Archives For plant pests

A gall, tumor, or burl is an abnormal growth on the leaves, stems, roots, buds, twigs, or crown of a plant. In most cases, the gall is unsightly but not damaging. In small plants, the vascular flow of water and food can be restricted, causing poor growth and making the plant more susceptible to other stresses. A large tree can be weakened by an infection over many years. Nematodes, mites, and insects cause 95 percent of galls. Bacteria and fungi cause the remaining five percent. In most cases, the gall-making organism can be identified by observing the structure of the gall and the species of the host plant.

Galls on a Flower

Galls on Rudbeckia laciniata ‘Herbstsonne’

Fungal galls are spread by ascospores in wind and water and can be found on many common trees including oak, maple, and common privet. Black knot affects many species of the genus Prunus—particularly cherries and plums.

Crown gall is a common problem caused by bacteria. The disease is spread by infested soil, transplants, or contaminated tools. The bacteria enters the plant through wounds caused by cultivation, pruning, or insects. Easy prevention methods are to plant only healthy stock (no suspicious bumps), to clean pruners between use on each plant with alcohol or a 10 percent bleach/water solution, and to take care not to injure plant stems. The bacteria stays active in the soil even after removal of infected plants, so place new, healthy stock elsewhere. Remove and destroy all infected plants. Galls caused by bacterial and fungus are more prevalent during wet years.

A gall can form in response to toxins injected during insect feeding or egg laying, or around a feeding larva. The hackberry leaf gall is caused by psyllids, which are tiny winged insects. Galls formed by insects usually do not affect the overall health of the tree unless they experience early defoliation over the course of many years. Parasites are an important control of this pest. Many oak galls are caused by gall flies and generally are not detrimental. A severe twig infection can, however, cause severe injury and even death. Spruce galls are often caused by several species of an aphid-like insect. If only a few galls are present, they can be cut off and destroyed before the insects emerge in midsummer.

Galls on a Flower
Galls on a Flower

 

Leaf galls on maple trees form because of feeding mites. Eriophyid mites produce a gall that resembles a felt patch and may occur on the upper or lower side of the leaf. The overall health of the tree should not be seriously affected.

Nematode feeding activity can injure roots and allow gall-forming bacteria into the plant. Nematodes can also form galls on carrots, camellia, fig, gardenia, okra, potato, roses, sweet potato, and tomato. Plants can be stunted, yellow, and wilted due to restrictions on the uptake of water and nutrients. Individual nematodes are invisible to the naked eye, but egg masses can be seen as pearly objects.  Roots can appear scabby, pimpled, rough, and have knots. Two important prevention methods are to rotate with nematode-resistant crops and to maintain rich organic soil.

Please contact Plant Information Service at (847) 835-0972 or at plantinfo@chicagobotanic.org for additional information.


©2017 Chicago Botanic Garden and my.chicagobotanic.org

Last year we discovered Viburnum leaf beetle (VLB) here at the Chicago Botanic Garden for the very first time. As I said then, “I strongly suggest you begin monitoring your viburnums for this critter” as once they move in, they become a perennial pest, just like Japanese beetles.

In early March, we monitored many of the Garden’s viburnums for signs of VLB egg laying and focused on areas where we observed VLB activity last summer. I had read recommendations for pruning out these twigs (with eggs) in the winter as a management technique and wanted to give it a go. To assist with this project, I called in our Plant Health Care Volunteer Monitoring Team; the more eyes the better. The six of us (all armed with hand pruners, sample bags, and motivation) began a close inspection with a focus on last season’s new twig growth for the signs of the distinctive straight line egg-laying sites. In less than five minutes, we found our first infested twig, pruned it out, and put it in a sample bag. After about three hours, we had collected about 20 twigs with eggs.

Truly, I was expecting to find a lot more. This was somewhat disappointing, as I had created a challenge to see which volunteer would fill his or her sample bag and collect the most. This turned out to be more like a needle in a haystack search, as it was a lot more difficult than I had thought. I also feel that the egg-laying sites would have been easier to see if we had done this in early winter, as the egg-laying locations had darkened with time.

Viburnum leaf beetle

Viburnum leaf beetle

Back at our lab, I dissected some of our samples under the microscope. When I removed the cover cap (created by the female after egg laying) material of a few of the egg-laying locations, I found about six orange, gelatinous balls (the overwintering eggs). These eggs were about a month or two from hatching.

For background on this new, exotic insect pest, please see my June 5, 2015, blog on the Viburnum leaf beetle.

American cranberrybush viburnum

American cranberrybush viburnum

  • The favored viburnums are the following:
  • Arrowwood viburnum (V. dentatum)
  • European and American cranberrybush viburnum (V. opulus, formerly V. trilobum)
  • Wayfaringtree viburnum (V. lantana)
  • Sargent viburnum (V. sargentii)
  • When to monitor and for what:
  • In early summer, you would look for the distinctive larva and signs of leaf damage from the larva feeding.
  • In mid- to late summer, you would look for the adult beetle and leaf damage from the beetle feeding.
  • In the winter, you would look for signs of overwintering egg-laying sites on small twigs.
  • Life cycle, quick review:
  • In early May, eggs hatch and larva feed on viburnum leaves.
  • In mid-June, the larva migrate to the ground and pupate in the soil.
  • In early July, the adult beetles emerge and begin to feed on viburnum leaves again, and mate.
  • In late summer, the adult female beetle lays eggs on current season twig growth in a visually distinctive straight line.

viburnum leaf beetle egg laying sites

Hopefully our efforts will lessen the VLB numbers for this coming season. We will see when we monitor the shrubs for leaf damage and larva activity in late May. If nothing else, it was a great learning experience with this very new, exotic insect.

