Unattacked willows

It is possible that the rapid drop in biomass of the assay Insects feeding on leaves from attacked and control trees coincided with reload of the test trees by chance, possibly due to the rapid spread of a pathogen through the laboratory Insect population (11). On the other hand, the possibility that reload of the test trees caused the biomass drop in both test and control assay Insects cannot be discounted. Both attacked and nearby unattacked control willows may have rapidly decreased the food quality of their leaves in response to a sudden Increase In biomass of Insects on the attacked trees. If so, this suggests that unattacked willows are sensitive to signals from the Insects or nearby attacked willows. This possibility was investigated in the following experiment. 

On the other hand, no differences were found in growth or mortality of tent caterpillars fed leaves from unattacked willows compared to those fed leaves from willows attacked by tent caterpillars (Figure IB). Reload of the attacked trees with additional tent caterpillars coincided with a rapid decrease in biomass and an increased mortality of Insects fed leaves from both attacked and unattacked control trees. This could have been by chance, but it suggested the possibility that both attacked and control trees exhibited a rapid decrease in food quality in response to the addition of more insects to the attacked trees. If so, this suggested that unattacked willows were sensitive to signals from nearby attacked willows or the attacking Insects. An experiment to test this hypothesis produced positive results (Figure 2). 

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