Chapter 51 Basic IPM

During the years from 1940 to 1972, the primary means of controlling insects was to spray with DDT or similar long-lived insecticides. Unfortunately DDT is the agricultural equivalent of a nuclear weapon: it kills ALL invertebrates (and over time, fish and birds). We learned some hard lessons as a result of this practice. First and foremost, not all insects are detrimental. For EVERY pest species, there is another insect that preys on it. Other animals (birds and spiders in particular) help control pest species as well. Second, killing both pests and their predators leads to MORE pests. Without predators, most pests cannot be eradicated entirely; they simply reproduce too fast. This is why trees “suddenly” become infested when they are moved indoors or to a new location. Pests were already there; either the predators did not follow, or their numbers were not high enough to keep the pests contained.

About the time DDT was banned, USDA formally changed its policy towards insect control from zero tolerance and routine calendar-based pesticide spraying to one of integrated pest management (IPM). IPM assumes that trying to eradicate all pests is expensive, unsafe, and often counter-productive. The goal instead is to keep pest populations below an acceptable threshold. Research showed that if almost all pests are killed then only those that have resistance will reproduce, making future population more resistant. Allowing a small population of a pest to survive at a reasonable threshold reduces the selection pressure that leads to resistance and dilutes the pool of resistance genes.

Greenhouse mealybugs are a good example of the wisdom of this approach. For many years it was standard practice in commercial greenhouses to spray all plants with high-potency synthetic insecticides on a set schedule. Over time many commercial greenhouses starting having problems with mealybugs and other sucking insects becoming resistant or immune to insecticides. The introduction of IPM strategies into commercial nursery production greatly reduced the incidence of chemically resistant greenhouse mealybugs, AND reduced production costs by reducing the need to purchase synthetic pesticides.

IPM also discourages repeated use of one type of control agent. Switching between different control agents further reduces the odds of resistance. Finally, IPM encourages combining multiple control strategies at the same time. These can be divided into:

  • Monitoring practices
  • Cultural and prevention controls
  • Physical controls
  • Biological controls
  • Chemical controls

51.1 Monitoring

Regular monitoring is the first step towards understanding how local pests and diseases behave. Post yellow sticky traps around your growing area to help attract pests that normally would be hidden or harder to spot. Keep records of what pests and diseases are encountered, when, and average air temperatures.

Most insects and many plant diseases develop at a rate that depends on temperature. Their appearance can be estimated based on ”degree-days:” the number of days above a particular temperature. With a little practice and personal data, you can use degree days to predict when a potential insect or pathogen outbreak is likely.

For example, I let my tropicals go through a short cold period before bringing them indoors each autumn. From experience I know that dormant mealybug eggs will hatch and start showing up after 30 days or so of temperatures above 70o>F. It happens around the same time every year, and I’ve learned to anticipate the mealybugs and respond so they do not get out of control.

51.2 Cultural and Prevention Controls

The goal here is to eliminate the underlying conditions that foster disease or pests. One of the first recommendations is to select cultivars that are resistant to local diseases. Unfortunately we do not have the luxury of choosing from a wide range of tree cultivars, and many of the species favored for bonsai are hosts to aggressive diseases like fire blight.

Cultural controls include general sanitation like removing diseased plant materials and burning or disposing of them offsite, and keeping debris cleared away where pests can hide. Cleaning pruning tools to prevent spread of infections is another routinely recommended cultural and prevention practice.

51.3 Physical Controls

Physical controls include insect traps and physical barriers that protect trees from infection. An example would be covering trees with woven mesh to prevent cicada damage, applying diatomaceous earth to soil to prevent slugs from reaching leaves, or spraying underneath leaves with a garden hose to kill aphids.

51.4 Biological Controls

Natural biological processes and materials can provide control, with acceptable environmental impact, and often at lower cost. The goal is to promote beneficial insects that eat or parasitize target pests. Biological insecticides derived from naturally occurring microorganisms also fall in this category.

I’ve written a separate chapter that has more information about how to recognize and attract beneficial insects.

51.5 Chemical Controls

Under IPM the goals are to use lower-toxicity chemical pesticides first, limit applications (both frequency and quantity), rotate the the agents used, and adjust spray protocols so they do not injure existing beneficial biological controls. Broad spectrum heavy duty insecticides like pyrethroids, organophosphates, and carbamates will kill most beneficial insects, but providing and not spraying refuge food plants helps keep them onsite and working to limit pests.

Again, I included more extensive discussion of chemical insecticides as part of the Arthropod Pests chapter.