No. 4 — April 15, 2019

Cross-resistance of Fruit Flies Drosophila spp. in New York

Last week in Fredricksburg, Texas, at the National Viticulture and Enology Leadership Conference, it was reported that fruit flies have evolved resistance to Mustang Max, Assail and malathion. The fruit flies were collected from a New York state vineyard in which Mustang Max had failed to control the fruit flies. This vineyard had been treated for three years with Mustang Max to control fruit flies. Hans Walter Peterson from Cornell said this particular population of fruit flies has developed cross-resistance to three different chemical classes (Table 1). This particular fruit fly population is susceptible to Delegate (spinetoram).

What does this mean for managing fruit flies in grape cultivars susceptible to sour rot? I strongly recommend that Delegate be used in rotation with other insecticides. If you have been using Mustang Max to control fruit flies for the past two years, then you should monitor these vineyards very closely after an application. If fruit flies are abundant after an insecticide application, this would provide some indication that fruit flies have developed insecticide resistance. Do not repeatedly apply Mustang Max to control fruit flies this season. Be sure to rotate insecticides from different chemical classes. See page 93 of the 2019-2020 Midwest Fruit Pest Management Guide for chemical classes of insecticides. For more information on chemical classification of insecticides.

Table 1. Insecticides in which fruit flies Drosophila spp. have developed crossresistance to in a vineyard in New York state.

Trade name Common Name IRAC Sub-group or
active ingredient
Assail acetamiprid 4A Neonicotinoids
Malathion malathion 1B Organophosphates
Mustang Max zeta-cypermethrin 3A Pyrethroids/Pyrethrins

The cost of managing fruit flies will increase as less Mustang Max is used in your fruit fly management program. Costs per acre per application will increase between 4 to 13 times more than an application of Mustang Max (Table 2).

Table 2. Cost per acre per application for selected insecticides that provide control of fruit flies Drosophila spp.

Product Unit cost1 Use rate Cost of product
per application
  $ acre $
Assail 30 SG 69.95 lb 2.5-5.3 oz 23.17
Delegate WG 9.55 oz 3-5 oz 47.75
Malathion2 42.95 gal 1.88 lbs ai 16.11
Mustang Max 115.95 gal 4 oz 3.62

1 Prices may very based on your chemical supplier.
2 Malathion is available from a number of chemical suppliers and formulations (Amount of active ingredient per gallon) varies by supplier. Often fruit flies or Drosophila may not be listed on the label within the label section for grapes. Typical formulations are 5 or 8 lbs active ingredient per gallon.

The fact that fruit flies have developed resistance to Mustang Max and other insecticides should not be all that surprising, especially to those in the entomology world. Early work by the renowned geneticists James Crow from the University of Wisconsin-Madison was instrumental in establishing Drosophila as the genetic model for resistance studies. At that time (1957) the interest was in understanding adult Drosophila resistance to DDT. It was a short time later (1962) that Rachel Carson published the book Silent Spring. The book Silent Spring and Rachel Carson are credited with the eventual ban of DDT. Interestingly, the Drosophila population James Crow had been evaluating had been exposed to DDT over a period of 25 years in the laboratory. At that time, it was shown that DDT resistance in adult Drosophila was the result of polygenic resistance. In field collected populations of Drosophila resistance is often the result of overtranscription of P450 genes. Briefly, cytochrome P450 monooxygenases are important in regulating the metabolism of pesticides as well as play an important role in other metabolic regulation. Cytochrome P450 monooxegenase mediated resistance is a very common type of metabolism based insecticide resistance. Since the fruit flies from New York have evolved cross-resistance to three different chemical classes of insecticides this would suggest that Cytochrome P450 monooxygenases are likely involved since Cytochrome P450 monooxegenases can confer crossresistance independent of the insecticides target site. On the other hand, if the New York fruit fly population was resistant only to a single insecticide such as Mustang Max this would provide strong evidence that resistance is the result of a target site modification.

Factors leading to insecticide resistance

A number of factors play a role in the selection of insecticide resistant fruit flies. The goal of applying an insecticide is to control the fruit flies. Within the population of fruit flies there are rare variant individual fruit flies that have the ability to detoxify the applied insecticide. These rare variants survive the insecticide application whereas the majority of the population is susceptible to the insecticide and die. Applying an insecticide to a crop is in effect applying selection pressure to the fruit fly population.

This selection pressure is impacted by the dose or rate of the insecticide applied as well as the number or times the insecticide is applied. Applying the recommended full label rate at three different application timings is greater selection pressure than applying the full recommended label rate one time. In addition to the rate and the number of times the insecticide is applied, the persistence or the insecticide or residual also results in selection pressure. The longer the insecticide persists and remains active the greater the selection pressure of the insecticide. Additionally, the rate of reproduction of the fruit fly plays a role in the development of resistance to insecticides. Fruit flies have tremendous reproductive potential. A single female can live for several weeks and lay 400 to 500 eggs. A new generation emerges approximately every 10 days at 77 degrees F. The population will increase from one female to 16 million females in approximately a month given the environmental conditions are right and all individual females survive. Therefore when a single fruit fly is resistant to an insecticide, the observation that a particular insecticide is no longer efficacious will be very pronounced. How? Fruit flies will still be swarming about the vineyard after an application of an insecticide.

The reproduction rate plays a very important role in insecticide resistance development. In contrast to the fruit fly, the Japanese beetle female lays approximately 50 eggs and there is a single generation per year. Additionally, unlike fruit flies, Japanese beetles are strong flyers and can travel several miles. This results in genetic heterogeneity or mating between the Japanese beetles that are naturalized within your vineyard and beetles flying into your vineyard from outside areas. This in effect may reduce the potential for selection of resistant Japanese beetles. Since a rare variant beetle that is insecticide resistant may mate with an insecticide susceptible beetle that has flown into the vineyard.

In conclusion, fruit flies in a New York vineyard have evolved resistance to insecticides from three different chemical classes. This population of fruit flies developed resistance in a vineyard that had been sprayed for 3-years with Mustang Max. At this time, the fruit fly population remains susceptible to Delegate. In Missouri vineyards in which Mustang Max has been applied for two years or more for the control of fruit flies in an effort to manage sour rot, a resistance management plan should be implemented. The plan should include a spray schedule that limits Mustang Max applications and rotates to insecticides from other chemical classes. Additionally, in vineyards in which Mustang Max has been applied for control of fruit flies for sour rot management, these vineyards should be monitored for efficacy after an insecticide application.

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