Neccessity of routine treatment of shrubs and trees with pesticides and hort. oil

Asked November 18, 2019, 10:21 AM EST

I got into using Tru Green for tree and shrub insect and disease control several years ago when I had a very bad infestation of bagworms on my arborvitae. (I also have a fungus on my cherry laurels but what they use does not make it go away anyway and so I treat only the cherry laurels with a fungicide, myself.) So! I have not had any bagworm issues for several years,probabaly at least 4. I pay for 2 horticultural oil treatments per year and two insect and disease treatments, a total of 4 treatments. I really wonder if I need to do any of this anymore. No bagworms for 4 years or so and I treat the cherry laurels for fungus myself. Would I be safe in discontinuing all applications in your opinion? (I hate using these things but am afraid of losing my arborvitae.) A big thanks for any help!

Baltimore County Maryland

9 Responses

If your shrubs are growing well and the pest/disease problems are under control, then you do not need to spray. No preventative treatments are needed.

All of our recommendations are based on IPM (integrated pest management), which always seeks the least toxic alternative for garden solutions. You need to become familiar with your plants, their growth habit and
necessary conditions for good growth. Monitor your plants and look for potential problems. Identify the pest, learn the life cycle, and the best time to take action. Start monitoring your arborvitae for bagworms in May and control if need be in mid June to mid July https://extension.umd.edu/hgic/topics/bagworms-trees-and-shrubs


Cherry laurel - most likely you are referring to cherry shot hole fungus. Cherry laurel is susceptible to a fungal leaf spot disease called cherry shot hole. You may be referring to this. This is a foliar fungal disease favored by wet weather. The infected leaf tissue falls out and the holes are left behind. The damage is cosmetic and no chemical controls are recommended.

There is nothing that you need to do and no control is necessary. The plant will recover. Rake up any fallen leaves. Make sure mulch is no thicker than several inches and keep away from the base of the trunk.
Take a look at our publication for more information on managing problems in your landscape https://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/hgic/HGIC_Pubs/Landscape/HG62...

Marian

Thank you so much. I will stop the routine treatments and monitor my arborvitae closely beginning in May. The cherry laurels, however, seem to have a more serious problem than shot-hole fungus. Some of the branches get covered in a powdery white substance, which I think is a fungus. Then those limbs and twigs die. I have cut out dead growth. But I feel I have to spray the shrubs with the fungicide (Clearys 3336 which was recommended by a Tru-Green employee.) Before I did this some of them died altogether

What you are seeing on your cherry laurels is not a fungus, but a type of sucking insect called scale. It is a common pest of cherry laurel and untreated can be quite damaging.
We recently published a blog post about this problem and how to best manage it.
See here: https://marylandgrows.umd.edu/2019/09/25/qa-what-is-this-white-growth-on-my-cherry-laurel-shrub/

As noted, Scale insects are difficult to control. To manage a scale infestation, prune out branches with severe symptoms. Parasites- meaning good bugs, i.e. beneficial insects generally provide control of many scale infestations. Unfortunately the type of spraying and blanket treatments used in the past likely killed your good/helpful insects.
To protect parasites, avoid spraying with insecticides during the summer. Also keep trees and shrubs watered during droughty periods and do not overfertilize.


Christine


Thank you so much for the correct diagnosis re: my cherry laurels. I have now used a brush and warm, soapy water on the scale. It has helped but I can see there is still some there. I will "scrub" again. Tru-Green will put a horticultural oil on soon and then I am going to stop all their applications (including the pesticides) and handle the scale myself. I still have these questions:
1. Is it ok to put horticultural oil on the cherry laurels at other times than now, e.g. in the spring or when I see the scale is coming back? I think it will be present permanently now to some degree or other.
2. Does one spray the horticultural oil on the leaves as well as the bark? The scale seems to be only on the bark.
3. Should I put the h. oil on shrubs that currently seem ok as a preventative? Some have had it but it is not showing, at least now.




You will have to monitor your plants for symptoms and the insects. You may have to apply a summer rate of horticultural oil during the growing season if you notice symptoms. There may be several generations a year in May, mid-July to mid-August, and September. The scale is not easy to control.
See our blog on white prunicola scale for more information and control. https://marylandgrows.umd.edu/2019/09/25/qa-what-is-this-white-growth-on-my-cherry-laurel-shrub/

Spray the leaves and the bark.

We do not recommend horticultural oil as a preventative. Again monitor your plants for symptoms.
You can place a simple trap to detect their activity; wrap a piece of double-sided tape around a few branches. When you see crawlers stuck to the tape, that is the ideal time to apply horticultural oil.

Marian

Thank you so much for answering all of my emails. I now feel I have a "recipe" to handle the ongoing treatment of the scale on my cherry laurels.

Well, here I go again with 2 more questions:
1. Do you ever recommend a particular brand of horticultural oil? Home Depot has 1: Neems. (I see I can make my own but I'd prefer not to.)
2. Re: spraying the leaves-tops AND undersides? Undersides will require some contortionist ability!
THANKS once again.

Neem is an insect growth regulator derived from the seeds of the neem tree. It also has repellent properties. This is Not horticultural oil.

There may be several brands of horticultural oil on shelves and we cannot recommend specific brands. Take a look in large garden centers, nurseries, hardware, and/or farm co-op stores. Horticultural oils are labeled for summer (growing season) and dormant uses. Follow label directions for rates and temperature.

It can be a challenge to spray the top and undersides of the leaves. You can look for a sprayer that is maneuverable. There may be some online. There are no easy answers.

Marian