped process on its inner face that points forward (see clarified mount below). The antennae have a terminal process that is 4.6-5.7 times the length of the base of antennal segment VI (cf. Phorodon humuli which has a terminal process which is 3.4-4.4 times base of antennal segment VI). The apical rostral segment (RIV+V) is 1.4-1.6 times the length of the second hind tarsal segment (HTII) (cf. Phorodon humuli in which RIV+V is 1.1-1.2 times HTII). The siphunculi are cylindrical with a thicker base, are curved outwards, and are 2.9-3.4 times the length of the cauda. The cauda has 6-10 hairs. The body length of adult Phorodon cannabis apterae is 1.6-1.8 mm.
All images above copyright Whitney Cranshaw under a Creative Commons Attribution 3.0 License.
The alate Phorodon cannabis vivipara (see third picture above) is yellowish green or reddish with large marginal and intersegmental pleural sclerites, a dark dorsal patch on tergites IV-VI and cross bars on VI and VIII. The antennae have 15-30 secondary rhinaria on segment III, 2-13 on segment IV and 0-2 on segment V. The siphunculi are dusky, but the cauda is pale.
Phorodon cannabis feeds on the undersides of the leaves and on the flower stems of various cannabis species. (Cannabis sativa, Cannabis ruderalis, Cannabis indica). Reports of Phorodon cannabis on hops and Prunus are probably misidentifications of Phorodon humuli since transfer trials have failed to establish Phorodon cannabis on hops. The species' distribution extends from central, eastern and southern Europe into Turkey and much of Asia and North Africa. The hemp aphid has been introduced into North America where it is widespread in Colorado and has recently been reported from both coasts and in Canada. Phorodon cannabis now is likely present in many areas of North America, at least in areas associated with some indoor production of host plants.
Phorodon humuli (Damson - hop aphid)
Phorodon humuli apterae are small to medium sized, whitish to pale yellowish green and relatively shiny. The abdomen is marked with three dark green longitudinal stripes (see first picture below). Antennal segment I has a protuberance on the inner side, and each antennal tubercle has a long forward-pointing projection (best seen on preserved specimens below). The siphunculi are pale with slightly dusky tips and are more than twice as long as the pale cauda. The body length of the Phorodon humuli adult aptera is 2.0-2.6 mm on plum, and 1.1-1.8 mm on hop.
The damson-hop aphid host alternates from blackthorn or plum (Prunaceae) to hops (Cannabinaceae). It does not cause leaf curling itself but often lives on plum leaves distorted by Brachycaudus helichrysi. Migration of winged forms to hops takes place from late spring. There is a return migration to the winter hosts in September, where sexual forms are produced and eggs laid. Phorodon humuli is indigenous to Europe and neighbouring areas, and has been introduced to North America and New Zealand.