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.

Strategies for minimising virus infection of potato crops

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This project report combines published information with the results from this work to provide a Decision Support System for minimizing PVY infection.

The project combines close collaboration with participating seed growers. It implements a system of within field aphid trapping and virus testing with complementary experiments designed to provide information to evaluate the risk of virus (principally PVY) infection and examine the merit of various testing strategies for crop management. The experiments were particularly concerned with evaluating the importance of mature plant resistance to PVY infection. The aphid species which have been shown to transmit PVY were given an efficiency index based on their ability to transmit the virus.

Aphid trapping can be a useful management tool for controlling PVY. In the two years prior to the date of this interim report, seed crops were burnt down promptly once a substantial rise in the ‘PVY index’ was recorded and in most crops either no PVY or a low incidence (1- 5 %) was recorded in post harvest tuber tests. The data obtained from these crops allows estimation of conditions which preclude substantial PVY spread but the absence of high incidence of infection limits the potential for modelling PVY ingress. Results from the virus movement experiment carried out over two years have been largely consistent. ELISA tests carried out on leaves and stems gave substantially lower incidence of PVY than the results from tuber tests. The incidence of infection in tubers was negligible for tubers harvested immediately after one week exposure to infection but one week later, infected tubers were detected and the incidence increased after a further week although to a lesser extent. The proportion of tubers infected tended to increase with increase in aphid index up to c. 50 but there was considerable variation in incidence of PVY in relation to aphid indices between exposure dates. Older plants had a lower incidence of infection than young plants, but even for old plants (exposed 6 weeks after emergence) up to 30 % were infected during weeks when the vector index was high. The differences in tuber infection between young and old plants confirms the importance of mature plant resistance for modelling infection in crops