A comparison of the insecticidal properties of New Zealand native and other plants

Abstract

Economically important plant species were selected from three different countries: New Zealand (12 species), Chile (11 species) and Australia (1 species). They were screened for insecticidal properties against the economically important insect pests: the lightbrown apple moth (LBAM) (Epiphyas postvittana walker), the tropical armyworm (TAW) (Spodoptera litura F.) and the pea aphid (PA) (Acyrthosiphon pisum Harris). The LBAM is a serious pest of fruits and ornamental crops in Australia and New Zealand. The TAW is a tropical and subtropical pest. The pea aphid is a cosmopolitan pest of leguminous crops and also a virus vector. Dichloromethane and methanol extracts of different plant parts (leaves, bark, seeds and fruits) were tested.

Forty plant extracts were screened for antifeedant activity against LBAM and TAW, using artificial diet squares in a two-way choice test. A feeding index (FI = (C-T / C+ T) X 100) was used, in which C = the mean weight of control squares eaten and T = the mean weight of treated squares eaten. 13 plant extracts had strong antifeeding activity against LBAM larvae and one plant extract against TAW. A strong deterrent effect from the leaf extracts of Dysoxylum spectabile towards LBAM (FI = + 89 %) and TAW (FI = + 71 %) and seed extracts of Azadirachta indica towards LBAM (FI = + 95 %) was recorded. In contrast, three plant extracts from Chilean Nothofagus species acted as feeding stimulants against LBAM larvae.

Thirty-nine plant extracts were screened for settling and reproduction effects on pea aphid adults, using a choice and a no-choice bioassay. Leaf extracts of two New Zealand and six Chilean Nothofagus species greatly reduced aphid settling. Bark extracts from six Chilean Nothofagus species significantly reduced aphid settling. N. glauca (bark) and N. alessandri (leaf) greatly reduced aphid settling by 61 % and 67 %, respectively, compared with controls.

Five extracts from New Zealand and 12 from Chilean Nothofagus species significantly reduced the number of pea aphid nymphs produced. N. truncata (bark), N. obliqua (bark), N. alpina (bark & leaf), N. leoni (leaf) and N. alessandri (leaf) greatly reduced nymphal production by over 75 %. Leaf extracts of N. truncata and N. solandri reduced aphid settling and reproductive behaviour in no-choice tests. Leaf extracts from N. menziesii did not reduce settling but reduced reproductive rate. Of the other New Zealand plant extracts all reduced settling and reproduction but Beilschmiedia tawa did not reduce settling significantly. All extracts reduced aphid reproduction by 50 - 88 %.

Twelve New Zealand plant extracts were incorporated into the larval diet of the LBAM and their extracts on larval mortality, pupation, adult emergence, egg fertility and egg mortality were evaluated. D. spectabile leaf extracts caused significant larval mortality and some of the adults which emerged from those larvae were malformed. Fecundity and fertility of the adults was also significantly reduced. N. fusca extracts had similar effects on LBAM. The advantages and disadvantages of the methods used are discussed, as are the prospects for the application of these results in agrochemical developments.