Answer to the exercise about tree sampling. In case of transect, spruces still have a better chance to be selected. Also, this forest could have some specific structure along the transect. So to improve method, one can use several transects and increase distances between selected trees.
Answer to the weevil question. In that case, first were always most active insects which piked the lethal dose of the chemical mush faster than less active individuals. Rule of replication was also broken here because one dish was used for the sequence of experiments. We think that if you read this explanation and understand it, it already became clear how to improve the experiment.