Always fun when you find a research paper that confirms what you’ve suspected all along. I ran across a paper last week in the Annals of Applied Biology entitled ‘Fertilisers and insect herbivores: a meta-analysis’ (Butler et al. 2012. Ann Appl Biol 161: 223–233). I’m interested in the topic because in recent years a dogma has emerged that if you fertilize a landscape tree it will be immediately devoured by insects. In this study the authors conducted a meta-analysis (basically a compilation of studies on a given topic and then combining and analyzing the aggregated results) and looked at dozens of studies of the response of insect herbivores to fertilization to answer the question, does fertilization increase insect damage? The answer was absolutely no surprise to me: It depends.
What does it depend on? First, what type of insect. Secondly, what kind of fertilizer. For example, fertilizing with nitrogen greatly increases populations of sucking insects. This makes sense when you stop to think that aphids and other sucking insects have to consume a lot of phloem sap –which is essentially sugar water – in order to get sufficient nutrients. Nitrogen fertilization did not significantly increase populations of chewing insects, however. This could be related to off-setting effects of improved nutritional quality of leaves versus increased presence of defense compounds or leaf toughness. For other fertilizer elements Butler et al. found that phosphorus decreased insect populations in 2/3rd of the studies (14 out of 21) and that potassium decreased insects in 7 out of 10 cases. As with nitrogen only, complete fertilizers (NPK) tended to increase insect populations, especially for sucking insects.
I should hasten to point out some limitations of the study as it relates to tree fertilization. First, of course, is the British spelling of fertilizer. Second, the study mainly dealt with fertilization in agronomic crops, not trees. Lastly, the authors only included studies on insect adults. In many cases insect larvae, not adults, are the most damaging life stage, especially for insects that affect trees. Nevertheless, the study highlights the difficulty of making generalizations when discussing host stress and insect interactions. In addition to type of insect and type of fertilizer, we could have added nutritional status of the plant before fertilization to the ‘It depends’ list. My rule of thumb is that trees shouldn’t be fertilized unless a problems is noted by visible symptoms, a soil test, and/or a foliar test – and preferably by more than one of these.
Bottom line: Before you buy into the notion that fertilizing a tree is going to increase insect problems make sure you know what type of pest you’re dealing with, what type of fertilizer and the current nutrient status of the tree.