College of Agriculture

Effects of S-Methoprene, Altosid® IGR, on fly numbers and weight gain in crossbred heifers in northern California

* Department of Animal Sciences, College of Agriculture, California State University, Chico


This study was conducted to determine the efficacy of S-methoprene for horn fly control and determine if control of horn flies improved weight gain in beef heifers.  The study took place in Orland, California using six to nine month old replacement heifers.  Heifers were randomly assigned to one of two treatments.  Treatment heifers received Right Now® Emerald mineral containing 0.01% (S)-methoprene and control heifers received Right Now® Emerald mineral without fly control, both of which were offered free-choice.  During period one heifers achieved optimum intake of mineral, whereas optimum intake was not achieved during period two of the study.  Results showed that (S)-methoprene did not offer significant control of horn flies during period one (p=0.2885) or period two (p=0.741).  Since horn fly control was not achieved, improved weight gains were not observed in methoprene treated heifers during period one (p=0.0918).  During period two, control heifers gained significantly more weight than methoprene treated heifers (p=0.0232).  Economic threshold fly populations were never reached in either treatment indicating the cattle used for the study did not require fly control to reach maximum genetic potential.

Key Words: Haematobia irritans, Methoprene, Weight gain, Cattle, Insect growth regulator


There are many pests that inhibit cattle from reaching their maximum genetic potential for growth and health.  The horn fly, Haematobia irritans (L.), has been estimated to cost the beef industry $876 million a year (Kunz et al., 1991).  The economic threshold for horn fly populations in a given environment is estimated to be 50 to 200 flies per animal with accepted levels varying by study (Andress et al., 2000 and Swanson et al., 2003).  When fly populations reach threshold cattle begin to drop off in weight gain and expend energy on production of new blood that is being depleted by the flies (Drumond et al., 1981).  Producers can receive premiums for their cattle when they can achieve maximum weight gain and market calves that are healthy and thriving.

Studies have shown that with the use of endectocides, fly counts can be kept below the threshold level (Andress et al., 2000; Miller et al., 2003; and Swanson et al., 2003).  External parasite control can lead to enhanced weight gains in yearling cattle (DeRouen et al., 2003; Sanson et al., 2003; and DeRouen et al., 1995).  Extensive use of pyrethroid and organophosphate fly control methods have allowed flies to develop a resistance to their lethal actions (Barros et al., 2001 and DeRouen et al., 1995).  Within the past ten years a new fly control product, Altosid Insect Growth Regulator (IGR) the active ingredient S-methoprene, was approved for use in California.  Methoprene mimics juvenile hormone naturally produced by insects and prevents pupae from molting into adults (Glare and O’Callaghan, 1999; Parthasarathy et al., 2006; and Stark 2005) but has not been well studied in northern California.  Furthermore, methoprene has a relatively short half-life in the environment and is converted to natural biochemicals in mammals (Glare and O’Callaghan, 1999 and Quistad et al., 1975).  Thus, the purpose of this study was to determine the efficacy of S-methoprene for horn fly control and determine if control of horn flies improves weight gain in heifers under field conditions in northern California.


The experiment was conducted at Quiet Hills Ranch in Orland, California.  Animals were allowed a 22 day acclimation period prior to onset of the study.  Period one of the study consisted of 19 days, and period two consisted of 57 days.


One hundred and twenty six commercial Hereford-Angus crossbred heifers approximately six to nine months of age served as the experimental units for period one of the study.  Heifer weights ranged from 148 to 327 kg.  Heifers were identified by individual ear tags and were given standard vaccinations for infectious diseases.  All heifers received a dose of injectable Dectomax for the control of internal parasites (Pfizer Animal Health, New York, NY).

Period two of the study was conducted using 20 treated heifers and 14 control heifers.  Heifer weights ranged from 250 to 330 kg.


During period one heifers grazed 350 acre lots of native rangeland pasture with ad libitum water via water troughs and reservoirs throughout the study.  Treatment groups were separated by one mile of pasture.

During period two heifers grazed 50 acre lots of irrigated pasture with ad libitum water via water troughs throughout the study.  Treatment groups were separated by a fence line.

Group allocation and treatment

During period one heifers were randomly assigned by lottery to one of two treatments, heifers remained in their respective treatments throughout both periods of the study.  One group received horn fly control in the form of an insect growth regulator, methoprene in a standard mineral granule, and one group received no horn fly control.  The heifers in the control group received free-choice Right Now® Emerald mineral.  The heifers in the treatment group received Right Now® Emerald mineral containing 0.01% (S)-methoprene offered free-choice.  All heifers were offered free-choice Right Now® Emerald mineral for 22 days prior to the start of the study to acclimate the heifers to mineral consumption.

Mineral Consumption

Mineral consumption was monitored on a weekly basis.  During period one mineral weight was only obtained as mineral was placed in tubs.  During period two, mineral remaining in tubs was weighed as was new mineral placed in tubs.

Determination of inhibition of adult emergence of the horn fly

On-animal estimates of adult horn fly numbers were made once per week beginning on day 0.  Horn fly numbers were estimated by randomly selecting 10 heifers from each treatment group and estimating the number of horn flies on one side of each animal.  All fly counts were made before 09:00 h with the aid of binoculars (DeRouen et al. 1995).  Final fly counts for period one were taken on day 19 and for period two on day 57.  Fly populations were considered to be under control when fly counts averaged less than 100 flies per side per animal.

Evaluation of weight gain of heifers

During period one individual weights of heifers in each treatment group were taken on days 0 and 19.  During period two individual weights of heifers were taken on days 0 and 57.  Weights were measured using a Powell digital scale.  Handling of the heifers was minimized to avoid shrink.

Calculations and Analysis of Data

Fly Counts and Body Weights.  The following equation was used to calculate percent horn fly control for the treated heifers.              

Replicated fly counts were analyzed using a two sample T-test (Statistix8, 2003).  Additionally, average daily gain data for each trial period were analyzed using ANOVA (Baldy, 2001) as a completely randomized design.  Pairwise comparison of mean weight gains between methoprene-treated versus non-treated heifers was made using Tukey’s multiple comparison test.  The accepted level of statistical difference was p≤0.05.


Mineral consumption

Label recommendations for intake of Right Now® Emerald mineral is 0.04 ounces per 100 pounds of body weight.  During period one, mineral consumption by heifers in the treatment and the control exceeded the label recommendation by 75.9% and 69.1% respectively.

During period two, treated heifers consumed 52.3% less mineral than was recommended, while the control heifers consumed the mineral at a rate of 53.6% below label recommendations.  Because heifers in both the treatment and the control were pastured with surplus cows, intake by designated animals was estimated from the entire 70 cow herd for the treatment heifers and the entire 64 cow herd for the control heifers.

Inhibition of adult emergence of the horn fly period one

Fly counts were not significantly different between treated and control groups during period one of the study (p=0.2885) (Figure 1).  Throughout period one fly counts in both the treated and control groups never reached the accepted economic threshold of 100 flies per side.

Figure 1: Mean number of horn flies per side of control and methoprene treatment during period one.  Treatment differences were not significant with p>0.05.  (two sample T-test)

Inhibition of adult emergence of the horn fly period two

As with period one, fly counts among treatments in period two were not significantly different (p=0.0741) (Figure 4).  Again, fly counts in both the treated and control groups never reached the accepted economic threshold of 100 flies per side.

Figure 2: Mean number of horn flies per side of control and methoprene treatment during period two.  Treatment differences were not significant with p>0.05.  (two sample T-test)

Weight gain of heifers

There was not a statistically significant enhanced weight gain in heifers treated with methoprene (p=0.0918) during period one of the study (Figure 3).  During period two heifers in the control gained significantly (p=0.0232) more weight than heifers treated with methoprene (Figure 4).

