Survey of Guthion Resistance in Codling Moth in Northern Utah, 2001

Diane G. Alston

Department of Biology, Utah State University

Objective: To investigate the level of resistance of adult male codling moths to the organophosphate insecticide, azinphosmethyl (Guthion), in northern Utah apple orchards. To compare 2001 results to those found in a mini-survey in 1992.

Background: Integrated pest management programs in conventional pome fruit orchards in western North America have depended upon the use of organophosphate insecticides to control key insect pests for most of the last 40 years. Guthion (azinphosmethyl) has been a keystone organophosphate used in Utah and other western apple and pear orchards for control of codling moth. The use of Guthion has allowed for integrated mite management in orchards because the indigenous predaceous mites have developed a tolerance to Guthion and other organophosphates. However, in the past ten years, sub-optimal field control of codling moth with Guthion has prompted the evaluation of resistance in field populations. A mini-survey of three codling moth populations in Utah County, UT in 1992 found that in two orchards with a history of fruit injury from codling moth at harvest, the codling moth population was more resistant (42.5-58.2% mortality) than a susceptible laboratory colony (88.2% mortality) and comparable susceptible orchards (with minimal codling moth injury to fruit at harvest) in Washington state (71.8-94.9% mortality). A third Utah Co. population from an orchard with a history of good codling moth control had a susceptible response to Guthion in 1992 (71.5% mortality). A field bioassay method was developed that would allow a fairly quick and easy measurement of a codling moth population’s response to a discriminating concentration of Guthion insecticide.

Methods: Eight apple orchards in Utah, Davis, Box Elder and Cache Counties with a range of Guthion-use and codling moth control histories were selected. Orchards were located in Santaquin (2 orchards; most southern sites), Genola, Payson, West Mountain, Kaysville, Perry, and River Heights (most northern site). A discriminating concentration of Guthion was incorporated into the adhesive of pheromone-baited male codling moth Delta style traps (75 ug a.i. per g adhesive). Ten traps were placed in each orchard site on three consecutive days and nights within two weeks following biofix. Traps were placed in Utah and Davis Counties from May 8-10 and in Box Elder and Cache Counties from May 15-17. The first generation of codling moth before insecticide control treatments were applied was targeted in both 1992 and 2001 resistance surveys. Five traps in each orchard contained a liner with adhesive + acetone (solvent for Guthion) + Guthion ("Guthion traps") and five traps contained a liner with only adhesive + acetone ("Control traps"). Traps were placed as five pairs of Guthion and Control traps across the orchards (2-5 acres in size) with 2 trees between each trap in a pair and at least 3 rows or 6 trees within a row between trap pairs. Traps were placed between 9 am and 4 pm on each placement day and retrieved between 8-11 am the following day.

Upon collection, liners with moths caught in the adhesive were placed into plastic boxes with grooved wooden boards over moist paper towels in the bottom. The liners were placed so as to prevent damage to moths and to maintain approximately 85% relative humidity in the boxes. Plastic boxes were kept in an air conditioned vehicle or indoor room at approximately 68-70F. Mortality of moths was scored on 1 pm of the third day after collection, or approximately 64 h after time of average moth catch (peak moth catch occurred at approximately 9 pm on the night following trap placement). Moths were scored as alive if they exhibited vigorous movement in antennae, head and thorax when probed with a fine needle. Moths were scored as dead if movement was weak or there was no movement.

Percentage corrected mortality was calculated for each pair of traps in an orchard based on Abbott’s formula ((% Guthion mortality - % Control mortality)/% Control Survival) and 95% confidence limits (CLs) based on the binomial distribution for percentages were compared among orchards. Analysis of variance (Proc Glm, SAS) was used to compare the mean corrected mortality among orchards (proportion data were arcsine square root transformed before analysis to help with normality). Survey mortality data from 2001 was compared with the three survey sites from 1992.

