Insects and Fertilization

Linda got a few comments and questions on her post a couple of weeks ago on fertilization and insect resistance.  This is an issue I’ve been peripherally involved with over the years so I wanted to share a few thoughts.  First, the relationship between plant nutrition and insect resistance is extremely complex.  We often have difficulty predicting how a plant is going to respond to fertilization, let alone predict how an insect is going to respond to how the plant responded.  I haven’t kept up but Koricheva (2002) reported over a dozen different theories have been proposed to explain insect response to plant nutrition.  One of the factors that makes it difficult to generalize about plant/insect interactions is that various insects feed on different plant parts in different ways; some are leaf feeders, some suck sap, some bore into wood, some feed on seeds or cones.  How an insect feeds can affect its response.  To stick with an illustration I’m more familiar with, we can look at insect response to plant drought stress.  Bark beetles are widely known to key in and attack pines and other conifers under drought stress but pine tip moths prefer succulent buds and new growth and are more likely to attack well watered trees.  It’s not unreasonable to think there are similar differences with nutrition.


Nevertheless, as noted above, there have been attempts to come up with general theories on the effect of plant nutrition on insect resistance.  One of the most widely cited is the Growth-differential balance theory proposed by Dan Herms and Bill Mattson  (“The dilemma of plants: to grow or defend.”  Q. Rev. Biol. 67: 283-335).  A quick check on Google Scholar indicated this paper has been cited by over 1,400 other papers, which is an astounding number and speaks to its influence.  The basic premise of the theory, as suggested by the title of the paper, is that plants make a trade-off between allocating carbohydrates for growth or allocating carbohydrates for secondary defense compounds.  Dan Herms subsequently applied the theory in synthesizing the literature on woody ornamentals in his 2002 paper,  “Effects of Fertilization on Insect Resistance of Woody Ornamental Plants: Reassessing an Entrenched Paradigm.” (Environmental Entomology 31(6):923-933.).  I have heard some arborists and others use this paper to argue that we shouldn’t fertilize landscape trees at all.  The problem is they oversimplifying the theory – which is understandable, this is pretty heady stuff.  They get the ‘trade-off’ idea; if plants grow fast they produce lots of yummy stuff for bugs.  But what is often overlooked – even though Herms makes a point to say it – is that when nutrition or other plant resources are low; there is no trade-off.

This figure from Herms and Mattson illustrates the idea.  If nutrients are deficient and we fertilize a plant the plant may increase growth and secondary compounds; it’s not always an either/or situation.  The bottom-line remains the same;  nutrient deficient plants can benefit from fertilization or correcting the factors (e.g., alkaline pH) that made them deficient in the first place.

Koricheva, J. 2002. The Carbon-Nutrient Balance Hypothesis Is Dead; Long Live the Carbon-Nutrient Balance Hypothesis? Oikos 98 (3): 537-539

Disney and Japanese Beetles

This past weekend I had the opportunity to speak at Epcot Center in Florida.  It was, without a doubt, one of the highlights of my career so far.  I spoke 6 times over the course of 3 days.  The focus of my talk was garden remedies.  I started by talking about Paris Green (a good story for next week), then made fun of some of Jerry Baker’s, Myles Bader’s and Joey Green’s recommendations, and then proceeded to talk about which homemade cures do and don’t work and why.  And then I asked the audience for questions.  I always ask the audience for questions at the end of my talks, but I’ve never had such a diverse audience before (both gardeners and non-gardeners from all across the country), so it was interesting to see which problems came up the most.  The winner was…. Japanese beetle with fire ants and deer coming in a close second and third.  With that in mind I thought I’d devote this column to Japanese beetle control.

Above is my newest prize possession — A statue of Mickey thanking me for speaking at Epcot

The first rule of Japanese beetle control is that you can’t control Japanese beetles.  Nobody has found a sure-fire cure yet and, if you try too hard, you’re going to poison yourself and everyone in your neighborhood.

The second rule of Japanese beetle control is like unto the first.  Don’t trust garden center employees to know a damn thing about Japanese beetle control.

The third rule of Japanese beetle control is not to make the problem worse that it already is.  Using a trap to lure Japanese beetles to their demise will kill a few — and may make you feel like you’re doing something — but you will be attracting more beetles to your yard than you kill.

The fourth rule of Japanese beetle control is that killing grubs doesn’t stop the adults.  In other words, while killing Japanese beetle grubs is possible (usually using imidacloprid), killing those grubs won’t prevent adults from flying into your yard after they’ve hatched from someone else’s yard.

The fifth rule of Japanese beetle control is that Japanese beetle control is dirty work. Most of the “organic” and biological controls just don’t work that well.  If you want to spray a concentrated dish soap spray on the beetles that will kill them, but it won’t last long and it will burn your plants.  Same thing with a spray of one of those citrus insecticides.  The organic insecticide pyrethrum will kill Japanese beetles, but it won’t last long and spinosad (another organic insecticide) which works for some beetles (it’s better on other insects)  just isn’t considered that great.  A biological control called Milky Spore Disease is supposed to kill Japanese beetles while they’re grubs, but the truth is that it usually kills less than 50% even in good conditions.

The sixth rule of Japanese beetle control is that, if you’re willing to go to a little bit of trouble, lose a few leaves, and use a little bit of a synthetic insecticide there is a way to protect your plants to some degree.  If you’ve ever been around these beetles then you’ll know that they prefer some plants over other — for example, they love roses, and so they’ll attack roses first.  If you spray roses with permethrin (a synthetic insecticide) you can get 7-14 days of clean roses, and, if you’re lucky, you’ll kill many of the Japanese beetles before they move on to other plants — this is called trap cropping.

The seventh rule of Japanese beetle control is that these beetles will seek revenge for their dead relatives.

Bags and Apples

One of my favorite garden “cures” is placing a clear plastic sandwich bag around apples when they are young to protect them from insects and disease.  It usually works great and impresses the heck out of people who see and eat the apples which are normally tough to grow without using  bunches of organic or synthetic pesticides.

Unfortunately this year was different.  Rebecca Koetter, the person who planted these trees and put the bags on the apples (on the University of Minnesota campus) discovered that birds may choose to ignore the bags.  And, as if that wasn’t bad enough, Asian lady beetles decided to get in on the fun!