I ran a two-way ANCOVA on the data below, in r-studio, to determine whether pupal weights Pupal.wt were affected by caterpillars' diet Trt, initial weights Initial.wt, and the amount of time it took each caterpillar to reach pupation Larval.period: aov(Pupal.wt ~ Trt*Initial.wt*Larval.period, data = GM). Is a Tukey HSD test sufficient to follow this analysis? If so, how can I change either the code or my data to make TukeyHSD() work (I get this error message when I try to run it:

ANCOVA <-aov(Pupal.wt ~ Trt*Initial.wt*Larval.period, data=GM)
TukeyHSD(ANCOVA) 

Error in rep.int(n, length(means)) : unimplemented type 'NULL' in 'rep3' 
In addition: Warning messages: 1: In replications(paste("~", xx), data = mf) :   
non-factors ignored: Initial.wt 2: In replications(paste("~", xx), data = mf) :   
non-factors ignored: Larval.period 3: In replications(paste("~", xx), data = mf) :   
non-factors ignored: Trt, Initial.wt 4: In replications(paste("~", xx), data = mf) :   
non-factors ignored: Trt, Larval.period 5: In replications(paste("~", xx), data = mf) :   
non-factors ignored: Initial.wt, Larval.period 6: In replications(paste("~",xx), data=mf):
non-factors ignored: Trt, Initial.wt, Larval.period

Here are my data:

> str(GM)
'data.frame':   44 obs. of  5 variables:
 $ Trt           : Factor w/ 2 levels "Infested","Uninfested": 2 2 2 2 2 2 2 2 2 2 ...
 $ Larval.period : int  14 20 18 18 26 14 22 26 20 18 ...
 $ Initial.wt    : num  0.181 0.318 0.218 0.195 0.25 0.181 0.213 0.255 0.236 0.298 ...
 $ Pupal.wt      : num  0.294 0.519 0.527 0.326 0.511 0.299 0.584 0.442 0.481 0.452 ...
 $ X..weight.gain: num  62.4 63.2 141.7 67.2 104.4 ...