The probing behaviour of the planthopper Delphacodes kuscheli (Hemiptera: Delphacidae) on two alternating hosts, maize and oat

Delphacodes kuscheli is the most important natural vector of Mal de Rio Cuarto virus (MRCV) in Argentina, a Fijivirus (Reoviridae) that causes important economic losses on maize production. Although this planthopper does not breed successfully on maize, virus transmission occurs when adults migrate from oat, a winter host and a reservoir of the virus, to maize. Probing behaviour on both hosts was recorded and analysed using the direct current electrical penetration graph (DC-EPG) system. Eight main waveforms were identified during probing behaviour by D. kuscheli. These waveforms were interpreted with reference to that described for delphacids and were related to putative probing activities: np (non-penetration or non-probe); N1, N2 and N3 (stylet pathway: penetration, salivation, stylet movement and extracellular activities near the phloem region); N4-a (sieve element salivation); N4-b (phloem sap ingestion); N5 (xylem activity); and N6 (derailed stylet mechanics). The EPG variables showed that on maize, D. kuscheli exhibited longer periods of non-probe and longer time from first probe to first phloem activity, with a higher percentage of probing spent in stylet pathway and xylem phase than on oat. Furthermore, on maize, fewer insects showed phloem phase activities (N4-a and N4-b) and sustained phloem ingestion than on oat, and the time on phloem ingestion was shorter than on oat. The constrained behaviour of D. kuscheli on maize suggests that plant resistance factors may be involved.