Special thanks to the Plant Health Care Volunteer Monitoring Team: Beth, Fred, Tom (x3), and Chris.


©2016 Chicago Botanic Garden and my.chicagobotanic.org

Viburnum leaf beetle is here, and he’s not a good neighbor!

Yesterday was an exciting (yet worrisome) day for me here at the Garden. We found viburnum leaf beetle here for the first time ever—although his arrival was not unexpected. Two separate discoveries were reported to me within just a couple of hours. One of our horticulturists made a discovery in one location, and one of our trained plant healthcare volunteer scouts found the beetle in another location. Both finds were on arrowwood viburnum (Viburnum dentatum), the beetle’s preferred host (and high on our watch list).

Click here to download the viburnum leaf beetle fact sheet with tips on managing the beetle.

If you live in the area, I suggest you monitor your viburnums for our new foreign friend. The sad thing about this critter is that once he moves in, he will become a perennial pest, just like Japanese beetles.

In ornamental horticulture (your home landscape plants), the viburnum leaf beetle seems to be on the verge of having a great impact in our area, as nearly everyone’s home landscape has viburnum. I’d like to take a moment to review this new critter.

Viburnum leaf beetle (Pyrrhalta viburni)

The viburnum leaf beetle (VLB) is native to Europe and was first found in the United States (in Maine) in 1994. It was first found in Illinois (Cook County) in 2009. In 2012 and 2013, the number of reports increased from Cook County and also from DuPage County. In late summer 2014, there were numerous reports from Cook County and some specifically from neighboring Winnetka, where complete defoliation was reported—only five miles from the Garden!

PHOTO: Leaf damage to Viburnum dentatum at the Chicago Botanic Garden by viburnum leaf beetle larvae.

Leaf damage to Viburnum dentatum at the Chicago Botanic Garden by viburnum leaf beetle larvae.

PHOTO: Viburnum leaf beetle (Pyrrhalta viburni) larva.

Viburnum leaf beetle (Pyrrhalta viburni) larva

PHOTO: Viburnum leaf beetle (Pyrrhalta viburni)

Viburnum leaf beetle (Pyrrhalta viburni) by Siga (Own work) [GFDL or CC BY-SA 3.0], via Wikimedia Commons

The VLB larva and adult both feed on foliage and can cause defoliation, and several years of defoliation can kill a viburnum. If you live in the area, I strongly suggest you begin monitoring your viburnums for this critter. There are many great university-created fact sheets for VLB that can be found online, or contact the Garden’s Plant Information Service for additional information. Please report new finds to the Illinois Natural History Survey, Illinois Department of Agriculture, or University of Illinois Extension Service.   

PHOTO: Plugged cavities on a viburnum twig containing egg masses of the viburnum leaf beetle (Pyrrhalta viburni).

Plugged cavities on a viburnum twig containing egg masses of the viburnum leaf beetle (Pyrrhalta viburni). Photograph by Paul Weston, Cornell University, Bugwood.org

Many people ask us: is it true that some viburnums will not be affected by the viburnum leaf beetle?

Viburnum leaf beetles prefer viburnums with little to no hair on the foliage. Plants grown in the shade also exhibit more feeding damage. The University of Illinois Extension has placed viburnums into four feeding categories: highly susceptible, susceptible, moderately susceptible, and most resistant. Viburnum species such as arrowwood (V. dentatum), European and American cranberrybush viburnum (V. opulus, formerly V. trilobum), wayfaringtree viburnum (V. lantana), and Sargent viburnum (V. sargentii) are in the highly susceptible and susceptible categories and can easily be destroyed by repeated infestations of the viburnum leaf beetle. Moderately susceptible species such as burkwood viburnum (V. burkwoodii), blackhaw viburnum (V. prunifolium), and nannyberry viburnum (V. lentago) may exhibit varying amounts of susceptibility, but are usually not killed, depending on the species. Other viburnums, such as Koreanspice viburnum (V. carlesii), Judd viburnum (V. x juddii), and doublefile viburnum (V. plicatum), are resistant to viburnum leaf beetle, will show little or no feeding damage, and are capable of surviving slight infestations. Please contact Plant Information Service at (847) 835-0972 or plantinfo@chicagobotanic.org for susceptibility questions on specific species.

The Garden is a member of the Sentinel Plant Network, a group that unites botanic gardens in monitoring and providing education on exotic plant pests and pathogens, and works in partnership with the National Plant Diagnostic Network (NPDN).

If you are a plant and bug person like me, please consider becoming a NPDN First Detector and help be on the lookout for these exotic invasive plant pests and pathogens. The NPDN offers an online training course to become a First Detector at firstdetector.org. It’s free, and upon completion, you even get a printable certificate!

©2015 Chicago Botanic Garden and my.chicagobotanic.org