Dear R-Sig-Ecology,

I'm analysing an experiment about mosquito larvae beeing treated with 3 conditions (no competitors added = "algae", few competitors added ="new", many competitors added ="old"). After 2 weeks I contaminated all conditions with 3 insecticide concentrations (no treatment = "control", low concentration ="low", medium concentration = "medium", high concentration ="high"). The effect of treatments was observed over a time period of 35 days. 
Now I would like to analyse, if there is a synergistic effect of condition and insecticide treatment. I used a gls model (see below).
(Important: the mosquito larval abundances differed significantly between conditions when insecticide contamination was applied. I'm not sure if this is a problem for the further analyses....)

Obviously there is a synergistic effect (condition * contamination, p<0.001), but I don't know how to interpret the output. Which treatments did the model compare?Does it mean, that the effect of medium and high contamination increased significantly at old-condition compared with the effect at algae-condition? (This I can hardly believe. Mosquito larval abundances were close to zero even without contamination at condition-old&contamination-control , hence an additional effect of insecticide treatment should be hardly detectable. I would rather expect an effect at condition-new in combination with contamination...)

I used:
mod1<-gls(log10(mosquito+1) ~condition * contamination , data=MK ,
 correlation = corAR1(form=~day |bucket),
 weights=varIdent(form= ~ 1|condition),
 method="REML")
summary(mod1)
anova(mod1)

The output was:

> anova(mod5)
Denom. DF: 258 
 numDF F-value p-value
(Intercept) 1 221.04772 <.0001
condition 2 243.79461 <.0001
contamination 3 24.71058 <.0001
condition:contamination 6 23.92481 <.0001

Coefficients:
 Value Std.Error t-value p-value
(Intercept) 1.9778939 0.1391071 14.218493 0.0000
conditionnew -0.7766661 0.1610017 -4.823963 0.0000
conditionold -1.8958920 0.1461519 -12.972068 0.0000
contaminationhigh -1.2825250 0.1840214 -6.969433 0.0000
contaminationlow -0.2044417 0.1840214 -1.110967 0.2676
contaminationmedium -0.7833268 0.1840214 -4.256715 0.0000
conditionnew:contaminationhigh 0.1011773 0.2129852 0.475044 0.6352
conditionold:contaminationhigh 1.2005231 0.1933408 6.209364 0.0000
conditionnew:contaminationlow -0.1917487 0.2129852 -0.900291 0.3688
conditionold:contaminationlow 0.1496570 0.1933408 0.774058 0.4396
conditionnew:contaminationmedium -0.1454105 0.2129852 -0.682726 0.4954
conditionold:contaminationmedium 0.7940461 0.1933408 4.106978 0.0001

Can anyone help me?
Many thanks,
Iris

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