Residual effects of selected pyrethroid insecticides on lone star ticks.

!!! BEGIN AUTH ABST Ticks transmit the causative agents of a variety of serious human diseases such as Rocky Mountain spotted fever, Lyme disease, and ehrlichiosis. Accordingly, reducing the number of cases of these diseases through tick control is a desirable goal for public health departments. This carefully controlled study evaluated the effects of three pyrethroid products against lone star ticks under field conditions. To test residual insecticide effects, live ticks were placed in the grass on three dates over a 15-day period inside artificially constructed tick enclosures which had been previously sprayed with the insecticides. Two of the insecticides, Permanone[R] 10 Percent EC and Suspend[R] SC, provided 100 percent tick control for the entire 15-day study period. The third product, Saga[R] WP provided 100 percent control for the first seven days. !!! END AUTH ABST

Introduction

In addition to having nuisance effects, tick bites transmit disease (1). Accordingly, tick control is a desirable goal for public health departments. Several tickborne diseases occur in Mississippi: Rocky Mountain spotted fever (RMSF), Lyme disease, ehrlichiosis, and tick paralysis. Approximately 20 to 30 cases of RMSF and about 10 to 20 cases of Lyme disease are reported annually in the state. Ehrlichiosis and tick paralysis are only rarely reported. Although cases of tickborne diseases are few relative to those of other diseases, they are consistent sources of morbidity and mortality each year. RMSF is especially severe, with a fatality rate of approximately five percent, even with treatment.

One goal of the entomology program at the Mississippi Department of Health is to explore ways of reducing the number of cases of tickborne disease. With the broader objective of setting up large-scale tick control projects in the Mississippi state parks, several small initial studies were undertaken to test insecticide efficacy against ticks and to identify ways to precision-target insecticides (i.e., to exactly place the chemical only in spots where pests occur). Earlier studies with the lone star tick, Amblyomma americanum, demonstrated that the tick is not evenly distributed in nature, but "clumps" or "clusters" instead (2,3). This behavior led the author to explore the idea of identifying spots of tick clustering in the woods and precision targeting those areas with insecticides. A question about the long-term effectiveness of precision-targeted insecticide was the basis for the present study.

Traditional organophosphate insecticides are certainly effective against ticks and have been used previously (4,5). Many of the newer pyrethroid insecticides have higher [LD.sub.50] values, however, and are generally considered safer. ([LD.sub.50] is measured in milligrams per kilogram of body weight and refers to the amount of pesticide that constitutes a lethal dose for 50 percent of a sample of laboratory rats. Thus, higher [LD.sub.50] values denote safer products.) Pyrethroids are pyrethrin-like materials, synthesized by chemists, whose action generally is improved in some way (such as photostability) over that of natural pyrethrins. In this case, the improvement is long residual action when the pyrethroid is used as a surface spray.

This carefully controlled study used artificial tick enclosures (pens) to evaluate three pyrethroids, all donated by AgrEvo Environmental Health (6). The purpose of the study was to evaluate long-term residual killing effects of the insecticides under normal outdoor meteorologic conditions (sunlight, wind, rain, etc.).

Materials and Methods

The basic idea was to create holding pens in an outdoor area, spray each one with a different product, and then systematically release ticks into the pens at intervals to evaluate how long each product would continue to kilt ticks. The only limiting factor in this study was the availability of live ticks. Two shipments, each of approximately 400 lone star ticks (equal numbers of male and female), were received from Dr. Matt Pound and co-workers at the U.S. Department of Agriculture (USDA) Livestock Insects Laboratory in Kerrville, Texas. The ticks were subdivided into separate vials containing 20 ticks each for release into the tick pens. Twelve tick pens i square meter ([m.sup.2]) each were constructed with thick, clear plastic in an area of high grass (see top photo on page 22). At the bottom of each pen the plastic was tucked inside and held down by large nails driven into the ground. Three products and one control were randomly assigned to the plots. This pattern was replicated three times, yielding a total of 12 plots. The products evaluated were Permanone[R] 10 Percent EC, which has a U.S. Environmental Protection Agency (U.S. EPA) registration number of 4816-688 (active ingredient permethrin); Saga[R] WE U.S. EPA registration number 432755 (active ingredient tralomethrin); and Suspend[R] SC, U.S. EPA registration number 432763 (active ingredient deltamethrin Deltagard[TM]). These products are commercially available to pest control operators. On May 5, 1996, each insecticide was mixed according to label directions and sprayed with a handheld 1-gallon sprayer in each assigned plot. Sufficient solution was applied to thoroughly wet the grass. Nothing was done to the three control plots. This was the only insecticidal treatment. Plots were open to sun, wind, and rain the entire time. The first batch of live ticks (20 per plot) was placed in the plots 24 hours later. Another 24 hours later, each plot was sampled for ticks with a 1 [m.sup.2] white flannel cloth. Sampling consisted of two careful swipes over the grass within each pen. The cloth was then examined for live ticks. Numbers for all three replicates were combined for each treatment and the control. This release/sample process was repeated on Day 7/8 and again on Day 14/15. Each time, the sampling occurred 24 hours after tick release. The number of ticks collected indicated product effectiveness. Accordingly, "no ticks collected" indicated a lethal residual effect.

