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Strawberry Root Louse, Strawberry Root AphidOn this page: Identification & Distribution Biology & Ecology Life cycle Ant attendance Natural enemies Other aphids on the same host Damage & Control
Identification & Distribution
Adult apterae of Aphis forbesi (see first picture below) are dark bluish green, sometimes with bluish-yellow mottling. The antennal tubercles are weakly developed. The apical rostral segment (RIV+V) is 1.6-1.9 times the length of the second hind tarsal segment (HTII) (cf. Aphis fabae & Aphis gossypii which both have a shorter RIV+V at 0.9-1.5 times the length of HTII). The cauda is tongue-shaped and longer than its basal width. The body length of adult apterae is 1.0-1.9 mm. Immature Aphis forbesi are yellowish green.
Aphis forbesi lives in colonies on the roots, shoots and bases of leaf petioles of strawberry (Fragaria spp.) where it is attended by ants. The ants build nests around the aphid colony, which makes fruit harvest and chemical control difficult (Araujo et al., 2013). Aphis forbesi does not host alternate. Sexuales, oviparae and apterous males, are produced in autumn. This aphid is native to North America, but was introduced into Europe about 1928. It is now also found in Kazakhstan, west Siberia, Japan and South America.
Biology & Ecology
Marcovitch (1925) describes the life cycle of Aphis forbesi. The strawberry root louse spends its entire life cycle upon strawberry, with no host alternation. In summer it migrates only to other strawberry fields. The insect passes winter in the egg stage either upon the pedicels of the leaves or on the underside of the leaves. In Tennessee hatching was from mid-February to mid-March. Upon hatching the new-born nymphs are pale green in color, with legs, antennae, and siphunculi (=cornicles) whitish, but they darken slightly over time. After passing through five instars the fundatrix (=stem-mother) becomes mature - this takes two to three weeks, depending upon the temperature.
The fundatrix gives birth to an average of two young per day for a period of 20 to 28 days. The immatures initially feed where they hatch, but after a short time they find their way to the tenderest leaves just coming out of the crown. The majority of the second generation of the insect becomes wingless viviparous females although some winged ones are often present and distribute the species to other strawberry plants or beds. The second and succeeding generations are usually found feeding on the pedicels of the young tender leaves, but are often deeply imbedded in the crown of the plant. They are sometimes located on the roots of the plants, but in Tennessee this is rare.
The males and oviparae make their appearance in Tennessee late in October, and may be found on the plants until February, when the eggs are ready to hatch. Marcovitch (1925) demonstrated experimentally that day length is the main factor affecting the production of sexual forms, mediated to some extent by temperature.
Weed (1889) described how Aphis forbesi were founds upon the roots and lower portions of the crown, where they were carefully attended by the brown ant (Lasius alienus), which carries them away in its jaws upon the approach of danger, treating them exactly the same way it treats the corn root louse (Protaphis middletoni - misnamed Aphis maidis in Weed's paper) in corn fields. From the discovery of aphid eggs about the strawberry roots he surmised that the ants take care of them through the winter as they do with Protaphis middletoni. In Tennessee Marcovitch (1925) observed the brown ant (Pheidole vinelandica) protecting the aphids. They do this by "building small crater-like mounds of dirt about the crown, so that the pedicels of the young leaves may be entirely covered and concealed from view. These little craters are often 2.5 inches [6.3 cm] high, and sometimes completely cover the crown of the plant. In such cases lice may be found feeding on the crown of the strawberry".
Sanderson (1900) recorded several hymenopterous parasites, among which were Lysiphlebus testaceipes (= Lysiphlebus myzi), Lysiphlebus salicaphis (= Lysiphlebus salicaphidis), and Lygocerus stigmatus (plus another species, Adialatus densleonis Ashmead, which we cannot find any reference to in the taxonomic literature).
Marcovitch (1925) identified the braconid parasitoid, Diaretus fuscicornis, and the syrphid, Paragus tibialis, as two of the most important natural enemies of Aphis forbesi. He also noted several ladybird beetles, such as Hippodamia convergens and Coccinella novemnotato, were frequent predators in strawberry louse colonies.
Araujo et al. (2013) recorded Lysiphlebus testaceipes parasitizing Aphis forbesi in strawberry crops in Brazil, and noted that it has been considered for the biological control of other aphid pest species. Previously recorded parasitoids include Adialytus fuscicornis, Lysiphlebus fabarum and Trioxys gahani.
Other aphids on same host:
Aphis forbesi has been recorded from 7 Fragaria species (Fragaria-×-ananassa, Fragaria chiloensis, Fragaria moschata, Fragaria vesca, Fragaria virginiana, Fragaria viridis, Fragaria yezoensis). Note: Fragaria-×-ananassa (the garden strawberry) is a hybrid of Fragaria chiloensis (=beach strawberry, Chilean strawberry, coastal strawberry) and Fragaria virginiana (=Virginia strawberry, wild strawberry, or common strawberry).
Damage and control
Blackman & Eastop (1984) note that Aphis forbesi is apparently not very injurious to strawberries, except perhaps in light sandy soils. Current control methods are focused on including the species in integrated control programmes (Bernard et al., 2013). In protected environments like glasshouses there is great potential for biological control using the parasite 'cocktail' approach (de Menten, 2011) and/or release of coccinellid larvae (Riddick, 2013, Seo & Youn, 2002).