Diverse plant choices and healthy plants fend off insects

We talk a lot about biodiversity. One aspect of biodiversity is to use a wide selection of different plants in your home garden.

Why is it that having many plants of the same species (called a monoculture) promote insect problems? Agriculture is a good example of a monoculture, where vast fields of a single crop is planted together. If an insect likes the plant, then it has a large food supply all in one place. By mixing plant species, insects must travel further for their next feed, so infestations tend to be localized and less severe.

Take poplar borer for example. This insect bores holes in the bark of aspen and poplars into which it lays an egg. The egg hatches and the larva tunnels and feeds on the tissue underneath the bark. This delicate tissue is how water and nutrients move through the plant. Once it is damaged, it cannot be repaired and the tree starts to decline and die, starting with the upper branches and eventually the entire tree. Swedish columnar aspen and tower poplar are popular tree selections, often grown in long rows: they grow quickly and provide screening from neighbours in a few short years. However, growing multiples of the same species makes them more prone to insect problems. A single infected tree becomes a “brood tree”, infested with many larvae, which mature and move easily to the tree next to it and will eventually damage or kill the entire row of trees.  The only solution is to cut down the brood tree to prevent the spread.

Monocultures are much more vulnerable to pest outbreaks than ecosystems with diverse plantings of trees, shrubs, perennials and annual vegetables, like the home garden. Furthermore, diverse ecosystems support different species of beneficial insects that keep pest insects in check. See the relationship? More plant diversity = more insect diversity. Monocultures = more pests.

What you plant matters too. Including native and near-native plants in the garden will attract and provide habitat for the natural enemies of insect pests. This does not mean that you should only grow native plants. It’s okay to mix it up with a few hybrids or other cultivated plants but do add more native or near-native plants as the opportunity arises.

Healthy plants can easily fend off minor insect problems. Insects always go for easy targets and are attracted to plants (especially trees) that are weak or stressed or in decline.

Decline is most noticeable in long-lived perennials like trees and refers to the loss of tree vigour and health. A thinning canopy, dieback of branches, and loss of needles are typical symptoms in trees. Decline happens over years and is the result of stress events that affect the metabolism and health of the tree. Consider the prairie drought conditions of 2021- 2024 and the extreme heat event in the summer of 2023. Many trees were not able to function normally and as a result produced less food stores (via photosynthesis) than in normal years. Insect and diseases often follow – and are easier to spot - but these are usually secondary problems.

Here are our top tips for keeping plants healthy and strong so they can fend off insect problems:

Choose the right plant for the right place. Before you plant, find out what your plant needs to thrive, beginning with the amount of sunlight needed by that plant. For example, vegetables and fruit bearing trees and shrubs need at least 8 hours of direct sun per day to be healthy enough to bear fruit. Less than that reduces fruit production and can cause them to decline.

Consider your soil conditions. Plants that need sandy soil with fast drainage like sedums, saxifrage and creeping thyme are not going to do well in clay soil which holds water for longer periods of time. However, a birch tree will thrive in a low spot in clay soil, provided there is ample water from rainfall or irrigation. 

Space plants properly when you plant. Overcrowded plants compete with nearby plants for sun, moisture and nutrients in the soil. Too little of these basic needs will weaken or stunt them.

Dense growth with poor air circulation makes for ideal habitat for certain pests such as whitefly or aphids. This is very common in dense, fast-growing vines like grapes or Virginia creeper. Thin out vines early in the growing season to promote better air flow.

Don’t rely solely on rainfall to water your plants. A good guideline for most plants is to water deeply but infrequently. Roots need oxygen as much as they need water, so allow soil to dry slightly in between watering. Apply 2.5 cm (1") of water every week unless it rains. To measure: set 1 or 2 empty tuna fish cans under the sprinkler. When the cans are full, you know you've watered enough. Deep watering encourages roots to grow deeply in the soil, making plants more resilient. This works well for vegetable gardens and most perennials.

Proper watering is especially important for high value plants like trees and shrubs. Trees benefit from 2.5 cm (1") of water every 7 – 10 days (unless it rains). However, use your best judgement in periods of extreme heat and drought and water more often, especially if the leaves are wilting. Water trees at the drip line (the area below and just beyond the widest part of the canopy) where the feeder roots are located.

As temperatures become cooler and days are shorter, water trees less often, perhaps every 14 – 21 days, unless your tree shows signs of wilting. Use your best judgement as it depends on your soil (sand/silt/clay) as well as your local weather conditions.  Too much water late in the season can delay dormancy which prepares trees for surviving winter temperatures. Once leaves start to turn colour, usually in mid-September, stop watering entirely.  After all the leaves have dropped, water again once or twice just before the ground freezes. Do not fertilize your tree in summer or fall as this can also delay dormancy which can lead to winter kill.

Weeds can be alternate hosts for insects. For example, common chickweed attracts lygus bugs or thrips. It’s much easier to prevent or reduce weed growth by using mulch in the garden. No bending and picking needed.  As a bonus, mulch provides habitat for beneficial insects, keeps moisture locked in the soil longer and improves the health of the soil.

Go easy on nitrogen fertilizer! Using compost and mulch in the garden promotes healthy soil and supplies ample nutrients to plant roots. Too much nitrogen from synthetic fertilizers applied late in the growing season puts on tender growth making it easier for insects to feed.

Plants that in the peak of health can resist insect problems. Diverse plantings with at least a few native or near-native plants ensure an ample supply of nectar and pollen to support a diverse population of insects including pollinators and beneficials. Best of all, adopting these practices will make you a better gardener!

 

 

This article was funded by the City of Saskatoon through our Healthy Yards partnership, the Cyril Capling Trust Fund of the College of Agriculture and Bioresources, and the Department of Plant Sciences. Thank you all for helping us grow!