Pest News for Week of June 10th

ORNAMENTALS AND TURF

From: Steve Frank, Extension Entomologist

Hibiscus Sawfly Damage

In Georgia last week I found severe damage by hibiscus sawfly. Larvae and adults were present on the plants I surveyed. The adults are active throughout the summer. The larvae feed on hibiscus and related plants. The larvae skeletonize leaves when they are young, but quickly defoliate plants as they grow. Contact insecticides such as bifenthrin and Orthene will kill larvae. Other insecticides such as spinosad, acetamiprid, azadirachtin and others listed can be used (http://www.ces.ncsu.edu/depts/ent/notes/O&T/shrubs/note07/note07.html). The larvae are not caterpillars so be sure to look for sawflies specifically on the label of the product you select. More information on this critter may be found at

http://www.ct.gov/caes/lib/caes/documents/publications/fact_sheets/entomology/hibiscussawfly.pdf.

New Pest: Daylily Leafminer Active Now

The daylily leafminer is a recent pest from Asia. It was first detected in 2006, but is now spread through much of the Southeast including North Carolina. I spotted some last week on a trip to Georgia. This fly lays its eggs in day lilies and the larvae produce relatively straight, vertical mines. Pruning infested leaves will help prevent the larvae from maturing and infesting new leaves. I do not know of any formal efficacy tests on this pest but other material targeting leafminers such as imidacloprid and pyriproxifen should help. A recent article about this pest is in American Nurseryman

(http://www.amerinursery.com/article-7705.aspx).

Cicadas in Nurseries and Landscapes

For folks in western parts of the state you may have periodical cicadas in your nursery or landscape. Of course this will depend on a number of things including the habitat surrounding your nursery. Areas with a lot of suburban development may have fewer than less disturbed areas.

Cicadas cause damage to trees when they lay eggs in branches. They use a knife-like ovipositor (egg inserter) to insert eggs into thin tree branches. This causes slits in the branch that could be 6 inches long or more. This long scar reduces plant aesthetic value, but also weakens branches. Scarred branches usually break and fall to the ground or break and remain hanging in the tree, but turn brown.

We have found that imidacloprid reduces oviposition in landscape trees (http://ecoipm.files.wordpress.com/2012/02/ahernfrankraupp2005jeconent.pdf). Females detect the insecticide with their ovipositor so treated trees have fewer scars and the scars are much shorter. Thus branches do not become as weak so there is less flagging. This is not to say you should treat every tree with imidacloprid. Most landscape trees over a few feet tall can withstand losing many branches with no negative effects on health. Even nursery stock could survive losing branches, but may need corrective pruning. Nursery stock can be pruned to remove scarred branches. 

Trees that are very valuable could be protected with mesh netting to keep cicadas off (http://ecoipm.files.wordpress.com/2013/06/cicada-tree-netting.pdf). This may apply to specimen trees in landscapes or to particularly expensive nursery trees. Japanese maples may be one species where shape is very important and it would be worth protection of some sort.

Cicada Management for Homeowners

With cicadas emerging, big box stores are overflowing with insecticides promising to kill periodical cicadas. This may be true. If you take a particular insecticide off the shelf and pour it on a cicada it will kill it. But these products will not ‘control’ cicadas. There are millions upon billions of them. 

There is no such thing as an insecticide that only kills cicadas. They also kill butterflies, bees, and other non-target organisms. It is important that homeowners consider the risks of these insecticides (some) compared to the benefit (none). Cicadas do not last long and pose no risk to people. Insecticides do.

The other problem with trying to manage cicadas with insecticides is that they are generally ineffective. Especially products available to homeowners provide so little benefit that the monetary cost and risk is just not worth it. If you managed to spray a whole tree with Orthene or Sevin for instance cicadas would likely colonize it again within hours or days.

Cicadas cause damage to trees when they lay eggs in twigs. They use a knife-like ovipositor (egg inserter) to insert eggs into thin twigs. This causes slits in the branch that could be 6 inches long or more. This long scar reduces plant aesthetic value, but also weakens branches. Scarred branches usually break and fall to the ground or break and remain hanging in the tree but turn brown.

Trees that are very small or that you just planted this year are at risk if they get many cicada oviposition scars. Cicadas prefer skinny branches (< 0.5 inch) so if your tree trunk is this skinny it could get damaged and this could kill your tree. Other trees will shed a few twigs and go on about their lives. You can protect trees with mesh netting to prevent cicadas from damaging them.

If you are unhappy about having cicadas on your porch or sidewalk just sweep them off. If you apply insecticide to these trapped critters (they don’t want to be on your porch) you will end up with a dead smelly pile of cicadas that you have to sweep up anyway. In addition, as you walk across the porch and sidewalk you will get insecticide on your shoes that will be carried into your house where kids and pets play on the floor. When you take your shoes off you get insecticide on your hands. Next thing you know you are eating a sandwich.

Insecticides have a place. That is to reduce economic or aesthetic damage to plants that we eat or enjoy.  Insecticide applied for cicadas won’t achieve this. So save your money and wait it out or try to enjoy it.

For more information contact your local Cooperative Extension Center and ask for the Commercial Horticulture Agent.