Nursery, Greenhouse, and Landscape News

Action Thresholds:
Keep Records on Pest Density and Plant Appearance to Help Conserve Natural Enemies

The urban landscape is rich in natural enemies that can keep late season pests like spider mites under control. However, natural enemies are often killed when insecticides are used to control early season pests that potentially threaten plant appearance or health. Pest managers can help conserve natural enemies by sampling to determine whether the early season pest density really constitutes a threat. Only when the pest density exceeds the preestablished action threshold the point at which plant damage will occur if the pest is not controlled should an insecticide be used.

Thresholds have not been developed for many pests of ornamental plants. Keeping records of pest density and plant appearance over the years can help you develop your own thresholds on specific plants. We experienced this first hand while managing pest problems on thornless honeylocust in nurseries participating in a pilot IPM program in Indiana. This tree is commonly planted in cities because of its ability to produce light shade with small leaflets that are easy to clean up in the fall. Originally touted as a pest-free plant, it now has three severe insect and mite problems in the Midwest. Early in the season, honeylocust plant bug distorts leaves as they unfold from their buds. In late June and early August, mimosa webworm* ties together leaves with webs. Honeylocust spider mite outbreaks can occur at any time from July through September, particularly when natural enemies have been killed by early season pesticide applications.

The mid-season pest, mimosa webworm, can be effectively managed with applications of the microbial insecticide Bacillus thuringiensis (Bt) without interfering with spider mite natural enemies. In contrast, applications of foliar insecticides against outbreaks of honeylocust plant bugs can wreak havoc with spider mite natural enemies and result in outbreaks. Although this plant bug is common each year it will only occasionally cause enough defoliation to force trees to refoliate in June. This refoliation severely stresses trees making them more susceptible to severe injury from thyronectria canker.

The spring of 1995 was a particularly bad year for honeylocust plant bug. We wanted to avoid spraying if at all possible in 3 honeylocust fields in one nursery with a history of spider mite problems. To help decide if honeylocust plant bugs were really numerous enough to cause damage, scouts inspected branches of trees to estimate the density of mobile stages of this plant bug. A threshold of 1 honeylocust plant bug per compound leaf (based on accumulated records of plant bug population densities and plant appearance in previous seasons in our pilot IPM programs) was used to determine whether to spray or not. This threshold was reached in two fields and the trees were sprayed with a foliar application of bifenthrin. The two fields that were sprayed for plant bug needed several miticide applications in July and August. Meanwhile, the trees in the unsprayed field looked great, with almost no apparent delays in leafing out. No miticides were needed later in the season on these trees.

We were able to conserve natural enemies and avoid a late season mite outbreak in one field by making insecticide spray decisions using a threshold based on records from previous years. One honeylocust plant bug per leaf was estimated as the pest density above which damage would occur if not treated. Landscapers and nurserymen worried about problems in 1996 may want to use this threshold in their IPM programs.

Keeping accurate records of the relative numbers of pest insects present on certain plants and the subsequent damage that is incurred (if any!) may help you estimate thresholds for use in your own IPM programs.

- Cliff Sadof, Purdue University

* Mimosa webworm does not occur in northern parts of the Midwest.
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