Colorado Potato Beetles and How to Predict Garden Pest Infestations

Colorado Potato Beetle laying orange eggs on stressed tomato plants.
Colorado Potato Beetle laying orange eggs on stressed tomato plants.

Sometimes there seems to be a disconnect. We forget that garden pests in one context are insects for nature or scientific study in another. This brings me to the Colorado Potato Beetle. This indigenous insect is just as fascinating as a ladybug or Solider beetle.

If it were named the Colorado Striped Beetle we would find it fascinating. I think it’s the word ‘potato’ that throws me off. And the fact it eats my food crops puts it in another class of being, the garden pest.

The Colorado Potato Beetle is originally from the midwest and western areas of North America. They have migrated eastward. When the Europeans started growing the American potato the beetle went along. The beetle is a serious pest in Europe. The main hostplant of the larvae are planted in the nightshade family. This includes tomatoes, potatoes, eggplant, and others.

Colorado Potato Beetle laying orange eggs on stressed tomato plants
Colorado Potato Beetle laying orange eggs on stressed tomato plants. Photo by Donna L. Long.

Since humans place a high value on those crops, heavy amounts of pesticides have been used to kill the beetles. The beetles that survived the bombardment of chemicals through a physical characteristic or for something other reason, reproduced. These beetles passed whatever genes or physical attributes that enabled them to survive the chemicals, onto their offspring. Now the chemicals used to kill Colorado Potato Beetles years ago have little or no effect on the current population. So, my organic, pesticide-free, remove- each-insect-by-hand techniques are the best method to protect my crops from an infestation.

The Colorado Potato Beetle overwinters near the site of the past summer’s infestation. It emerges in the spring, finds a mate, and lays eggs underneath the leaves of nightshade hostplants. The lovely orange eggs, hatch and the larvae begin to devour the host plant. The larvae grow and reach the stage when they must pupae into adults. The larvae drop to the ground and for two weeks, change into the striped backed beetle we see in our fields and gardens. The adults emerge or overwinter in the ground until the following late spring when they emerge, mate, and lay eggs. The cycle begins again.

The leaves of a plant are where you will spot the beetle. The small holes eaten on the interior of an infested plant are made by the larvae. The large holes along the edge of the leaves are made by the adult beetles. If left untreated, all the leaves of the plant will be eaten. The plant will not have the food making factories that are the leaves.

I noticed the beetle on plants that were already stressed. There were several eggplants and tomatoes I hadn’t planted in the ground yet. The plants were stressed from drying out, too much sun, too small a pot, and perhaps little nutrients left in the small containers. These stressed plants, not the thriving ones already planted, hosted several beetles.

5 June 2019 Donna's Nature Journal
5 June 2019 Donna’s Nature Journal

I need to watch out for the Colorado Potato Beetle next year. That is where phenology comes in. I used my indicator plant, the Shadbush tree growing in my backyard, as a natural guide. When I first spotted the Colorado Potato Beetle, the Shadbush berries were dark, ripe, and the birds had started to eat. I also noticed strange growths on the shadbush berries. The growths looked like galls infecting the fruit. The Catawba tree across the back drive was dropping the last of its’ blossoms. And the Red mulberry tree near my community garden plot, berries were turning from green to red.

5 June 2019 Donna's Nature Journal
5 June 2019 Donna’s Nature Journal

Now I know when to check my young crops for the mating adults and bright orange eggs of the beetle in question. And that is what phenology is: observing the life stages of the plants and animals when a natural event happens.

Observation and Studying the Colorado Beetle

After observing the Colorado Potato Beetle, several questions come to mind.

  • Is there a connection between a stressed plant and the Colorado Potato Beetle choosing that stressed plant?
  • Does a stressed plant give off a chemical signature?
  • Does the female beetle go by taste, scent, touch or sight? We know female insects use sensors in their feet to identify a suitable host plant, but is plant health part of the choice?
  • How long after mating does the female lay her eggs?
  • How long does it take for the eggs to hatch? How long is each stage of development?
  • How long does an adult beetle live?
  • How long does a beetle live after emerging in the spring?

I doubt I’ll study the beetle to find these answers, but others might. I bet agricultural scientists already have the answers to these questions. But, then again maybe not.

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