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All viviparae of Monelliopsis are alate. The alate vivipara has the front of the head a little produced in the middle with poorly developed antennal tubercles. The epicranial suture is sometimes weakly developed. Hairs on the head are sometimes on wart-like elevations or processes. The antennae are six segmented. The terminal process is often shorter than, sometimes subequal to, or a little longer than the base of antennal segment VI. Secondary rhinaria are suboval to narrow elliptical without cilia. In some species the pronotum, mesonotum and metanotum have spinal setae on wartlike elevations. The wings are held upright at rest - which is the most obvious distinction between Monelliopsis species and Monellia species which lay their wings flat. The forecoxae are greatly enlarged, their margins visible at sides of pronotum. The empodial setae are spatulate. There are spinal, pleural and marginal setae on each of the tergites, with those on the anterior tergites often on mammiform processes. Marginal tubercles are developed on tergites I-V, those of tergite V often with a black spot. The siphunculi are nearly poriform, with marginal seta of tergite VI appended to the posterior margin or placed behind it. The cauda is knobbed, and the anal plate bilobate. Viviparae are without full melanistic forms, although pigmented spots on the abdomen are more commonly developed in the late seasonal form.
There are ten species in the genus Monelliopsis. Most feed on walnuts (Juglans species), although some also visit pecan trees (Carya spp.), and three only feed on pecan (Monelliopsis bisselli, Monelliopsis pecanis, Monelliopsis pleurialis). They are all native to North America, but at least two species have been introduced to walnut or pecan producing countries in Europe, Asia or Africa.
Monelliopsis caryae (American walnut aphid)
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).
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.
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, 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 on Juglans mandshurica.
Monelliopsis pecanis (Yellow pecan aphid)
All adult viviparae of Monelliopsis pecanis are alate. These alatae (see first picture below) are yellow with reddish eyes, and have the dorsum either unmarked or with a few dark spots. The antennae are pale apart from the apical parts of segments and the section of antennal segment III bearing secondary rhinaria which are dark (cf. Monelliopsis bisetosa, which has the antennae mainly dark except at the bases of segments). The rhinaria-bearing section of antennal segment III is only slightly swollen: up to 1.5 times the basal diameter of the segment (cf. Monellia caryella, which has the rhinaria-bearing section conspicuously swollen: it is more than 1.5 times the basal diameter of the segment). The subcoxa (= coxa base) of the hind leg has no transverse mark (cf. Monelliopsis bisselli, whose subcoxa of hind leg usually has a dark transverse streak). The distal ends of the hind-femora (and to a lesser extent the fore-femora) are marked with conspicuous black patches. The tibiae are mainly pale (cf. Monelliopsis bisetosa, which has dark tibiae).
First image above copyright Domingo Zungri under a Creative Commons Attribution 3.0 License.
The alatae of Monelliopsis pecanis usually have 2-5 spinopleural hairs on each of abdominal tergites I-V in spring, and 4-6 spinopleural hairs on each segment in midsummer; in summer a few of the hairs may have dark bases, especially on tergite V (cf. Monelliopsis nigropunctata, which has similar numbers of hairs but most hairs have dark bases, so it usually has more dark spots per segment than Monelliopsis pecanis). The spinopleural hairs on abdominal tergites I-V are often hardly raised above the surface, especially those on tergites I & II (cf. Monelliopsis pleurialis, in eastern North America, which has the spinopleural hairs on tergites I & II arising from low processes). The body length of the adult Monelliopsis pecanis alate is 1.2-1.7 mm.
The yellow pecan aphid lives and feeds on the leaves of pecan (Carya illinoiensis, see picture above) and bitter pecan (Carya aquatica), and does not host alternate. Populations are concentrated along the main leaf veins. Sexual forms develop in autumn, and the oviparae deposit overwintering eggs in bark crevices of pecan. Monelliopsis pecanis is indigenous to USA and Mexico, but has been introduced to Egypt, Sicily and South Africa.