Biology, images, analysis, design...
|"It has long been an axiom of mine that the little things are infinitely the most important" |
Holcus grass 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 Schizaphis holci (see first picture below) are greenish yellow or pale yellow usually with a faint darker green spinal stripe. The apical rostral segment (RIV+V) is 0.8-1.0 times the length of segment II of the hind tarsus (cf. Schizaphis agrostis which has the apical rostral segment 0.75-0.88 times the length of segment II of the hind tarsus). The siphunculi are pale with dark tips (cf. Metopolophium dirhodum, which has the siphunculi completely pale). The siphunculi are 1.25-1.4 times the length of the pale cauda (cf. Schizaphis agrostis which has the siphunculi 1.3-1.5 times the length of the cauda). The body length of adult Schizaphis holci apterae is 1.6-1.9 mm.
The alates of Schizaphis holci (see second picture above) are brownish-green in colour with black-tipped siphunculi. The pictures below show an adult aptera and an alate in alcohol.
Schizaphis holci is found on the undersides of leaves of Yorkshire fog (Holcus lanatus) and creeping soft grass (Holcus mollis). Colonies are usually attended by ants. Oviparae and apterous males develop in late September and October. In the UK Stroyan (1984) describes this species as rare but perhaps overlooked.
Biology & Ecology
Until recently Schizaphis holci (and Schizaphis agrostis) have been considered rare in the UK (Kati et al., 2013). However, suction trap catches suggest they have become more common since around 2000.
The younger instars often live inside the rolled new grass shoots where they are largely protected from weather and natural enemies.
Colonies develop on Holcus grass in May, and by June quite large colonies may have developed on a single grass leaf.
All the colonies we found were in early June - so far we have found none later in the year. Suction trap catches show a peak of alate females in late May/early June (Kati et al., 2013). Numbers then decline with sexuales produced in late September and October.
Schizaphis holci are often, but not always, attended by ants. Most of the colonies we found in East Sussex were attended by Lasius niger.
There was usually only one ant attending per colony.
None of the aphids found by Kati et al. (2013) were attended by ants.
Schizaphis holci colonies in Sussex have been attacked by at least two species of braconid parasites (so far unidentified), one producing a brown mummy and the other a black mummy (see pictures below).
The black mummy was probably produced by an Aphelinus species. Japoshvili & Abrantes (2006) also listed Aphelinus abdominalis and Aphelinus mali as Schizaphis parasitoids.
Other aphids on the same host
Schizaphis holci has been recorded from 2 Holcus species (Holcus lanatus, Holcus mollis).
Damage and control
Feeding by Schizaphis holci has a phytotoxic effect, and leaves hosting a colony turn a characteristic pinky orange-brown (see picture below) and soon die.
Such leaf discoloration in May or June is usually the best indicator that the aphid is present.
Are they a threat to crops
The increasing numbers of winged (alate) aphids morphologically resembling the greenbug (Schizaphis graminum) caught in 12.2 meter high suction-trap samples of the Rothamsted Insect Survey (RIS) prompted a study by Kati et al. (2013) to establish their identity. The results from the host-choice and life history studies performed on the Schizaphis species found in the UK on Holcus lanatus suggested that the species involved was actually Schizaphis holci and hence unlikely to be a threat to crops.
Morphometric characteristics of alatae caught in suction traps indicated that the suction-trapped alatae were Schizaphis agrostis or Schizaphis holci, with no Schizaphis graminum being found. However, two UK specimens had identical cytochrome c oxidase subunit I (COI) sequence to the US sorghum-adapted form of Schizaphis graminum, suggesting this form of Schizaphis graminum may be present in the UK. - However those suction-trapped alatae may have been carried by the jet-stream air currents from North America. There are no records of Schizaphis graminum from field crops in northern Europe.