Dealing with Garden Pests: A Scientific Approach

Every garden has unwanted visitors. Sometimes they are large, like the squirrels who keep planting walnuts and pecans among my flowers. Sometimes, they are small with too many legs. The large ones do little damage; they are just an annoyance. The little ones, though, can really cause problems.  

The two worst small pests are whiteflies and aphids. The whiteflies live on the catmint, and the aphids take to black-eyed Susans. All other plants share lacewings, mealy bugs, and beetles— none of which ever get out of hand. But, the aphids? They were so thick on the Susans that I once mistook them for the plant’s natural color. An errant brush against the catmint brings up a cloud of flies, so many that if one happens to be talking while pruning, a lot of spitting and epithets will ensue.  

All gardening books provide pest advice. All advice works well, right up until it doesn’t. The pests never hear the advice, so none know they are supposed to succumb to the various methods for getting rid of them. By July, the pests are in high season. Each has selected a favorite plant and started having families, and the struggle reaches a high point. I have sprayed neem oil until I got hand cramps. The whitefly population remains unchanged.  

Aphids on black-eyed Susan leaves

Complicating matters are the bees. Since I want bees to visit, I don’t dare use any insecticide that would harm them. Insecticidal soap is more threatening and much less effective than it sounds. So, what is a gardener to do? Frustrated, I decided to try a different approach. Since my peace of mind and enjoyment of the garden were at stake, I decided to determine, scientifically, how I might deal with pests without going off the deep end—like Bill Murray in Caddyshack. Also, I wanted a tool that would tell me when it was okay to simply give up. Here are my rigorously tested findings. 

First, we need to calculate how long it will take to develop pest-based fatalism (TPbF). Use these equations: 

TPbF (Time to Pest-based Fatalism)
LaborIntensityPredictor = GA/P
MiseryFactor = Bugs x (1/HeatHumIndex)
TPbF= (Labor Intensity Predictor x Misery Factor)/30

GA/P is the garden area (GA) in square feet divided by the number of plants (P). If your garden is spread over a large area with relatively few plants, the pests can’t congregate and help each other, so you have a chance to win. But in a small plot like mine, where they are as thick as thieves, the likelihood of winning the battle is not so great. Don’t despair yet, though; there are a few other factors that could work in your favor.   

For the Bugs value, just plug in any number over 1000. There are really many more than that, but we are lying to ourselves and putting 1000 because it seems like a big number, and we don’t want to know or even consider the actual number. The 1000 also accounts for mosquitos that appear out of nowhere. (Yes, fudging like this is allowed in science—google “dark energy”). Now, everyone knows that it is easier and more pleasant to work outside when the humidity is low and the temperature moderate. (This is useless knowledge during summer garden season.) Ideally, you want the HeatHumIndex to be no higher then 1.0, anything above 1.50 is bad. If you are in the South, your HeatHumIndex is 1.95. C’est la vie… 

Assassin bug nymphs–good guys

Multiply the HeatHumIndex by the Bugs value to obtain the MiseryFactor. I know this is a lot of math, but you are doing fine. Finally, multiply the LaborIntensityPredictor by the MiseryFactor and divide the result by 30 (days) to get the TPbF.  

Here is a worked example. For a 10 x 10 foot garden with 50 plants, the LIP = 2. The MiseryFactor in someplace like Vermont is (1000 x 1/1) = 1000, making the TPbF = 2000/30 = 66 days before fatalism is likely to occur. In the South, this would be (2 x (1000 x 1/1.95))/30 = 34 days

A reasonable TPbF is 60-90 days, which counting from mid-April, is mid-July. At that point, people who are giving up will start thinking fall is coming, and all this stuff will be dead anyway—textbook pest-based fatalism. If you are approaching this psychological milestone, you are likely wondering what to do. Before we get to the advice part of the post, you need to know about the Plant Replacement Index (PRI).  

The PlantReplacementIndex is determined by: 1) the number of viable plants you have left and the cost to replace any plant, including labor, 2) how likely it is the local garden center will have that same plant next spring, and 3) the number of bites, mosquitos or otherwise, you are likely to incur while replacing a plant. This is a quantum-ish calculation involving dark energy, so I’ll give you a simplified way to arrive at the PRI. If the plants in question are common varieties that go on sale every spring, the PRI is 45.324 ergs.  

To wrap up this treatise–if the ToF-PRI >= 0, then you probably live in the northern parts of the country, and a hot day is in the upper 70s. Go battle the bugs and enjoy the gentle breezes on your skin. If ToF-PRI < 0, then you really only have one viable course of action— grab a beer or glass of cool chardonnay and focus only on the plants that are doing well. The others are toast, so why let them ruin your summer? Next year will come, and plants go on sale every year.

As a final step, start turning your gardening pest battles into stories and anecdotes that you can use on your blog or share with friends over drinks. Isn’t gardening great!!!???


  1. I was never very good at math beyond the basic four methods, and your calculations look a lot like algebra, but even I can see that here in Northern Michigan, as in Vermont, 66 days to fatalism may well coincide with first frost. What luck!

    1. Author

      Great luck indeed!

      As it is, I have to wear a long-sleeved jacket covered in insect repellent and gloves just to water. So, I have about five more weeks before I give up for the year.

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