Scale infestation of azaleas and holly bushes and practically everything in the garden

Asked May 30, 2020, 12:48 PM EDT

Hi, Here is UMD's advice for management from Mostly, the infestations we have match the cottony camelia scale pictures although some of the azaleas have azalea bark scale. Both the azalea and holly bushes both have sooty molds growing on the undersides of their leaves. "Scale insects are difficult to control. To manage a scale infestation, prune out branches with severe symptoms. Parasites generally provide control of many scale infestations. To protect parasites, avoid spraying with insecticides during the summer. If treatment is necessary, spray the tree with a dormant rate of horticultural oil during the dormant season to control overwintering scales. Also keep trees and shrubs watered during droughty periods and do not overfertilize." We have some questions. - Are there ways to encourage (not simply not kill) parasites? Which parasites should we encourage? - What is a good horticultural oil? Is neem oil horticultural? Related to this, what is the dormant season? - What other strategies - other than watering during drought - can you suggest to strengthen the plants? We don't fertilize at all, so perhaps a little fertilizing would be a good thing. Finally, is there anything else we could be doing? We have just planted a vegetable patch and now realize that the holly bushes on both sides are infested. - Do you have any ideas for protecting the young vegetable plants? Thanks in advance for your help, Be well, Andrew

Montgomery County Maryland

1 Response

Hello Andrew,

It would help us to see photos of the pests in question to make sure the correct approach to treatment is followed. Scale outbreaks have been widespread this spring, and beneficial insects seem to have been slower in following with regards to attacking prey. It's possible the colder-than-average spring weather caused this; they should be catching up soon and starting to control pest populations.

Habitat diversity encourages beneficial insects, especially when a variety of flowering plants are used to offer insects alternative food sources. This publication overviews a number of proactive gardening tactics, including using plants to attract beneficials (on page 6). Any beneficial insects (predators, parasitoids, parasites) would be useful to attract, and a range of flowering plants should attract as many different kinds of them as possible. Some work best at certain temperatures and humidity levels, some emerge at different times of year, and so on, so the more you can draw, the more effective they will be as a group.

Horticultural oil comes in many forms; any commercially-produced product (there are many brands) should work so long as its label includes the pest you need to control (listings may be generalized, such as "soft scale" or just "scale"). This is in opposition to home remedies, which we do not recommend since their ingredients are not intended for use on live plants and may cause damage; in addition, they may also simply not work if the ingredients are not appropriate for pest control. Neem oil is a particular kind of horticultural-use oil that is extracted from the tropical Neem tree. One of its chemical components has insecticidal or insect-repelling effects, but it is not universally effective against all insect pests.

In our climate, the dormant season refers to winter, when plants are not in active growth and, for deciduous plants, leaves have been shed. This distinction is used when oils in heavier concentrations that are safe to use in winter would otherwise damage plant tissues when the plants are not dormant. These formulations are often simply called "dormant oils" or oils used at a "dormant rate." Instructions on the label will indicate if a product can be used in this way. Sometimes, they are sold as separate formulations.

Practices to boost plant health depends on what the plants in question are. The simplest approach is to use plants that are best suited to the existing conditions, rather than trying to adjust conditions to suiting certain plants. Light levels and soil conditions are the hardest to adjust; moisture can be supplemented in areas that are dry, but improving drainage sufficiently in wet areas can be challenging if the desired (or existing) plants are sensitive to poor drainage. By not causing a plant chronic stress from unsuitable growing conditions, it will be more resilient in the face of periodic pest or disease outbreaks. Disease cycles are often highly weather-dependent, as the correct conditions need to occur for spores to spread and successfully infect tissues before succumbing to exposure to the elements themselves. Warm, dry spring weather can result in few disease issues where cool, wetter weather can permit various infections, as has been the case this year.

Other care factors, such as timing of pruning, need for irrigation, benefits of fertilizer, and so on, similarly depend on the plants in question. In general, fertilizer is not often needed for established perennials, shrubs, vines, and trees, but a soil test would help illuminate any nutrient excesses or deficiencies. Soil testing information and a link to area labs can be found on this page: Excessive nutrient levels can be damaging to plant health, so understanding the current soil conditions is useful to avoid both root damage as well as groundwater pollution and wasted product. In addition, poor root health can manifest as foliage nutrient deficiencies when none exist in the soil itself; in such a case, it's the root's inability to absorb what it needs that causes the symptoms.

Watering during drought is vitally important, yes, especially for those plants that flower on old wood (meaning that flowers develop the summer or fall prior to the year they open). Insufficient hydration during the critical period when next year's leaf buds and flower buds are developing can result in bud loss and dieback over winter, and poor growth and/or flowering the following year. Any causes of root loss - often moisture-related from an overly-wet site or drought - will also result in bud loss and dieback in the branches, since too little water is reaching the canopy. Improper watering is one of the most common causes of plant failure or chronic stress.

Scale found on hollies would not feed on vegetable plants; they aren't among their host plant palette. Potential pests of vegetable plants vary widely, depending largely on the type of vegetable. You can look up potential problems for vegetables here: