🚀 go-pugleaf

Is the interest really in comparing the time courses of mortality (eg
time to 50% mortality), as suggested by the experimental design? Most
of the entomological pesticide resistance work I have seen has focused
more on dose response relationships, eg LC50 = the concentration giving
50% mortality (in some defined time period of exposure, eg 24 hours).
Then sometimes ratios of LC50s are compared for resistant vs
susceptible strains. This would entirely eliminate concerns about
nonindependence of the time course.

On the other hand, the time course (rate of dying) might really be of
interest: most household bug sprays have ingredients specifically to
give a quick knock-down, since consumers don't want that bee or roach
to continue roaming around for even a few minutes. In this case perhaps
survival analysis (modelling the time to death of individual roaches)
would be the way to go.

In either case you will have to deal with the fact that the
experimental unit really is the batch of 10. Persumably this would
require some sort of nested model.

Hope this helps.

- Andy Taylor

***********************************************************************
*  Andrew D. Taylor                                                   *
*  Department of Zoology                 phone: (808) 956-4706        *
*  University of Hawaii at Manoa         FAX:   (808) 956-9812        *
*  2538 The Mall - Edmondson 152         ataylor@lala.zoo.hawaii.edu  *
*  Honolulu, HI 96822                    taylor@uhunix.bitnet         *
***********************************************************************


>       I have been confronted with yet another interesting challenge.  I have
> an entemologist that is using pesticides on field strain and susceptible
> strain cockroaches to test if there is increased resistance to the pesticides.
> Currently, nine batches of ten roaches each are placed in containers with the
> pesticide.  They are observed every 15 minutes and the numbers of dead roaches
> are counted at each observation time.  When graphing the cumulative number of
> dead roaches by observation time, the graph makes a fairly good sigmoid curve.
> A probit approach is currently being used, but I know that it is not
> appropriate since the counts at the observation times are not independent. So
> how should I proceed from here? My entemologist wants to be able to compare
> the response from the field strain to the susceptible strain as well as
> compare how the field strain may change over time ( more exposure to the
> perticide). In addition, how might I calculate whether the nine groups of ten
> is an adequate sample size?
> Robyn Lee
> lee_rb@mricd1.apgea.army.mil
>