Like many people these days, although I’m not strictly organic, I prefer not to use chemical sprays in my garden, particularly on any plants that produce edible fruit or foliage. One significant drawback of not using chemicals, is that timing of pest control becomes much more critical. With chemicals pests can usually be easily vanquished. With softer sprays, certain life stages and heavy population pressure, pests can overcome the attack.
Fortunately, many of the nasties happen to emerge about the same time when the weather warms up in late spring. This is the time to hit them hard, when the bugs are at their most vulnerable and before they can start the next round of breeding. Key pests I focus on are scale, citrus white fly, vegetable white fly, potato psyllid, mealy bug, passion-vine hopper or aphids. These are all sap-sucking pests that significantly reduce production, infest plants with viruses and create conditions for other diseases to develop, such as black sooty mould.
I use a sprayer filled with a mix of neem oil and insecticidal soap (such as Yates Natra-Soap). This is a good all-purpose spray for most plants in the garden. You may need to dilute it more than indicated on the label for plants with sensitive leaves, like tomatoes and soft leaved ornamental plants like ferns.
Another good combination is horticultural spraying oil (such as Eco-oil, essentially vegetable oil with an emulsifier like detergent) and Eco-fungicide (also known as Eco-carb; baking soda is an alternative). This is effective on plants that suffer fungal diseases, such as black spot on roses and apples, and powdery mildew on vegetable crops.
The key is to make several applications about 7-10 days apart in late spring. I usually do my first spray straight after Labour Weekend, or a week or two later if it has been a particularly cold or wet spring, as the bugs will emerge later in these conditions. This year is one of those. If pest pressure remains high after Christmas, these sprays can continue to be applied, but control will be much less effective on more mature populations and as pests spread from other parts of the garden or from your neighbours.
Make sure the pressure is turned up on the sprayer and get into the middle of shrubs and trees with the nozzle so that the spray covers all surfaces, including the underside of leaves. This is important as these sprays are largely contact sprays, that is they need to land on the pest or disease to smother them or disrupt their feeding. Many pests concentrate on the underside of leaves and the interior of shrubs to give them more protection.
These types of sprays are more effective when they are wet than when they dry out, so the longer they remain wet the better. This is the opposite of many chemical sprays where the idea is to get the spray to dry as quickly as possible. I prefer to spray on cloudy, humid days towards the evening, but avoid spraying where rain is imminent. Although these sprays are generally safe on bees and other beneficial insects, they will still disrupt their feeding for a short time, so evening spraying helps avoid this too.