Results and Discussion

It rained on the plots two times during the 15-day study period - approximately 0.10 inch on May 9 and 0.50 inch on May 11. Weather on the other days consisted of varying amounts of sunshine and mild temperatures (range 49 [degrees] to 88 [degrees] F), typical for mid-May in Mississippi. One might wonder why mid-May was chosen, and what the outcome of the test would have been if it had been conducted in the middle of summer. The activity of lone star ticks (both adults and nymphs) peaks in the southern United States from late April through late May. Accordingly, precision-targeting of insecticides for lone star tick control would be most effective if accomplished during this time of greatest activity.

Ticks seemed to survive well in the control pens during the study period and could easily be observed on grass or leaves inside all three of those pens (see bottom photo on page 22). As might be expected, ticks were collected in control plots during each sampling period [ILLUSTRATION FOR FIGURE 1 OMITTED]. Not one live tick was collected during the 15-day study period from the Permanone 10 Percent EC and Suspend SC test plots, even though 20 ticks were released three different times into each plot (times three replicates, for a total of 180 ticks) [ILLUSTRATION FOR FIGURES 2 AND 3 OMITTED]. This result indicates complete tick control by the residual effect of these products. Saga[R] WP, on the other hand, began to lose effectiveness somewhere between Day 8 and Day 15 [ILLUSTRATION FOR FIGURE 4 OMITTED].

Results of this study demonstrate that at least two commercially available pyrethroids are sufficiently lethal to ticks to provide 100 percent control in the natural environment for at least two weeks during late spring. Armed with this information, we can be confident that spraying for ticks will be effective (if the chemical comes into contact with the ticks) for at least two weeks.

Corresponding Author: Jerome Goddard, Ph.D., Medical Entomologist, Bureau of Environmental Health, Mississippi Department of Health, P.O. Box 1700, Jackson, MS 39215.

REFERENCES

1. Goddard, J. (1996), Physician's Guide to Arthropods of Medical Importance, 2nd ed., Boca Raton, Fla.: CRC Press.

2. Jackson, L.K., D.M. Gaydon, and J. Goddard (1997), "Seasonal Activity and Relative Abundance of Amblyomma americanum in Mississippi," Journal of Medical Entomology, 33(1):128-131.

3. Goddard, J. (1997), "Clustering Effects of Lone Star Ticks in Nature: Implications for Control, Journal of Environmental Health, 59(10):8-11.

4. Bloemer, S.R., D.R. Barnard, A. Morris, G.A. Mount, E.L. Snoddy, and R.H. Zimmerman (1990), "Management of Lone Star Ticks in Recreational Areas with Acaricide Applications, Vegetative Management, and Exclusion of White-Tailed Deer," Journal of Medical Entomology, 27(4):543-550.

5. Sardelis, M.R., L.R. Boobar, H.J. Harlan, E.G. Milstrey, K. Neidhardt, and M.J. Pench (1989), "Reduction of Amblyomma americanum Population at Ft. A.P. Hill, Virginia, by Aerial Application of Diazinon Granules," Journal of Medical Entomology, 26(5):494-496.

6. Permanone[R] 10 Percent EC, Suspend[R] EC, and Saga[R] WP are registered trademarks of AgrEvo Environmental Health Company, Montvale, N.J. Other synthetic pyrethroids are available. The Mississippi Department of Health does not endorse any particular commercial product.

Did you know . . .

One female tick can produce 2,000-4,000 eggs.

(Source: "Ticked Off" Tip Sheet, Oakland's Channel 5, http://www.kocotv.com/5oys/ticktips.html)