The probing activities of the most important leafhopper vector of maize streak virus, Cicadulina mbila (Naude) (Hemiptera: Cicadellidae) were studied on different host-plants. Host preference was determined by giving the insects a choice of six plants, Digitaria sanguinalis, Zea mays, Zea perennis, Paspalum conjugaium, Oryza sativa and Saccharum officinarum (all Gramineae). Cicadulina mbila showed a strong preference for D. sanguinalis. Histological examination of plant tissues for salivary sheaths was used to reveal the preferred tissue. Cicadulina mbila tended to ingest from the phloem of its preferred hosts, and switch to mesophyll or xylem tissue on less preferred hosts. The suitability of each host was assessed by measuring the weight gain and amount of honeydew produced on each host by newly emerged adult females. Both parameters were greatest on the preferred host. Probing behaviour was also monitored electronically by recording the different waveform patterns produced when insects were probing, ingesting or not probing on the selected hosts. Cicadulina mbila probed less frequently, ingested longer and spent less time in non-probing activities on its preferred host, D. sanguinalis, although these responses were similar on maize and sugarcane. Rice induced poor probing responses and altered behaviour. This interpretation of the waveforms was supported by histological examination. As maize streak virus (MSV) is not found in phloem sieve elements, prolonged ingestion from sieve cells would not account for efficient acquisition from maize, which would require ingestion from other types of cells. D. sanguinalis was the host in which most ingestion from the phloem occurred, but it is the poorest source of MSV.

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SummaryThe feeding activity of Aphis craccivora (Koch) was monitored on cowpea plants of aphid‐susceptible (Vita 7) and aphid‐resistant (TVu 801) cultivars, using an AC electronic feeding monitoring system. Waveforms corresponding to salivation, phloem ingestion and non‐phloem ingestion were observed. Aphids probing on the resistant cultivar showed a significantly reduced ingestion of phloem sap compared with those aphids which fed on the susceptible cultivar. The insect also made more brief and repeated probes on TVu 801. In addition, the duration of non‐probing activities and non‐phloem ingestion was shorter on the susceptible than on the resistant cultivar.

The cause of a streak disease of pearl millet (Pennisetum glaucum), originating from Nigeria, has been attributed to a geminivirus belonging to the 'African streak virus' cluster. A full-length, infectious clone of the virus was obtained which was transmissible by the vector Cicadulina mbila (Naud6). Analysis of the complete nucleotide sequence of the coat protein gene of this virus shows it to be most closely related to sugarcane streak virus. The possible evolutionary implications of this finding are discussed.