Summary

Insekticidresistens hos væggelus i Danmark

The purpose of this project is to clarify whether bed bugs, Cimex lectularius, collected in Denmark have developed resistance to the insecticides most applied for control of bed bugs.

During the past three to five years focus on bed bugs has been enhanced – in particular in the USA, Australia, and England. The extent of the Danish bed bug problems have not been well established, however, there is a great deal indicating that they are growing. In any case it is to be expected that the prevalence in Denmark will increase when it increases in countries visited by many Danes. Travelling is considered the most important factor for import of bed bugs, and a rise in the number of travels – especially travels to more exotic destinations – may add to the bed bug problems. In Denmark only two classes of insecticides have been approved for bed bug control, i. e. pyrethroids and organophosphates. It is important to have several different classes of insecticides to choose from as the different classes should be used alternately in order to avoid resistance at all, and if resistance has occurred to one of them, the other can be tried. It is the very existence of resistance to the two remaining groups of insecticides used for control of bed bugs in Denmark that this project aims to map.

The survey has been conducted by means of a collection of bed bugs in various locations in Denmark and by a test of their susceptibility to the two groups of insecticides: pyrethroids, in this case permethrin and deltamethrin, and organophosphates, in this case chlorpyriphos. The susceptibility of the field collected bed bug strains was compared with a susceptible strain. Two types of testing were applied: a discriminating topical application test and a tarsal test on different surfaces. For the topical application test individual bed bugs are externally administered a drop of insecticide of a concentration considerably higher than what it takes to kill susceptible bed bugs. The tarsal test is less accurate; but on the other hand it is more realistic as the insects move on different types of surfaces treated with the insecticide like in a practical control.

A total of 20 field-collected bed bug strains were tested in the topical application with permethrin. Only one of the 20 field-collected strains showed total mortality for all the treated individuals, two strains had approximately 60% mortality, and the remaining strains showed no effect at all. These results indicate that often permethrin will have no effect at all in a practical control. When treated with chlorpyriphos, however, the susceptibility of the bed bugs is only slightly reduced. Only one of 14 strains measured had a mortality below 50%. This indicates that in all probability a practical control with chlorpyriphos will be effective, because the normal doses used are considerably higher than necessary to kill one individual in a topical application test. The products on the Danish market for control of bed bugs contain chlorpyriphos in either a lacquer or in a micro-encapsulated formulation with the effect that the insects in both cases are subjected to very high doses.

In fact, the tarsal test confirms the results of the topical application test. Neither permethrin nor the other pyrethroid, deltamethrin, proved particularly effective on the field-collected bed bugs. The pyrethroids had effect on only one of six tested field strains, and it was one of these strains that resulted in a 60% mortality in the topical application test. The micro-encapsulated chlorpyriphos product was 100% effective – a result obtained despite the fact that a dose of only 25% of the recommended was applied.

Due to the fact that only 20 field collected bed bug strains were included in the test, it is not possible with certainty to conclude that the results represent the situation among all bed bug populations in Denmark. However, we do believe that some general considerations about the practical application of the available insecticides can be made based on the present results. Chlorpyriphos, in particular the micro-incapsulated formulation, has proved sufficiently effective against all the tested populations and can thus still be considered suitable for bed bug control. As all the tested populations showed moderate to high resistance against permethrin and deltamethrin there is a high risk that other bed bug populations in Denmark will also be resistant against synthetic pyrethroids. As chlorpyriphos probably disappears from the Danish market during the next couple of years, all that is left are the pyrethroids which, in many cases, might be inefficient according to this project. Consequently, it is very important to examine the possibilities of both new control methods and methods for preventing and detecting bed bugs.