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The Effects of Pesticides on Mites

As the growing season is quickly approaching, most growers are looking at their spray programs from last year and trying to figure out what strategies they are going to use this upcoming season. Some of the decisions growers make on products used are based on efficacy, past experiences, and environmental effects; but what about the effects of pesticides on mite populations?

The use of insecticides, fungicides and miticides can have a direct affect on predatory mite populations in apple orchards. Research has been conducted on the effects of pesticides on mite populations (pest and predatory mites) for decades. However, with the increase in registration of newer chemistries for pest management we are constantly trying to learn more about these products and how they effect mite populations in the orchard ecosystem.

The two main ways in which pesticides affect mite populations in the orchards are: by elevating pest mite populations in the orchard (through stimulation of reproduction also known as hormoligosis), or by reducing the predatory mite populations in the orchard (a result of killing or sterilizing adult predatory mites, or by reducing their egg hatch). As a result of increasing pest mite populations or decreasing predatory mite populations, growers may need to apply a miticide. Miticides are very expensive and there are very few options available for controlling mites. Pesticide resistance develops very quickly in mites. Miticides should be used as sparingly as possible.

Most consultants and growers are aware that the use of pyrethroids can result in an increase in phytophagous mite populations in the orchard. Pyrethoids have been shown to cause an increase in egg laying in European Red Mite populations, and are also considered to be toxic to beneficial mites. However, there are significant differences in the toxicity of different pyrethroids to beneficial mites. While the application of a pyrethroid in apple orchards is never an IPM friendly option, it should be avoided at all costs. There are some instances when the use of a pyrethroid may be necessary to control a pest (i.e. oriental fruit moth). So what can growers do to reduce the negative effects of pyrethroids on their mite populations? If the application of a pyrethroid is necessary, try to make the application as early in the season as possible (preferably no later than petal fall). Some of the species of predatory mites found in Ontario orchards overwinter on the orchard floor, and do not move up into the trees until mid July. Thus by applying the pyrethroids early in the season, there are fewer predatory mites in the tree, and as a result we would expect to have fewer mites affected. The residue from early season applications of pyrethroids will degrade over time, and will hopefully have reduced effects on the predatory mites when they move into the tree canopy.

Research has also shown that pyrethroids are toxic to predatory mites. However there are differences between the pyrethroids and their toxicity. Dr. Noubar Bostinian, a researcher at Agriculture and Agri-Food Canada has studied the residual toxicity of pyrethroids and found that lamda cyhalthrin (Matador) creates less than 50% mortality of the good mites (10 days after treatment), while other pyrethroids such as cypermethrin (Cymbush) and deltamethrin (Decis) caused >86% mortality in Amblyseius fallacis. Permethrin (Pounce) is considered to be the least toxic synthetic pyrethroid. Always keep in mind that orchards which have been treated with an application of a pyrethroid after petal fall may experience high populations of mites the following year.

Although one would think that miticides would be extremely harmful to beneficial mites, research suggests that this is not always the case. The oil sprays that are applied before bloom (pink through 'til petal fall) often have little effect on the beneficial mites since they are hidden away in crevices on the bark of the tree, or are found on the orchard floor. While some miticides such as Carzol are considered to be harsh on beneficial mites, others such as Pyramite do not cause significant mortality, and do not affect fecundity or egg hatching in A. fallacis.

Most growers don't think about the effects on mites when developing fungicide programs for their orchards. However, some fungicides can be highly toxic to beneficial mites. In general, fungicides do not affect the different developmental stages of A. fallacis. Studies conducted by researchers in Quebec (Larocque, Bostanian, Racette, and Lasnier) have shown that Captan, Nova, Polyram, and Sovran do not have a toxic effect on A.fallacis (adults, nymphs, eggs or fecundity). However fungicides such as Dithane, Manzate, and Equal are considered to be toxic to A. fallacis. While the use of these chemicals is often necessary for managing scab in apple orchards, growers should look at methods to minimize the negative effects of these fungicides to beneficials? Altering fungicide programs to incorporate the use of these products early in the season (prior to mid-July) when fewer predatory mites are present in the canopy will reduce their negative effects. Also minimizing the spray run off into the ground cover will reduce the effects on beneficials, which are present on orchard floor during early season applications.

It is important to remember that the toxicity of pesticides can differ between species of mites. For example Pyramite is considered to be non-toxic to A. fallacis, however it is toxic to other species of mites such as Agistemus fleshneri which is a predatory mite in Quebec apple orchards (but is not currently known to be present in Ontario orchards). The effects of pesticides on beneficial mites can also vary from orchard to orchard. In some orchards the application of an insecticide or fungicide can be highly toxic to beneficial mites, while in another orchard the application does not appear to have any effect on mite populations. These variances in toxicity are probably a result of different chemical programs used by the producers.

The pest management program used in apple orchards can have a significant effect on mite populations. As a result the effect of pesticides on mite populations should be taken into account when developing a pest management program for an orchard. Also careful monitoring of mites by a qualified consultant is important to preventing mite populations for getting out of hand.

Toxicity of selected pesticides on important apple orchard predatory mites in Quebec.

Insecticides:

Miticides:

Fungicides:

Thinners:

* = Non toxic if used no more than up to 15 days after calyx
** = Non-toxic at a reduced rate on colonies of predatory mites
X = Toxic? = Unknown

Authors: Jacques Lasnier / Noubar J. Bostanian / MartinTrudeau / Gaétan Racette

⁽¹⁾ Where Stigmaeidae include Zetzellia mali which are bright yellow with orange markings and somewhat diamond shaped (0.5 mm length)

(2) Where Phytoseiidae include Amblyseius fallacis wich are clear or translucent, sometimes with brown marking on back, tear shaped, very fast mvoing (0.3 mm in length)