Neonicotinoids and pyrethroids target-site resistance in Italian populations of the green peach aphid (Myzus persicae)

The green peach-potato aphid Myzus persicae (Sulzer) (Hemiptera: Aphididae) is one of the key pests of many agricultural and horticultural crops, causing significant damage both by direct feeding and by the transmission of many plant viruses. This polyphagous aphid is distributed worldwide and it is responsible for major economic losses. In Italy the most damaged cultivations are peach orchards and open field cultures such as sugar beet, potato and tabacco. Its control is mainly achieved by chemical treatments and in the recent years neonicotinoids and pyrethroids have been key components of the pest management strategies used by growers. Recent resistance monitoring programmes in Southern Europe have shown the widespread distribution of resistance populations of the green peach aphid, posing a serious threat to the long-term efficacy of these insecticide classes. The present work described the current Italian status of the main target-site mutations associated to neonicoitinoid and pyrethroid insecticide resistance. M. persicae populations were collected from spring to autumn 2012 and 2013 in different areas of Italy, mainly from peach orchards, primary host of this aphid, but also from a few secondary herbaceous hosts. Genomic DNA was extracted from a single specimen by a “salting-out” protocol and the presence of single point mutations was assessed with allele-specific polymerase chain reaction amplification (PASA-PCR) or pyrosequencing. The presence of the R81T mutation in the loop D region of the nicotinic acetylcholine receptor B1 subunit, linked with neonicotinoids resistance, was found in more than half of the analysed populations, with a great precentage of the homozygous resistant genotype. These data describe a critical situation especially in Emilia-Romagna, the most specialised peach-growing area of Italy. Furthermore, this mutation was also found for the first time in samples collected from herbaceous crops (eggplant and pepper), underlying that resistant aphids, spreading on secondary hosts, can affect any neonicotinoid-based management strategies. The presence of L1014F (knockdown resistance: kdr) and M918T (super-kdr: s-kdr) in the voltage sodium channel gene, that are correlated with pyrethroid resistance, was found in almost all the populations collected, confirm the compromised effectiveness of this class of insecticide for the control of this pest. In addition, one further single nucleotide polymorphism in the s-kdr locus was identified. This new non-silent mutation (M918L) was recently described only in heterozygous form and in the absence of the classic kdr or s-kdr. Its involvement in pyrethroid resistance was also shown. In our samples this mutation was found with two alternative nucleic substitutions (a/t, as described in literature, or a/c), in both heterozygous or homozygous form and also in the presence of the classic kdr and s-kdr. Our data describe a quite worrying situation that must be considered and monitored, due to the spread of the resistant populations and also to the changes in the distribution of the main target-site mutations in the last few years. The development of rapid and accurate molecolar methods to detect target-site resistance is important to determine the resistance status of this pest, in order to avoid the ineffective insecticide applications and provide correct information to support coordinated control strategies regarding resistance management.