Well, You Have Some Gall! (plus invasives)

September-October 2006

THE ORNAMENTAL HORTICULTURE SECTOR has had its fair share of impacts from introduced pests in the last decade. We have almost one new pest for each of the last 10 years. Thankfully, not all of these have proved to be as threatening as we had feared. International trade has been the pathway for the introduction of several alien organisms into North America. Once these organisms find themselves out of their native habitat and without their native predators/parasites, some seem to take on a more invasive role in their new environment. Often, they will feed on different host material than they do at home, causing more damage and mortality than they do in their native range – hence the term “invasive alien species” (IAS).

Emerald ash borer, Asian long-horned beetle and Sudden Oak Death are just a few examples of IAS that have had tremendous impacts on our ecosystems and on the green industry. Since CFIA is contributing some information on EAB and ALHB this issue, I thought I would cover some of the others that are pertinent to landscape and arboriculture. For more information on EAB, go to www.inspection.gc.ca. Under “Hot Topics,” click on “Invasive Alien Species.” Click on your choice of IAS and click on “Latest Information.” You can also call 1-800-442-2342.

Sirex Wood Wasp
You probably haven’t heard as much about Sirex wood wasp (Sirex noctilio) as other IAS insects. It is also an exotic pest in other countries including New Zealand and Australia. This species of wood wasp attacks trees of the genus Pinus. All infested trees have been in unmanaged, unthinned and stressed stands where there is canopy closure. According to its behaviour so far, it doesn’t seem to be a threat to landscape trees, nursery or Christmas trees under cultivation.

This wood wasp (“horntail”) is a robust insect, up to 3.75 cm long with a distinctive posterior spine coming from the end of the abdomen of both adults and larvae. We have several native woodwasp species that look very similar to Sirex noctilio, which can make identification very challenging. Our native species attack only dead/dying trees whereas Sirex noctilio can attack healthy pines (although it is attracted to stressed trees as well). Symptoms include wilted and flagging needles (green to brown), resin dribbles down bark (egg laying sites) and round exit holes (6mm).

This IAS was detected in Oswego County, New York State, in 2004. Monitoring and trapping were carried out on the opposing shores in Ontario last year and multiple specimens of Sirex noctilio were subsequently identified. The USDA found this pest in four more counties in 2005 and an additional 13 counties in 2006. It was also recently identified in Pennsylvania. Unfortunately, we also found Sirex noctilio in two additional areas (Cambridge and Uxbridge), here in Ontario. For more information, visit the following website www.na.fs.fed.us/spfo/pubs/pest_al/sirex_woodwasp/sirex_woodwasp.htm.

Sudden Oak Death (SOD)
This disease (Phytophthora ramorum) was observed on Viburnum nursery plants in Europe and on dying oaks in California in the early 1990s. It wasn’t until 2000 that Phytophthora ramorum, the fungal-like pathogen, was identified as the causal agent of the dying oaks in California and blight of Viburnum in Europe.

Since its discovery, SOD has been responsible for the mortality of hundreds of thousands of coastal oak species along the coast of California and southwestern tip of Oregon. Although it is the oak trees that are dying, this disease tends to be worse in naturally infested forests in the presence of heavy sporulating understory species (e.g. bay laurel, rhododendron). Somehow, the disease has been transmitted to ornamental nursery crops under cultivation and infected ornamental crops have been intercepted in various retail garden centres and wholesale nurseries. The result is unnatural, long-distance spread of SOD in the United States and into British Columbia.

Since SOD was discovered, there have been 48 positive finds in nine states. So far in 2006, there have been multiple finds of SOD across the U.S. including California (25), Oregon (13), Washington (3), Florida (2), Alabama (1), Georgia (1), Indiana (1), Maine (1) and Mississippi (1). This year there has been only one positive wholesale nursery found in B.C. The first incidences of SOD on nursery stock were found on Camellia plants shipped from a California nursery. Other high risk host genera that are most often linked to positive tests for SOD include: Rhododendron, Viburnum, Pieris, Kalmia and Syringa. Since 2004, CFIA has surveyed nurseries across Canada for the presence of SOD and has only detected the disease at facilities in B.C. The British Columbia Nursery and Landscape Association has been working diligently to create and implement an effective SOD Certification Program to reduce the incidence and spread of this disease on nursery stock in B.C.

