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Identification & Distribution:Apterae of Neotoxoptera formosana are shining magenta-red to dark reddish brown or almost black.. The body length of the aptera is 1.6-2.3 mm. Winged forms are very dark red to black with the wing veins heavily black-bordered. The borders on the wing veins of Neotoxoptera formosana are of rather constant width along the lengths of the veins (cf. Neotoxoptera oliveri where the borders widen out at the base and apex of each vein).
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The clarified slide mounts below are of adult viviparous female Neotoxoptera formosana : wingless, and winged.
The onion aphid does not host alternate but spends it entire life cycle on onion (Allium), either on the leaves or on bulbs in store. There are no sexual forms, so the species reproduces parthenogenetically all year round. Neotoxoptera formosana is native to east and south-east Asia, but is invasive in Australia, New Zealand, Hawaii, North & South America, and north-west Europe.
Biology & Ecology:Neotoxoptera formosana is not considered to be normally resident in Britain, so we were surprised to find a flourishing colony on an onion purchased at a supermarket in Inverness, Scotland in August 2013 (see picture below). This is only the second record of the species from Scotland since it was first reported from Fife in 2008 (Reid, pers. comm.).
The onion aphid was first reported in Britain on onion at RHS Wisley in October 1999 (Halstead, 2000 ). Attempts to eradicate Neotoxoptera formosana appeared successful, although the pest was again found on Allium at Wisley in May 2000 having probably overwintered on garlic cloves in a cold frame. Infested plants were sprayed and the pest was considered eradicated at Wisley. It has since been trapped several times in 40ft aerial suction traps (MacLeod, 2007 ), but it is not known whether these are migrants from resident British populations or from mainland Europe.
Neotoxoptera formosana was first found in Europe in 1984 in France (Leclant, 1999 ). Since then, the onion aphid has been observed regularly in low numbers in Britain and other European countries. Interception of Neotoxoptera formosana by Finland on onion bulbs from The Netherlands suggests that it may have been present in The Netherlands in 1994. Van Dijk (1993) reported that various Allium species grown in pots in the open and in glasshouses at Wageningen in The Netherlands, were at times found heavily infested with aphids which may have been Neotoxoptera formosana. In July 2000, Neotoxoptera formosana was reported for the first time in Italy. It was found on Allium schoenoprasum (chives) grown under glass as a continuous crop (Barbagallo & Ciampolini, 2000 ). It was reported from Germany in 2008 (Schrameyer 2008 ), and confirmed from The Netherlands in 2008 (Piron, 2010 )
Rather few observations seem to have been made on the biology of Neotoxoptera formosana. Costa et al.(2012) assessed the development and reproduction of the chrysopid Chrysoperla externa when fed with Neotoxoptera formosana. They concluded that the aphid is a viable food source for C. externa and that the predator's potential for pest management programmes should be investigated. Piron (2010) observed that the 'crowding effect' - the development of winged forms when food runs out - did not to seem to operate for onion aphids since in his food-stressed colonies no alates developed. However, different aphid species uses different environmental cues (for example day length) and one cannot conclude (as Piron did) that the aphid has poor dispersal abilities.
Hori & Komatsu (1997) investigated the olfactory behaviour of Neotoxoptera formosana to host and non-host plant odours. They were attracted by odours of Allium species, and repelled by odours of some non-host plants, namely rosemary and pennroyal. Hori (2007) identified the two sulphur compounds in Allium plants, dipropyl trisulphide and diallyl sulphide, responsible for attracting the aphids, and concluded that they used these as olfactory cues to find the host plants.
Damage and controlNeotoxoptera formosana populations can build up rapidly to damaging levels. Feeding damage on Allium schoenoprasum, reported in Italy, include wilting, yellowing and desiccation (Barbagallo & Ciampolini, 2000 ). Neotoxoptera formosana may also transmit plant viruses. In Japan, it has been shown to transmit Garlic latent virus (GarLV) between Allium species (Sako et al., 1990 ). It can also transmit Alstroemeria mosaic virus (AlMV) (Yasuda et al., 1998 ) and an unidentified virus that produces mosaic symptoms, leaf curling, yellowing and dwarfing in garlic (Abiko et al., 1980 ).