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Hemp aphid, Cannabis aphidOn this page: Identification & Distribution Biology & Ecology Other aphids on the same host Damage & Control
Identification & Distribution
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 (see second picture below of immature), or pink with or without stripes (see third 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, the terminal process of 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. The images below show (first) a clarified mount of a Phorodon cannabis aptera and (second) a colony of this aphid on the underside of a hemp leaf.
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.
Biology & Ecology
Müller & Karl (1976) carried out field studies of Phorodon cannabis in the German Democratic Republic where the aphid is found wherever hemp is cultivated. The larvae of the fundatrices hatch in early April. Cultivated hemp is an annual plant and practically the only host in Central Europe - hence host finding is rendered extremely difficult for the fundatrices, which hatch from overwintering eggs. The two generations following the fundatrix produce around 80% alatae or more. As a result the build-up of the populations is delayed and large populations seldom occur until August.
Image above copyright Whitney Cranshaw under a Creative Commons Attribution 3.0 License
In autumn the sexual forms (egg-laying females and winged males, see first picture below) develop, and after mating the females start to deposit eggs.
Images above copyright Whitney Cranshaw under a Creative Commons Attribution 3.0 License.
Phorodon cannabis eggs are yellow-green when first laid, but as they harden they turn black - as most are in the second picture above. The white egg has been laid not by the aphids but by a syrphid fly, the larva of which is a voracious aphid predator. The oviparous female aphids prefer to lay their eggs lay their on the hemp plants' seed capsules. In an open-air insectarium the total egg-laying in the populations spanned the period from mid-September to late November.
Halbert et al. (2016) reported that the most important predators of Phorodon cannabis in the field were convergent lady beetle (Hippodamia convergens, see first picture below) and multicoloured Asian lady beetle (Harmonia axyridis), with at least four other coccinellid species common in Colorado hemp fields. Three species of green lacewing were present, with Chrysopa oculata and Chrysoperla plorabunda the most common. Larvae of at least four species of hover fly (Syrphidae) and Aphidoletes aphidimyza (Cecidomyiidae) also were observed feeding on cannabis aphids in hemp fields. There were also numerous generalist predators including the bugs Orius insidiosus (Anthocoridae), Nabis alternatus (Nabidae), Chlamydatus associatus (Miridae) and the big-eyed bug (Geocoris punctipes) (Geocoridae, see second picture below).
First image above copyright Katja Schulz under a Creative Commons Attribution 2.0 Generic license.
Also present were larvae of undetermined species of Diptera in the genera Platypalpus (Hybotidae), and Condylostylus (Dolichopodidae). Parasitism by braconids and entomopathogenic fungi have also been observed but only infrequently.
Other aphids on the same host
Phorodon cannabis has been recorded from 4 Cannabis species (Cannabis sativa, Cannabis gigantea, Cannabis indica, Cannabis ruderalis) and possibly Humulus lupulus (common hop or hops), which is also a member of the Cannabaceae - plus an unidentified Artemisia species.
The rice root aphid, Rhopalosiphum rufiabdominale mainly lives on the roots of cannabis and is the most common aphid reported from indoor marijuana production in Colarado. Plants supporting large populations can show a significant decline ( Cranshaw et al., 2019).
Damage and control
Feeding by Phorodon cannabis is reported to cause wilting and yellowing of cannabis (Oregon Department of Agriculture, 2017). However, Halbert et al. (2016) noted that although large numbers of the aphid were observed in hemp fields, accompanied by production of large amounts of honeydew, no obvious plant injury was apparent. The outbreaks were on large, well-irrigated and well-established plants nearing maturity, conditions that favour plant tolerance of aphid injuries. In addition, numerous natural enemies (see above) were observed predating Phorodon cannabis on field-grown hemp. These natural enemies were found to build populations that could effectively control the aphids present in the late-season outbreaks.
In protected environments such as greenhouses biological control of Phorodon aphids is feasible by release of (for example) lacewing larvae, as described by McPartland et al. (2000). If there are fewer than 10 aphids per leaf, one should release 25 larvae per square metre in hotspots and 10 per square metre in surrounding areas. For 11-50 aphids per leaf, the release rates are 50 larvae per square metre in hotspots and 25 per square metre in surrounding areas.
There appears to have been little work done regarding chemical control of Phorodon cannabis. The California Department of Food & Agriculture (2019) report that the Department of Pesticide Regulation (2015) lists azadirachtin, horticultural oil, insecticidal soap, rosemary and peppermint oils, and the pathogen Beauveria bassiana as potential agents for the control of aphids on cannabis, including Phorodon cannabis. Phorodon cannabis is reported to vector at least two plant viruses (cucumber mosaic virus and alfalfa mosaic virus) to Cannabis sativa (Schmidt and Karl, 1970).