Mass rearing Pteromalus puparum on Pieris rapae to assist eradication of Pieris brassicae from New Zealand

Pieris brassicae L. (Lepidoptera: Pieridae) was first detected 14 May 2010 in Nelson City, New Zealand, and became the target of an eradication programme from Nov 2012 to May 2016. Methods were available for killing P. brassicae eggs, larvae and adults, but not the elusive pupae. This project aimed to mass-rear a parasitoid of P. brassicae pupae, Pteromalus puparum L. (Hymenoptera: Pteromalidae), and release it at Nelson locations where the Department of Conservation (DOC) considered there was a high risk of P. brassicae pupae being present. The efficacy of the parasitoid releases was estimated by seeding sites where releases had and had not been made with unparasitised P. rapae ‘sentinel’ pupae L. (Lepidoptera: Pieridae), and monitoring their parasitism rates. The P. rapae colonies produced five generations of laboratory reared butterflies before viable egg production ceased and signalled the end of the release program. A total of 837 parasitised P. rapae pupae were shipped to Nelson at weekly intervals from late February to early April 2015; over 85% produced parasitoids, resulting in releases of an estimated 14,280 Pt. puparum adults. No sentinel P. rapae pupae placed at control sites were parasitised by Pt. puparum. The first evidence of parasitism in sentinel pupae placed at release sites occurred on 21 May when parasitoids emerged from a single pupa exposed during the last release on 7 April. The longer than expected development time indicated that insect development had slowed down. Hence remaining healthy, but potentially parasitised, sentinel pupae were returned to Lincoln and maintained at 20°C and 16: 8 hr L: D. By late August, parasitoids had emerged from a further 5 sentinel pupae. Although parasitism of sentinel P. rapae was low at release sites, even very small numbers of P. brassicae pupae parasitised by released Pt. puparum could have significantly benefited the eradication programme. Furthermore, our observation that parasitised pupae that are in diapause produce viable parasitoids on exposure to warmer temperatures and longer days bodes well for the survival of Pt. puparum over winter and the ongoing fight against remaining P.brassicae.