Leaf Galls on Trees
Late summer is a great time of year to monitor for insects. In amongst those pest species, you can also find an exciting variety of predatory/pathogenic/parasitic organisms that can have a significant impact on pest populations. Then there are those insects and mites that seem to be causing a lot of damage, but it turns out that their feeding has very little impact on the overall health of the tree. Leaf galls are a very good example. September is a great time to see all the weird and wonderful galls on landscape trees. Many of which are caused by midges, wasps and mites.

The genus Quercus is probably the most noted for its leaf galls. Indeed, some of the earliest botanical drawings of Quercus foliage and twigs included the various leaf galls that we see today. If you read the literature about leaf galls, more often than not you’ll see statements like “galls do not have a negative effect on the tree… chemical control is not necessary.” So why do we get so freaked out about them? Maybe because they are strange-looking. Some are down-right ugly and we can’t help but worry that this is some nasty new disease that is killing our trees!

There are far too many leaf galls to characterize than I could possibly write about in my column, so let’s try to nail down some general guidelines to determine the potential impact of plant galls. Take a good look at the infested leaves. Are the leaf galls green? Is the area in between the leaf galls still green? Does it remain green throughout the growing season? If tissue is still green, then leaf tissue is still photosynthesizing and manufacturing energy for the plant (as well as the developing gall insects). If it stays green all summer, great! If it stays green most of the summer and turns brown in August/September, then that tissue has done its job for the most important part of the growing season. If the gall tissue is red, that might not be so bad either since anthocyanins (the pigments responsible for the red hues) are also photosynthetic. Most of the leaf galls on trees fall into these categories and thus result in negligible reductions in growth. However, some leaf and twig galls may result in leaf drop and dieback (e.g. oak twig and stem galls). Obviously, a moderate to severe infestation is going to have a very negative impact on the tree’s health.

Leaf galls on evergreens may be a little more serious since galls that turn brown in August are replacing green shoots that were meant to persist and function for several years. Then its time to take a look at the percent of the foliage that is affected by the gall, most evergreens can handle a 10-15% loss in foliage with no side-effects on health. Supplemental watering during hot, dry conditions can help these trees overcome foliar losses.

Pruning out galls and destroying them before the infesting creatures have a chance to complete their life cycle can help reduce populations but the amount of labour can be cumbersome. Many of the insects and mites that cause galls will be mating and laying their eggs in developing leaves during leaf emergence. Delayed dormant or applications of summer oils (e.g. Landscape Oil) can help reduce those populations while having minimal effects on non-target organisms. Always read the pesticide label for caution statements that warn against use on certain sensitive plant hosts. You know what the cool thing about gall pests is? The insect/mite stimulates gall formation by secreting compounds that mimic plant hormones. These hormones actually trick the plant into thinking the gall is a seed and the plant responds by sending sugars and other compounds to the gall. The result is a cozy home with a never-ending supply of food for the developing gall-maker pests until they are ready to emerge as adults. Sound familiar?

Contact Information

This column is written by Jen Llewellyn, Ontario Ministry of Agriculture and Food and Rural Affairs (OMAFRA) Nursery Crops Specialist

P: 519-824-4120 ext. 52671 • F: 519-767-0755
E: jennifer.llewellyn@omafra.gov.on.ca

OMAF website: http://www.omafra.gov.on.ca/scripts/english/crops/agriphone/index.asp

Nursery Landscape Agriphone: 1-888-290-4441

Our mission is to enhance and promote the care and benefit of trees for present and future generations in Ontario through education, research and awareness.