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Rhododendron aphidIdentification & Distribution Biology & Ecology Damage & Control
Identification & Distribution:Both winged and wingless Illinoia lambersi exist as green, pink and yellow forms, which can occur in mixed populations. The images below show an adult Illinoia lambersiaptera and an alate of the green form. The apices of the antennal segments and the tips of the legs are dark. The siphunculi are mostly pale but with darkened apices. They are slightly swollen, strongly attenuated near the tip, and 2.0-2.5 times longer than the cauda. The cauda is mainly pale but dusky towards the tip. The body length of Illinoia lambersiThe second segment of the hind tarsus is rather short and less than 1.5 times longer than maximum diameter of the swollen part of the siphunculi. This is in contrast to its close relative Illinoia azaleae (also found on Rhododendron) which has the second segment of the hind tarsus more than 1.5 times longer than that diameter. Other differences are that Illinoia azaleae is mainly found on azaleas and does not exhibit colour polymorphism (all are green).
The alate of Illinoia lambersi has rather indistinct abdominal marginal and intersegmental pleural sclerites. The antennae are only slightly longer than the body and have 21-30 secondary rhinaria placed along the whole segment, not precisely in a row (cf the antennae of Illinoia azaleae which have 7-21 secondary rhinaria in a row).
The clarified slide mounts below are of adult viviparous female Illinoia lambersi : wingless, and winged.
Colonies of Illinoia lambersi are mainly found on the new shoots and flower buds of rhododendron (Rhododendron), but have also been found on holly (Ilex aquifolium). Illinoia lambersi is indigenous to North America where it is widely distributed and sexual forms occur. It is now also well established on rhododendron in South America (Chile), Japan and parts of Europe including Denmark, England and the Netherlands where it overwinters as viviparae.
Biology & Ecology:
First reports of Illinoia lambersi outside its native North America came the Netherlands (Hille Ris Lambers,1973 ), and then from Britain (Stroyan, 1971 ) both in the 1970's. The species is now established over much of Europe.
Over the last few years we have found Illinoia lambersi on evergreen rhododendrons (mainly Rhododendron ponticum) in many locations in southern England, more or less wherever rhododendron occurs. Large populations develop on the young shoots in June/July when rhododendron produces its new flush of leaves after flowering. Aphid numbers decline as the leaves age, but occasional new shoots are rapidly colonized through summer and autumn. They overwinter as viviparae sheltered inside developing buds.
Of the three different colour forms, the green form (see pictures above) is the commonest,
The yellow form (see pictures above) is rather less common than the green form.
The pink-red form seems to be the rarest of the colour forms, but can nevertheless usually be found in most populations.
The different colour forms often occur mixed together in the same colony as can be seen in the picture above. As with many cases of red-green polymorphism in aphids, the functional significance of the different colour forms is not obvious (see red-green polymorphisms )
Rhododendron protects itself against herbivores by producing a sticky exudate containing toxic phenols, which covers young emergent lead buds (CABI, 2015 ). This exudate catches and kills herbivorous insects and any of a wide range of other flying insects that make the mistake of landing on it (see first picture below).
The rhododendron aphid is generally very adept at not getting trapped by this exudate, but just occasionally one can find the odd nymph which has become trapped (see second picture above).
Despite having observed many colonies of Illinoia lambersi, we found little or no evidence of parasitoid activity. A similar observation was made by Hille Ris Lambers (1973) . Syrphids do however lay eggs near colonies (see first picture below), and the rhododendron aphid is sometimes affected by parasitic mites, most likely of family Trombidiidae (see second picture below).
The toxic phenols in rhododendron undoubtedly limit the activities of potential natural enemies of the rhododendron aphid, which is itself presumably able to detoxify or sequester the toxins - at least in the concentrations present in young growth.
Damage and control
Rhododendron is grown for ornamental and landscape purposes, and heavy aphid infestations can detract from its appearance.
The leaves of infested shoots remain undeveloped or are distorted, as shown in the picture (above right). Nevertheless aphid control is unlikely to be worthwhile.