Results: Mean percentage corrected mortality of the eight codling moth populations in response to the discriminating concentration of Guthion in 2001 surveys ranged from 31.3-100.0% (Table 1 and Fig. 1). There were significant differences in codling moth resistance among sites (Table 2) that were most likely related to Guthion-use history in the orchards, proximity of less managed codling moth populations (i.e., untreated reservoirs of moths in urban areas or abandoned orchards), general intensity of organophosphate use on the farm, and age of orchards and nearby fruit production area. There is no definitive cut-off line where we can say that certain orchards have resistant codling moth and others do not, however, corrected mortality below approximately 56% in Payson, Santaquin and Perry sites indicate some likely resistance in these populations (Table 2 and Fig. 1). Moth mortality of approximately 77% and above in the North Farm, Genola, West Mountain, Kaysville and River Heights sites indicate these populations are probably not resistant to Guthion. Mortalities of 77-87% for Genola and River Heights sites likely indicate some movement of the population toward a mild resistant response, but these populations would not be classified as resistant at this time. The response of each population tested is not a general reflection of all orchards in that area, but may be an indicator for other orchards managed similarly and with similar conditions on the same farm. The field bioassay method used appears to provide biologically meaningful results that fit with Guthion use histories and codling moth control levels observed, however, the method does have variation in its resistance detection abilities. The 95% confidence limits presented in Table 1 indicate the amount of variation in mortality for each population. The results from the field bioassay are to be used in comparisons among sites and years, and do not provide an absolute measure of resistance.

There was a loose relationship between higher male moth catches in Guthion-treated codling moth traps and greater resistance in the population to Guthion (Table 1). Very low moth catch in North Farm and West Mountain orchards in 2001 (16 and 27, respectively) make calculations of mortality fairly unreliable. However, all the moths caught in Guthion-treated traps were killed, whereas, very few died in the controls, so estimation of 100% mortality is not unrealistic.

Comparison of Guthion resistance found in the 1992 mini-survey with that in the larger 2001 survey indicate some small changes in codling moth population responses over a 9-year period (Table 1). The same Payson orchard site was used in both years. There was a very small increase in resistance (decrease in mortality) from 58.2% in 1992 to 55.7% in 2001. The 95% confidence intervals for these two means substantially overlap, so they are not different. This orchard has a history of high codling moth populations and less than satisfactory control of codling moth with Guthion (i.e., fruit injury). The West Mountain sites were on the same farm, but different orchards in the two survey years. The decrease in resistance (increase in mortality) from 71.5 to 100.0% over the 9 years is most likely reflective of the change in codling moth management practices from primarily organophosphate-based in 1992 to primary use of mating disruption and much reduced use of organophosphates by 2001. The confidence intervals between the two surveys do not overlap indicating that resistance substantially declined. The most resistant site in the 1992 survey, Orem, was not surveyed in 2001. This was an older orchard that had been surrounded by urban sprawl in 1992. The orchard had a history of high use of organophosphates, poor codling moth control with Guthion and high moth populations.

In conclusion, the field survey method results should not be used to infer absolute measures of codling moth resistance to Guthion; however, the results are useful for comparison among sites and in evaluating changes over time. At least three sites surveyed in 2001 (Santaquin, Payson and Perry) indicate some likely resistance of codling moth to Guthion. Such information should be used by the growers to modify their codling moth management programs and to lower selection pressure for resistance. The longevity of Guthion registration for use in orchards appears to be limited at least in some fruit crops. There are new classes of insecticides becoming available. Future codling moth management programs should incorporate new management techniques and classes of insecticides to manage for resistance.

 

Table 1. 2001 and 1992 survey of Guthion (azinphosmethyl) resistance in codling moth populations from orchard sites in northern Utah.

Year

Site

No. moths treated

Mean % corrected mortality

95% Confidence limits

2001

Santaquin

113

42.8

33.8-51.8

North Farm

16

100.0

91.0-100.0

Genola

71

77.6

68.6-86.7

Payson

537

55.7

46.6-64.7

West Mountain

27

100.0

91.0-100.0

Kaysville

45

97.2

88.2-100.0

Perry

1017

31.3

22.3-40.3

River Heights

58

87.3

78.3-96.4

1992

Payson

340

58.2

52.7-63.2

West Mountain

142

71.5

63.9-79.1

Orem

113

42.5

33.9-52.6

 

Table 2. Analysis of variance comparison of mean percentage corrected mortality among orchard sites in 2001.

Site

Mean % corrected mortality

North Farm

100.0 a

West Mountain

100.0 a

Kaysville

97.2 ab

River Heights

87.3 bc

Genola

77.6 c

Payson

55.7 d

Santaquin

42.8 e

Perry

31.3 e

P>F

<0.0001

Proportion data were arcsine square root transformed before analysis. Means were separated with Waller-Duncan k-ratio t-test. Means followed by the same letter are not different from each other.

Figure 1. Mean percentage corrected mortality of codling moth populations caused by a discriminating concentration of Guthion (0.75 ug a.i. per g adhesive) in eight orchards in northern Utah in 2001.