One parched, hot summer day about 15 years ago, my husband and I ventured to the annual plant sale at the Arboretum at Flagstaff. We were recent transplants from Southern California and couldn’t wait to get our hands on some landscaping plants native and/or adapted to the region. Other than a jumble of weeds, our property was nearly devoid of vegetation.
The sale was delightful, and we returned with armfuls of perennials and shrubs. We had other places to go that day, so we set the plants down in the shade next to the house. We returned a couple of hours later to discover nearly all the foliage of our new treasures had been devoured by grasshoppers. We had never encountered this before. What happened?
Since then, we’ve experienced several summers of insect infestations of one kind or another, and I have often wondered: Can increased insect infestations correlate with climate change?
The research I have come across shows that the answer to this question is complex. Let’s look at what we already know: Climate change results from increased greenhouse gases like carbon dioxide and methane in the earth’s atmosphere. It’s causing our planet to warm, which brings on droughts, floods, extreme and fluctuating temperatures, and more. We also know that insects are ectothermic (cold-blooded), meaning their body temperature is relatively the same as their environment. Additionally, the greater the environmental heat, the faster insects grow. Thus, entomologists predict that as the earth warms, insect herbivores will consume more and develop quicker.
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Compounding the issue, plants in their natural environments may not be adapted to increasing temperatures, and as in the case of the American Southwest, may simultaneously suffer from drought. And so, some species of native plants and trees are dying. In response, insects are spreading out in search of greener pastures, so to speak, like weeds, crops, watered landscapes, and gardens.
Some studies have shown that weed control in fall and spring may alleviate summer grasshopper infestations. It has also been shown, however, that it is unwise to destroy weeds already teeming with grasshoppers, as this will cause the critters to find nourishment elsewhere — like your veggies. (I suggest treating our gardens with Nolo bait while the hoppers are still young.)
In pursuit of answering my question, I came upon some fascinating research exhibiting that fungal growth on grasshopper eggs may prevent them from hatching. And since parching, hot weather inhibits fungal growth, more grasshopper eggs are enabled to survive. (This could indeed be a clue as to why my grasshopper infestation was so bad about 15 years ago.) Winter weather, on the other hand, doesn’t seem to affect grasshopper eggs since this region doesn’t get cold sufficiently.
Unlike grasshoppers, aphids and spider mites can develop more than one new generation a year. Thus, these sap-sucking critters may increase exponentially in one hot, dry season! This also holds true for some other pestiferous arthropods.
Let’s not forget about increasing levels of CO2. Data has shown that many arthropods like Japanese and Mexican beetles are more apt to scarf down plants grown under high levels of CO2 because these plants may manufacture more sugar. But heightened leaf damage and frass(poop) caused by heightened insect activity also draws more predators, with the predators ofttimes winning the battle.
Adding fuel to the fire, two or more concurrent plant stressors like weather extremes and/or elevated carbon dioxide levels are likely to become more prevalent across the globe. Studies reveal that these double stressors may have a synergistic effect on insect crop damage. One case in point is in southern Africa, where the destructive fall armyworm has been proliferating due to a combination of increased heat and precipitation.
Getting to the bottom of my question, “Can increasing insect infestations correlate with climate change?” I would say it is highly likely, given that insects eat more as the mercury rises, and given other evidence I have cited in his column. The answer, however, is multifaceted, and we must take caution in making generalized conclusions. Scientists are delving into the subject. We can only hope they present us with a broad scope, extensively researched data along with practicable solutions soon. Our planet’s life may depend on it.
Cindy Murray is a biologist, co-editor of Gardening Etcetera. and a Coconino Master Gardener with Arizona Cooperative Extension.
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