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American walnut aphidOn this page: Identification & Distribution Other aphids on the same host Damage
Identification & Distribution
All adult viviparae of Monelliopsis caryae are alate. Adult Monelliopsis caryae are yellow-green with banded antennae, but their other markings vary seasonally: Spring alates (see first picture below) have no dark markings apart from the banded antennae - which may cause problems if keys assume they are marked. Summer alates (see second picture below) have a dusky median streak on the head, 4 transverse rows of dark spots at the bases of dorsal abdominal hairs, and a black transverse streak on the hind subcoxa (= proximal part of coxa) (cf. Monelliopsis quadrimaculata on Juglans major in Arizona, and Monelliopsis pecanis on Carya pecan, neither of which have a black streak on the hind subcoxa). The antennal terminal process is 1.0-1.3 times the length of the base of antennal segment VI (cf. Monelliopsis quadrimaculata, which has the terminal process 0.7-0.8 times the base of antennal segment VI). The pronotum and metanotum are without processes (cf. Monelliopsis tuberculata on Juglans nigra in New Mexico, Idaho and Utah, which has well-developed processes on the pronotum and metanotum).
Note: the unpigmented specimen (first image below) was identified as Monelliopsis caryae by Eric Maw of Agriculture et Agroalimentaire Canada. The fully pigmented specimen (second below) was identified by Natalie Hernandez from Bill Keim's photograph in BugGuide (see also Biodiversidad).
The abdominal tergites I-VI have flattened hair-bearing processes each bearing one pair of spinal hairs (cf. Monelliopsis pallida on Juglans hindsii in California, and Monelliopsis nigropunctata on Juglans throughout USA, both of which have at least 4 spinopleural hairs on each of tergites I-VI). In summer the distal ends of the fore- and hind-femora are marked with conspicuous black patches. The tibiae are pale (cf. Monelliopsis californica on Juglans californica in California and Utah and Monelliopsis bisetosa on Juglans major in western USA, both of which have dark tibiae). The siphunculi are reduced to pore-like chitinized rings. The cauda is yellow, short and globular. The body length of adult alatae is 1.2-1.8 mm.
Immature Monelliopsis caryae are pale yellow to orange with from six to eight rows of prominent tubercles, each bearing stiff long capitate hairs. The tubercles are pale early in the year (see pictures below), but by midsummer are dusky or dark and very conspicuous.
Images above by permission, copyright Claude Pilon, all rights reserved.
Monelliopsis caryae is found on the undersides of the leaves of black walnut (Juglans nigra) and white walnut (Juglans cinerea). It does not host alternate, only feeding on walnut. Despite the production of abundant honeydew, which accumulates on the upper surface of the leaves below colonies (Tasheva-Terzieva, 2006), there is no ant attendance. Sexual forms develop in autumn and, after mating, the oviparae deposit overwintering eggs. Monelliopsis caryae is widely distributed in North America. It has also been (accidentally) introduced in about 1984 to Europe - it was first recorded in France in 1998, Portugal in 1998, Spain in 2002 and Bulgaria in 2006. More recently the American walnut aphid has been recorded in South Korea (Lee et al., 2018) on Juglans mandshurica.
Other aphids on the same host
Monelliopsis caryae is found on 3 species of Juglans (Juglans cinerea, Juglans nigra, Juglans mandshurica).
Walnut aphids reduce walnut yield by reducing nut quality, inhibiting the growth of seedlings and causing sooty mould to grow because of the abundant honeydew production.