The role of monitoring, reduced-rate insecticides and insecticide placement in the control of carrot rust fly (Psila rosae F.) in Canterbury, New Zealand

Abstract

The carrot rust fly (Psila rosae) is a serious pest of carrots and other umbelliferous crops in New Zealand. Carrots suffer damage from mining by the fly's larvae. Currently, carrot growers in New Zealand rely on 'calendar'(prophylactic) application of insecticides.

Flight activity of the carrot rust fly was monitored with yellow sticky traps (Rebell®) in the North and South Islands of New Zealand over a three-year period from 1993-96. Three distinct periods of adult flight activity were observed, indicating three generations per year. At Lincoln (the South Island site), the first flight peaked in mid-late October, followed by a second peak in mid-late January, with a third in mid March-mid April. At Feilding (the North Island site), the peak catches of second- and third-generation flies were 10 and 22 days earlier than at Lincoln, respectively. However, variation in flight activity occurred from region to region and year to year.

Two field experiments were conducted in 1993-94 to investigate the efficacy of the insecticides phorate, diazinon and furathiocarb at field rates and at 0.5, 0.33 and 0.125 field rates for the control of carrot fly larval damage. Phorate granules drilled with the seeds effectively controlled the first-generation larvae, even at one third recommended rate, and half rate was equally efficient as field rate in controlling second-generation larvae. Third-generation damage was not significantly reduced by any rate of phorate granules applied at drilling. Phorate applied as soilsurface, mid- or late-season granules gave better control of second-generation damage, even at half rate, than did full rate at drilling. The furathiocarb seed coated treatment at 4% controlled the first-generation damage, but had no significant effect on the second generation. A diazinon mid-season spray at this site controlled third-generation damage at the field rate but not at lower rates.

Pitfall and Malaise trapping was carried out to study the predatory arthropod fauna in carrot fields in Canterbury. Staphylinidae (rove beetles), Linyphiidae (money spiders), Lycosidae (wolf spiders), Phalangiidae (harvestmen) and Carabidae (ground beetles) were the most abundant predators. Spiders, staphylinids, harvestmen and carabids comprised 68, 13, 11 and 2.6 % of all predatory arthropods in pitfall traps, respectively. Overall, a total of 33 staphylinid species was recorded, including one, Ocypus ater (Gravenhorst), which was new to New Zealand. The total number of predators caught in pitfall traps declined in March and April. The seasonal activity and abundance of the dominant species are described.

Predator abundances were assessed by pitfall trapping in replicated plots (each 10 m x 5 m) in two experimental carrot fields, under a range of insecticide regimes. Abundance of predators was greatly reduced by the field rates of diazinon and phorate (by 92% and 52% respectively), but at lower rates reductions were not significant. The recovery of predator numbers depended on dose rate and predatory group, and most of the predator populations had recovered by 40 days after treatment.

Carrot rust fly damage levels at different distances from the crop edge into the carrot field were studied. Overall root damage was up to 40% at crop edge, but was 0 - 5% at 30 m into the field. Application of phorate granules at reduced-rate only around the edges of carrot fields (10 m-strip), controlled larval damage effectively, requiring approximately 10% of the recommended rate (i.e., the mid-field remaining untreated). Neem seed extract (NSE) and neem oil were evaluated in laboratory experiments against carrot rust fly oviposition, egg hatch, pupation and adult emergence rates. Neem oil and NSE showed oviposition repellent effects against carrot rust fly. The number of pupae developed from eggs exposed to NSE was reduced by up to 96 %. Neem oil and NSE decreased egg hatch and egg development. NSE did not significantly reduce adult emergence. However, malformed adults were recorded in the neem treatments. The management of carrot rust fly in New Zealand is discussed with knowledge gained from this study and other New Zealand work and in relation to European research.