Michael E. Reding, Michael G. Klein, Ross D. Brazee, and Charles R. Krause, U.S. Department of Agriculture, Agricultural Research Service, Application Technology Research Unit, Wooster, Ohio.
The authors are investigating various aspects of the biology and management of the exotic white grub complex in nurseries in northern Ohio. Our projects include studying the life history of oriental beetle (OB), natural enemies of OB and Japanese beetle (JB), and testing subsurface applications of insecticides as rescue treatments against white grubs in field-grown ornamentals.
There are several exotic scarab beetles (white grubs) that are pests in ornamental nurseries in northern Ohio. In the past, these beetles were considered primarily as contaminant pests in ornamentals, which was related to quarantine issues. However, during 2001 and 2002, we received more reports of damage to woody ornamentals caused by the root feeding of grubs.
In autumn 2001, European chafer (EC) grubs were killing PJM
Rhododendrons in a commercial nursery in northern Ohio. Another incident
of EC grubs killing plants occurred in a field of Canadian hemlocks
(Tsuga canadensis) in a different northern Ohio nursery during
autumn 2002. Both fields were about two acres and were infested with
about three EC grubs per plant.
We conducted trials to test rescue
treatments in both sites. In the rhododendrons, we tested subsurface
applications of Dursban TNP. We had three applicators that allowed us to
inject the insecticide into the soil. Two of the applicators had
single-point nozzles that were inserted into the soil, and the
insecticide was sprayed below the soil surface. The third applicator had
a high pressure nozzle with a splash guard that injected insecticide
into the soil from the surface using high pressure. Treatments with all
three applicators were effective in the rhododendron field, reducing
grub numbers by 95 to 100% compared with untreated plants.
In the hemlock trial (autumn 2002), a single-point subsurface treatment of Dursban TNP was compared with surface drenches of Dursban TNP and Dylox 80SP. The subsurface treatment of Dursban and the Dylox drench had significantly lower numbers of grubs than the untreated trees, reducing grub numbers by 95% and 69%, respectively. The Dursban drench treatment had grub numbers similar to the untreated trees. Dursban tends to get bound up with organic matter in the soil. Consequently, when used as a drench, Dursban does not penetrate the soil far enough to reach the grubs in the roots of trees and shrubs.
Knowledge of pest biology and ecology is necessary for the development of non-chemical management strategies. This information is also important for making chemical-based management as efficient and effective as possible.
In general, members of the exotic white grub complex have a one-year life cycle with the following life stages: egg, three larval (grub) stages (instars 1 to 3), prepupa (transition stage from third instar larva to pupa), pupa, and adult. These species overwinter primarily in the third larval stage with a small percentage of some species overwintering as second-stage larvae. Some individuals that overwinter as second-stage larvae take two years to complete development (two-year life cycle).
In 2001, we noticed a larger percentage of OB overwintering as second-stage larvae than expected. If a large number of OB overwinter as second-stage larvae, some management tactics might be less effective.
Imidacloprid (Merit, turf and landscape; Marathon, ornamental nursery) and halofenozide (Mach-2) are reduced-risk insecticides commonly used to control scarab grubs in turf and ornamentals. These materials are most effective when they are timed to target the eggs and first-stage larvae. In northern Ohio, this timing is generally late June through mid-July.
However, the two-year life-cycle individuals are second- or third-stage larvae at that time and would be less susceptible to treatments of insecticides, especially imidacloprid or halofenozide. If large numbers of OB have a two-year life cycle, treatments of imidacloprid or halofenozide may not prevent damage. In addition to their potential to cause damage, the surviving grubs may be contaminants, resulting in quarantine issues.
In 2002, the authors started a project that focused primarily on studying the life history of OB in nurseries of northern Ohio. In addition, we collected information on the occurrence and life histories of the other main members of the white grub complex, Asiatic garden beetle (AGB), EC, and JB. Previously, most of the information concerning the biology and management of white grubs came from research in turf.
We surveyed five nurseries, and OB, AGB, EC, and JB were found in four of them. In one nursery, only EC and JB were found. In general, the life histories of the four species were very similar, with EC developing about two weeks earlier than the rest. In addition, the time period of adult activity was much shorter for EC than the other species.
Life history of OB was studied extensively at two nurseries. At one nursery, the percentage of two-year life-cycle OB was within the range of previously reported levels (<15% of the population). At the second nursery, the percentage was very high (about 40%). These two nurseries were only about a mile apart. These data suggest that OB has a very flexible life history, which may allow it to adapt to a variety of climatic conditions.
During the 1920s and 30s, a natural enemy (Tiphia vernalis, a parasitic wasp) of JB was transported from China and Korea and released into the eastern United States. During the 1940s, there were a few releases made in Ohio from colonies that had established in the United States. In 2001, we found this parasite in a Lake County nursery. At that time, we discovered it attacking OB as well as JB.
In 2002, we surveyed several nurseries in Lake County to determine how common it was and which species of grubs it parasitized. We found T. vernalis in three nurseries. At one nursery, it was attacking JB and OB. At another nursery, it attacked only JB, but the OB population was low there. At the third nursery, we found adults but never found any parasitized grubs. This nursery had a lot of OB and very few JB.
We will be continuing all of the previously mentioned projects in 2003 and will add several others.