Figure 3: Mean weight gain of control and methoprene treatment during period one.  Treatment differences were not significant with p>0.05.  (Tukey-Kramer test)

Figure 4: Mean weight gain of control and methoprene treatment during period two.  Treatment differences were significant with p<0.05.  (Tukey-Kramer test)


Despite a lack of difference in fly counts during period one between the treated and control herds, it should be noted that fly numbers in both treatments never went above the economic threshold of 100 flies per side.  Due to late rains the fly season was delayed, which may have impacted the results obtained during period one.  Methoprene mimics juvenile hormone action in the horn fly and stops the horn fly life cycle at the pupae stage.  Methoprene is not effective at killing adult horn flies, thus it may take up to 21 days to eliminate initial fly loads.  Since period one only lasted 19 days this was not an adequate amount of time to evaluate the efficacy of methoprene for reducing horn fly numbers.  Lack of range feed made it necessary to end period one on day 19 and relocate the study to irrigated pasture.

Upon relocating heifers to irrigated pasture heifers were mingled with cattle that had not previously received fly control treatment.  Furthermore, those cattle on methoprene for control of horn flies were surrounded on all sides by cattle receiving no horn fly control treatment (including the control group for the study).  Fly migration from untreated herds is a likely explanation for the lack of difference in fly counts between the methoprene treated and control heifers during period two.  Another possible explanation for the lack of difference in fly populations is the lower than label recommended intake of methoprene by the treated cattle, which would indicate improper levels of methoprene in the manure of these heifers, thus, leading to decreased control effects.

Previous studies investigating horn fly control in yearling beef cattle (DeRouen et al., 2003 and Floate et al., 2001) showed percent control levels ranging from 68% to 100%.  During period one of this study an overall horn fly percent control of 16.7% was achieved.  Period two of this study provided an overall horn fly percent control of 3.6%.  The lower percent control percentages obtained in this study could be explained by the fly populations being below the economic threshold in both the control and treated groups and may not be a good indicator of the efficacy of methoprene for fly control.

Despite the lack of significant difference in horn fly control throughout this study, several studies demonstrate the efficacy of S-methoprene for control of various insects.  A study conducted by Young and colleagues (2004) found S-methoprene significantly reduced cat flea populations in beagle dogs.  Another study demonstrated effective emergence inhibition of the parasitoid Cotesia congregate with the use of methoprene (Beckage et al., 2002).  Methoprene in combination with diflubenzuron was shown to effectively control S. oryzae and R. dominica in stored sorghum (Daglish and Wallbank, 2004).

Although there is overwhelming evidence suggesting methoprene is an effective control tactic for insects, a study in Florida showed an isolated population of mosquitoes has developed some level of resistance (Dame et al., 1998).  Prolonged use of methoprene to control the mosquito, Ochlerotatus nigromaculis, showed higher lethal concentrations (LC) 50 and LC 90 were required and lower levels of control were achieved (Cornel et al., 2002 and Cornell et al., 2000).  Furthermore, a study conducted on field populations of the house fly showed some resistance to the IGRs diflubenzuron and methoprene (Kristensen and Jespersen, 2003).  In order to prevent methoprene from suffering the same fate as pyrethroid, organophosphate, and endectocide insecticides it would be wise to consider a horn fly control protocol that either rotates products from year to year or uses products in combination.

Further studies investigating the efficacy of methoprene should be conducted at several ranch locations in the northern California area during years when horn fly infestations are heavier.  While this study ended in August, horn fly populations also exhibit a peak in the months of September and October, so it would be wise to conduct a study that continues for the entire fly season.  To address the intake issue associated with this study, it may be wise to consider formulating a methoprene sustained release bolus to ensure the proper concentrations of methoprene end up in the manure to adequately prevent pupae from molting into adults.


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