Rose Leaf Miner

(Question)

Hi, I work at a regional building maintaining gardens. The roses on site have been damaged by a pest, and I am wondering if you can identify it and perhaps give info on how to eradicate the pests. They spread to different beds on the property each year, by mid summer the rose is pretty much dead looking. Thanks.

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(Answer)

Hello, Thank you for contacting the Toronto Master Gardeners with your inquiry,

The bright green worms (larvae) seen clearly in your photo on the surface of the leaves are likely from eggs laid in the ground or on the plant itself and hatching this spring.  They could have been laid by moths, flies (likely the rose sawfly), or occasionally beetles.   Infestations that occur yearly require extra vigilance on the gardeners’ part.  Once spotted, pick as many of the larvae off the plants’ leaves as you can. Then remove and throw them and as many infected leaves as possible into the garbage. Do not place infected material into the compost or yard waste bags instead, throw it directly into the garbage.   Spray the plant with an approved insecticide.  Ontario has regulations which prohibit the use of chemical pesticides by home gardeners.  However, if you work for a commercial landscape company,  please contact Landscape Ontario for information as to the type of insecticide your company is able to use. http://landscapeontario.com.

You seem to have a chronic major outbreak which may require a commercial grade pesticide.  However the use of these types of pesticides may also kill beneficial insects such as wasps and birds that help control the spread of these larvae.   Your photo shows that you have mulched the garden with wood chips. It is possible that these chips contained the insect or its eggs which then migrated to your plants.  Consider removing the chips, cultivating the surface of the soil  in order to disturb the life cycle of the bugs, and replacing the chips with two to three inches of good quality triple mix.  Do this only after most of the larvae have been destroyed and infected leaves removed.  You may want to install some sticky traps to catch egg laying adults.

https://en.wikipedia.org/wiki/Insect_trap

Worms perforating leaves on rose bush(es)