Guidelines for preventing and managing insecticide resistance in aphids on potatoes

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The guidelines apply primarily to the peach-potato aphid (Myzus persicae) as this is the aphid species with the most significant insecticide resistance issues associated with it.

Insecticide resistance is well-established in Myzus persicae but there is as yet no evidence of field resistance to insecticides in Macrosiphum euphorbiae. Resistance mechanisms can be divided into two main categories:

(a) Metabolic – pests with this type of resistance make increased amounts of certain enzymes which break down or ‘mop up’ insecticide molecules before they reach their target sites (these are primarily in the insect nervous system); in Myzus persicae overproduction of enzymes called esterases confers resistance primarily to organophosphates, although carbamates and pyrethroids are also affected to a lesser extent. (b) Target site – in pests carrying this type of resistance, a mutation occurs in the protein that insecticides normally bind to and inactivate, rendering them no longer sensitive to insecticidal effect; two target site mechanisms known to exist under this category in Myzus persicae: MACE (Modified AcetylCholinEsterase), which confer strong resistance specifically to some carbamates (Pirimicarb is the only insecticide approved in the UK that is affected by MACE resistance – Aphids are categorised as either MACE or non-MACE), and Knockdown resistance or kdr, which can arise through one of two genetic mutations, usually denoted as ‘kdr’ and ‘super kdr’ and are associated specifically with resistance to pyrethroids – Aphids are categorised as either kdr or non-kdr (kds).

Status of resistance in Myzus persicae: Aphids with high esterase, MACE and kdr were widely distributed on potato crops in eastern England in 1996, but with the exception of kdr, resistance levels then declined by 2000. However, MACE aphids resurged in central and eastern Scotland in 2001, and in the last five years MACE aphids have become more common in England. So far, there has been no indication in the UK or elsewhere of resistance to neonicotinoid insecticides including those used on potatoes (thiacloprid, acetamiprid and thiamethoxam). There is currently no evidence of resistance to pymetrozine or flonicamid in Myzus persicae.

Status of resistance in Macrosiphum euphorbiae: Field populations of this species have not currently demonstrated any practical resistance to any insecticide, but this may change since elevated levels of esterases have been detected in some individuals indicating the potential to become resistant to commonly used insecticide groups.

 

The Implications of resistance in Myzus persicae for aphicides approved on potatoes has been discussed. The report provides practical recommendations for potato growers based on Monitor chitting sheds for the onset of aphids, alternation of single products on the growing crop, number of application, type of products and their affectivity, important factors to bear in mind when making product selections, recommended products on seed crops that are least likely to harm beneficial insects, monitoring treatment efficacy, application frequency of insecticides and recommended rate.

Fungicide Resistance Action Group UK (FRAG-UK) Potato late blight: Guidelines for managing fungicide resistance

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This guide sets out actions to minimise the risk of resistance development to existing fungicides and new active substances for control of late blight.

The introduction in the late 1970s of the phenylamides brought a new dimension to blight control but there was a rapid development of resistance in the blight pathogen. This quickly brought about a change in the number of recommended applications. Of the current approved fungicides, resistance in Phytophthora infestans has developed only to the phenylamides, which was first identified in the UK in 1981.  The report provides information on the use of fungicide, the types and occurrence of fungicide resistance and how to reduce the risk of resistance developing in a pathogen population. A list of 12 fungicide groups, their mode of action and mobility, common name of active ingredient, product name, resistance risk and use is being presented. The Impact of new strains on fungicide use is also being addressed.

Managing fungicide resistance can be summarised as follows: where possible use a blight resistant cultivar, avoid growing large areas of highly susceptible cultivars, eliminate sources of blight (infected seed, ground-keepers and dumps), monitor local weather conditions, apply fungicides as protectants, select fungicides to suit local conditions, use mixed formulations of active ingredients with different modes of action or from a different fungicide families, or target specific products in blocks to appropriate growth stages, do not chase the epidemic with fungicides, burn off early if blight levels are significant and delay lifting until the haulm has been dead for at least 14 days.