Effects of acaricides, pyrethroids and predator distributions on populations of Tetranychus urticae in apple orchards

Abstract

We sampled mites in three apple orchards in Nova Scotia, Canada, that had been inoculated with pyrethroid-resistant Typhlodromus pyri and had a history of Tetranychus urticae outbreaks. The objective of this study was to monitor populations of T. urticae and phytoseiid predators on the ground and in trees and to track dispersal between the two habitats. Pesticides were the chief cause of differences in mite dynamics between orchards. In two orchards, application of favourably selective acaricides (abamectin, clofentezine) in 2002, coupled with predation by T. pyri in trees and Neoseiulus fallacis in ground cover, decreased high T. urticae counts and suppressed Panonychus ulmi. By 2003 phytoseiids kept the tetranychids at low levels. In a third orchard, application of pyrethroids (cypermethrin, lambda-cyhalothrin), plus an unfavourably selective acaricide (pyridaben) in 2003, suppressed phytoseiids, allowing exponential increases of T. urticae in the ground cover and in tree canopies. By 2004 however, increasing numbers of T. pyri and application of clofentezine strongly reduced densities of T. urticae in tree canopies despite high numbers crawling up from the ground cover. Another influence on T. urticae dynamics was the distribution of the phytoseiids, T. pyri and N. fallacis. When harsh pesticides were avoided, T. pyri were numerous in tree canopies. Conversely, only a few N. fallacis were found there, even when they were present in the ground cover and on tree trunks. Low numbers were sometimes due to pyrethroid applications or to scarcity of prey. Another factor was likely the abundance of T. pyri, which not only competes with N. fallacis, but also feeds on its larvae and nymphs. The scarcity of a specialist predator of spider mites in trees means that control of T. urticae largely depends on T. pyri, a generalist predator that is not particularly effective in regulating T. urticae.

Effect of the width of the herbicide strip on mite dynamics in apple orchards

“…Finally, there is the matter of ambulatory dispersal from one plot to the next. Firstly, both P. ulmi and T. pyri are absent or at very low numbers in orchard ground cover (Hardman et al 2007). As for T. urticae, numbers counted in ground cover quadrats were quite low, possibly because of control by N. fallacis, which was found in that habitat (unpublished data from this study).…”

“…The worst scenarios would occur where rates of ambulatory or aerial dispersal would be sufficient to induce strong correlations in mite counts in adjacent plots having different treatments, thereby obscuring the impact of the two factors that had most influence in this study: leaf chemistry and predation by phytoseiids. Risk factors could include the following: a strong contrast in tetranychid densities between trees in one plot and those in the neighbouring plot, particularly in early season; low densities of specialist phytoseiids such as N. fallacis in the ground cover, which would allow build up of T. urticae in that habitat, followed by higher immigration into trees; and low densities of phytoseiids such as T. pyri in trees, which would allow tetranychids moving into lower density plots more opportunity to multiply in the tree canopy (Hardman et al 2007). In these cases, use of larger plots with guard rows would be a wise precaution.…”

“…In others, T. urticae suddenly appeared in the trees in July or August and densities quickly exceeded the economic threshold of five motile mites per leaf despite the presence of T. pyri (Hardman 1999). To uncover the causes of T. urticae outbreaks and to develop remedies, detailed surveys of mites in the ground cover vegetation, on tree trunks and in tree canopies were conducted in commercial apple orchards (Hardman et al 2005(Hardman et al , 2006(Hardman et al , 2007. Causes of outbreaks included (1) the near absence of Neoseiulus fallacis (Garman), a specialist predator of T. urticae, in tree canopies (Hardman et al 2005(Hardman et al , 2007; (2) prolonged warm, dry weather, which increases T. urticae populations in the ground cover, promotes dispersal from the ground cover into trees, and increases the rate of population growth in the tree canopy (Hardman et al 2005); (3) application of pesticides that suppress T. pyri populations in the tree canopy (Hardman et al 2006); and (4) the presence in herbicide strips of large numbers of reproductive host plants of T. urticae coupled with application of herbicides that induce the mites to crawl up tree trunks into the canopy (Hardman et al 2005).…”

Diversity and abundance of Phytoseiidae (Acari: Mesostigmata) in three crop management strategies of citrus orchards in Tunisia

“…Many studies related the negative effects of pesticides on Phytoseiidae (i.e. Kreiter et al, 1998;Childers et al, 2001;Chen et al, 2003;Hardman et al, 2006Hardman et al, , 2007Bonafos et al, 2008;Meyer et al, 2009;Peverieri et al, 2009). In surveys comparing treated and untreated apple orchards in North Carolina, Farrier et al (1980) showed that there were twofold more species on untreated trees compared to treated ones.…”

Effect of two-spotted spider mite population (Tetranychus urticae Koch) on growth parameters and yield of the summer apple cv. Katja

“…It should be noted that in apple orchards, the max. populations of spider mites usually occur in July and then decrease as a result of natural processes, as well as the activity of predatory mites or the use of selective pesticides (Hardman et al 2007;Duso et al 2009). Thus for late-season apple cultivars, in contrast to earlyseason varieties, harvesting can take place up to about 2-3 months after the max.…”