JB populations on the rise

Steven Shimek, Nursery Program Coordinator

Minnesota has seen an increase in population and spread of Japanese beetle (Popillia japonica) to more locations around the state over the past few years.  As a result, nursery stock has become more commonly infested both by the adult beetles, and in a recent incident as grubs and pupae in container grown nursery stock. 

Japanese beetle grubs spend their lives below ground feeding on roots.  They most commonly occur in turf. Turf that is fertilized and irrigated is particularly attractive for adult beetles to lay their eggs and for grubs to happily munch away until they pupate and emerge from the soil as the adult beetles the following year.  Grub feeding can cause damage to turf.  The adults can cause significant damage to a number of plants feeding on and skeletonizing leaves and feeding on flowers and fruits. 

It is important to note that even stock that is in compliance with the Japanese Beetle Harmonization Plan (JBHP) for container accreditation can become infested.  This certification method requires that container grown stock must be kept on an impervious surface such as gravel.  This is meant to discourage egg laying in and around nursery containers.  More details on the JBHP that was revised in 2016 and became effective January 1, 2017 can be found at: http://nationalplantboard.org/wp-content/uploads/docs/jbhp 2017_update.pdf

Surveying for JB in containers is done by removing plants from pots, examining the potting media for life stages, namely grubs and pupae.  Some samples can be destructive in nature, though staff try to minimize the operational and economic impact.

Observations from recent surveys of accredited container stock

There seems to be no common denominator as to why containerized stock grown under the JBHP guidelines became infested during the past year.  Production history or location of the stock to potentially infested surrounding areas did not seem to influence the likelihood of infestation.  Stock that was positive included: Physocarpus, Taxus, Thuja, Juniperus, Hosta, Heuchera, Atrium, Rosa, Rhus, Celastrus, Vaccinium, and Rubus, several of which are not known to be commonly fed upon by JB larvae or adults.

During a survey of accredited containers at the end of June, larvae were most often found in the bottom 1/4 of the pots.  Larger pots were favored.  Some loose potting media at the bottom of the pot seemed to be more likely to harbor beetle grubs.  Plants that had more or heavier roots were not favored by the beetle.  When one larvae was found, it was likely that more than one was present, up to 6 or 8 per pot.  Most of the grubs found were late instar in late June, and many pupae were also found.  Grubs and pupae were confirmed as Popillia japonica by MDA personnel.  Adult beetles were also found in traps placed nearby to determine the beginning of the adult flight period.  

Regulatory Action

Blocks of plants where grubs were found are restricted pending treatment and control.  Follow-up inspections will be conducted to verify efficacy of treatments.

Lirula Needle Blight (Lirula macrospora)

David Simmons

This fungus kills needles of White Spruce (Picea glauca), Black Hills Spruce (Picea glauca var. densata), and Colorado blue spruce (Picea pungens).  Picea glauca are the more susceptible species.

Needles killed by Lirula macrospora remain attached until they have weathered off after several years.  The life cycle of the fungus is protracted, taking years to complete.

• Year One:  Needles become infected; the fungus overwinters within the needle.
• Year Two:  May/June - infected needles release spores and spread by splashing rain or even center-pivot irrigation.  New needle growth is the critical time frame of susceptibility.  Eventually, a raised black line appears on the midrib, underneath the needle.
• Year Three:  Spores are released and infect third year needles.

Apply a protectant fungicide of chlorothalonil or mancozeb when new needles are half the length of mature needles, and a second time later.  Read the chemical label, especially concerning prevention of fungicidal immunity and rotate the Mode of Action (MOA) accordingly.  Lastly, because this is a 3-year disease life cycle, fungicide applications must occur for 3 consecutive years. In other words, don’t skip a year.  It’s a continuous process.

Northern and Southwestern Region - Deborah Davis Hudak

Southern inspections began the first week of May in 4" of snow.  High winter injury from lack of snow cover was witnessed throughout the region. Cool temperatures throughout the Northern Region kept nursery stock pests at bay with inventory levels still high going into June.  In the Northern Region, fungal problems weren't as huge a problem (unlike the southern area) until the heat came on (mid-June).  White smut on gaillardia (blanket flower) caused confusion at a few places. This disease is caused by a fungus (Entyloma polysporum) and is quite common.  Plant spacing and sanitation helps.  Warm temps also brought aphids, and spider mites were observed on spruce and aspen.  Bronze birch borer damage resulted in birch being removed from sale at one location, and two-lined chestnut borer was found last week on pin oak. Pine needle scale and eastern spruce gall adelgid seem to be on the rise this year.

West Central and Southern Region - David Simmons

Oystershell Scale (Lepidosaphes ulmi) was found in Otter Tail County on rose 'Winnipeg Parks'.  Plants are injured by this armored scale insect when it removes plant fluid with its piercing-sucking mouthparts.  Twig or branch dieback is common with an infestation, and occasionally a tree or shrub will die as the result of a severe infestation.

Overwintering eggs are under the cover of the dead mother scale.  Nymphs mature in mid-summer to mate, eggs in late summer and early fall.  There is one generation per year.