With the last substantial rains being a distant memory, summer temperatures and low humidity are ideal conditions for spider mites – a scourge of the season for roses and lots of other plants.
Spider mites are related to spiders and ticks. With eight legs and an oval body 1/20 of an inch-long, they look like moving dots to the naked eye. Living in colonies that may include hundreds of individuals mostly on the undersides of leaves, they collectively spin fine webbing for protection from natural enemies and weather.
First you might see tiny, yellow, or white dot-like patterns appear on rose leaves. The upper leaf surface can take on a bronze or rusty color, ultimately turning yellow and dropping off. Shake leaves that appear to be infested over a piece of white paper – if you see tiny dark specs moving on the paper – you have spider mites.
Populations can build up to huge numbers in a short time. A generation can be completed in less than a week if the conditions are right – hot weather, low humidity and dust and dirt on leaf surfaces. Low numbers may make plants just look bad. Heavy infestations slow plant growth and cause leaves to drop prematurely, and severe infestations can kill plants, especially if plants are suffering from water stress.
Prevention is the best protection against spider mites. Focus on keeping plants healthy, well-watered and free of dust on leaves. That can be a challenge with water restrictions. Get creative! Capture the water from your shower and put it into a pressurized sprayer. The water mixed with air can easily knock the pests to the ground and give your plant a bit of extra moisture. As with any application of water to foliage, do it early in the day so leaves can dry.
Encourage natural enemies – they’re the most important weapons against spider mites. There are the general predators – lacewings, big-eyed bugs, and minute pirate bugs, and a few specialized predators. The shiny black, oval-shaped spider mite destroyer lady beetle, Stethorus picipes, and its larvae, consume about half a dozen mites per day. Western predatory mites, Galendromus occidentalis, are about the same size as plant-feeding mites but have longer legs and are more active; they also feed on pollen and other food.
Avoid using insecticides in the garden. They do not control mites (as they are arachnids, not insects), they may kill off natural enemies and can stimulate mite reproduction, especially when applied during hot weather, causing dramatic spider mite outbreaks within a few days.
If you have an infestation now, prune out damaged portions of plants or those with significant webbing. As the seasons change, temperatures drop and rains return, high mite populations generally decline rapidly and take a break until next summer. Implement preventive measures early next year and you can keep spider mites at a tolerable level.
By Nanette Londeree, Master Rosarian