Climate Smart Farming
#4 Bugs in our changing climate. What is eating my beans?
Between 2017 and early 2020 a severe drought gripped southern Australia. The south east region of NSW suffered badly with very low rainfalls; long periods of no rain at all; strong, hot drying winds, heatwaves, very low humidity and devastating bushfires. This region already experiences a very variable climate, but unfortunately an increase in hot dry periods is what we can expect with climate change.
Extreme conditions are stressful for farmers, backyard growers, plants and animals. And with increased climate extremes and hotter temperatures, conditions are favouring some pests species. The ranges of many species are changing - moving polewards (south in Australia) and to higher altitudes, and some pest outbreaks are increasing.
I have been grown climbing beans in rotations on the fences of my market garden for years without any real problems except sometimes early or late frosts. These beans have helped feed my family and friends, provided shade to improve the microclimate of my summer gardens, protected other plants from the wind and added some nitrogen back into the soil. In the last few years, I expanded plantings of these summer staples to sell into a regional produce box scheme and local markets.
But by early summer 2018 my climbing beans were not looking good. The leaves were starting to look like this:
Photo: Michigan State University
What was happening?
After a bit of research, I decided that spider mites were probably eating the leaves. Spider mites attack many plants across the world, including fruits, vegetables, hops and other vines, ornamentals and indoor plants. Spider mites are very tiny (<1mm). I didn’t manage to get enough magnification in the garden to see one, but the risk of attack from spider mites increases in the weather we were experiencing - hot conditions, exposure to drying winds and low humidity. It also increases with lack of habitat for beneficial insects. The damage to the leaves and the fine web on the leaves seemed unmistakable.
What should I do?
I removed and destroyed the infested plants. I thought the beneficial insects might be struggling in the climate conditions we were experiencing and decided to buy and release a predator, Phytoseiulus persimilis, a beneficial insect which eats spider mites.
Persimilis (red mite) and two spotted spider mite. Photo Dennis Crawford. Obtained from Spider mites - Bugs For Bugs
I thought the Persimilis would thrive and be effective. Surely we had a good microclimate. Our place is protected from the worst winds. Our summers are often moderate. We are about 850 m above sea level and catch a little of the cooler easterlies in the evenings. We also have companion plants, herbs and flowers growing nearby and fruit trees and vines to increase shade and habitat. So, after releasing the Persimilis, I planted more climbing beans.
The seedlings started to emerge, but the rains didn’t come, the hot, dry winds continued to blow, my efficient drip irrigation system, buried under mulch, continued to deliver water to the root zone, but the air all around the beans was hot and dry. The extremely low humidity and hot winds meant the spider mites outcompeted the Persimilis which couldn’t find enough humid places to reproduce well near my climbing beans.
But in another bed distant from the climbing beans, I had planted some dwarf bean seeds amidst a riot of self-sown rocket and spinach plants, with some shade from a young pear tree nearby. The Persimilis established here and seemed to be doing their job. A month or so later these beans were starting to look good, while my next lot of climbing beans over on the fence were struggling. I removed the climbing beans and considered us lucky to have any beans at all.
What did I learn?
Most plants (and animals) start to suffer with temperatures in the mid to high thirties, with many plants starting to shut down their transpiration by the time temperatures reach about 35 degrees Celsius. Low humidity and hot dry winds make this stress worse. When they are stressed, plants are much more susceptible to pests and diseases and some pests thrive.
Pests usually need to be managed quickly. I learnt to make more careful observations on my early morning review of the gardens, and if anything didn’t look quite right, to come back and check it at different times, to look more closely, talk to other local growers, research and to be ready to act quickly.
Extreme conditions also affect insects, reducing microclimates and restricting feeding and reproduction of some, but creating conditions for outbreaks of others. Beneficial insects are important to controlling other insects as well as to pollination of many species. But to thrive and work in our gardens, they need habitat and microclimate. It may be worthwhile to import some beneficial insects if poor conditions and lack of habitat means their populations are depleted, but they will not do well unless you improve the habitat for them.
Our garden has multiple vertical layers – grasses, herbs, flowers and vegetables, vines and fruit trees. These provide spaces for insects, reptiles and birds, all of which can eat pest insects. They also create patches of shade in hot summer conditions and establish a complex root zone with multiple levels under the soil. In this root zone more moisture and nutrients leach and nutrients are captured by the roots than in a more simply structured garden, and leaves falling in autumn return more of those nutrients back into the soil, increasing fertility. But we still did not have enough of the upper shade levels for long hot dry periods in our garden. Shade cloth can help but is not as good as a tree which provides more cooling and humidifying effect through its own transpiration as well as habitat and nutrient cycling.
The plants that did best in these extreme conditions were the ones that grew together amongst others, increasing the humidity in the immediate area and providing habitat for beneficial insects. I learnt that companion planting means more than a few flowers or herbs in a bed of one single vegetable species. Instead, I have started experimenting with different species planted more intensively together.
These changes to my planting design require more research and sometimes more work. Sometimes they fail, but mostly they help reduce climate change risks through diversifying produce. The more I rely on just one or two crops, the higher the risk that I will lose my cash crops and reduce the variety in my family’s diet and the biodiversity in my garden. I like to experiment but, if I diversify too far beyond my labour and knowledge base, all things can start to suffer. Finding a balance is important.
With climate change, extreme weather is increasing, but not always in the same way. Every season is likely to produce different challenges and different pests. This summer, 2020/21, I haven’t seen evidence of spider mite, but in the wet La Nina conditions, periods of high humidity have led to fungal attacks and air flow and drainage have become very important. The mostly cooler La Nina weather has restricted pollination and fruit set of some crops, such as eggplant, and there have been outbreaks of the sap sucking Harlequin Bug (Dindymus versicolor) which have damaged fruit, tender seedlings and some plants.
Conclusion
The high variability in our climate requires different approaches at different times. But in almost all conditions, it is important to create habitat to encourage beneficial insects like lacewings, ladybirds and Persimilis.
Other strategies include:
- designing some parts of the garden for hot, dry times and other parts to channel and increase air flow in humid conditions,
- increasing shade to reduce heat impacts and sunburn,
- using covers to minimise the impacts of sudden cold spells on heat loving plants,
- creating windbreaks with plants and structures,
- importing beneficial insects if there aren’t enough in the garden,
- rotating crops to reduce disease risk,
- observing closely, and often, what’s going on in the garden and
- being ready to make changes if plants aren’t growing well.
References and Resources
Bugs for Bugs: What’s your pest? – Bugs For Bugs
Bugs for Bugs: Bio control agents – Bugs For Bugs
Controlling Spider Mite - Fact Sheets - Gardening Australia - GARDENING AUSTRALIA (abc.net.au)
