Nursery, Greenhouse, and Landscape News

Entomopathogenic Nematodes Control Iris Borer

Iris borer, Macronoctua onusta, is a common pest of iris that kills plants by boring into the rhizomes. Eggs laid on foliage in the fall spend the winter on old leaves. In early spring the eggs hatch, bore into the leaves, often moving through stems and flower buds before they reach the rhizomes. The pinkish larvae have a brown head capsule and can be up to 2 inches long before they emerge as dull brown moths in late summer and early fall. Sanitation, or the removal of leaves in the fall does not always remove enough of the overwintering eggs to control the problem. Home gardeners and professional growers will often apply a systemic insecticide to get their desired level of control.

Research conducted at the University of Maryland indicates that entomopathogenic nematodes can control iris borer as well or better than the chemical alternatives. Commercial formulations of two nematodes, Steinernema carpocapsae and Heterorhabditis bacteriophora were used in the study along with two insecticides. Dimethoate (Cygon) was used because it is the standard insecticide used in the industry. Imidacloprid, a relatively new but popular insecticide was used because its systemic action has enabled it to provide lasting control for a wide variety of pests. The best control in the study (100%) was obtained by S. carpocapsae. Both insecticides and H. bacteriophora also provided good (87%) control.

Nematodes in this study were effective because they were applied properly. Application was delayed until soil temperature was above 50F when nematodes are most active. Enough water was applied (1 quart/ft2) to allow the nematodes to swim close enough to the pest so that they were within their effective hunting range. Finally, the pest was in the larval stage so that the infective juvenile nematodes could enter the pest through the spiracles or anus and colonize them with insect-killing bacteria. Individuals seeking to try biological control for the first time are likely to find success with iris borer if they are given the correct guidelines.

Source:

Gill, S. A. and M. J. Raupp 1997. Evaluation of biological and chemical applications for control of iris borer. J. Environ. Hort. 15:108-110.

Natural Enemies of Balsam Twig Aphids on Christmas Trees

Balsam twig aphid, which feeds on the buds and elongating shoots of balsam fir, is traditionally controlled by insecticides. Insecticides need to be applied just after egg hatch, long before the noticeable colonies develop, to prevent shoot damage. However, pesticide applications often occur too late to prevent feeding damage but could kill natural enemies associated with the aphids.

Studies in Wisconsin show that insecticides do little to reduce shoot infestation when sprays are applied after aphid-infested shoots and their distinctive feeding damage (curling and deforming of the needles) is noticed. Fourteen days after treatment, the percentage of infested shoots on unsprayed trees was equal to that of sprayed trees, and 9 months later there was no significant difference in the proportion of damaged shoots on the two groups of trees. The treatment did reduce subsequent generations, evidenced by a low number of aphids on treated trees in the following season.

Populations of predators, primarily lady beetles and syrphid fly larvae, were greatly reduced by the insecticide application. In contrast, numbers of predators on unsprayed trees increased immediately after the treatment date, possibly in response to the availability of aphids. Predator populations tend to be small early in the season, so pesticide sprays applied after egg hatch might have less impact on natural enemies than applications made when damage is visible. Conservation of these predators could prevent resurgence of the aphid population.

Source:

Kleintjes, P. K. 1997. Midseason insecticide treatment of balsam twig aphids and their aphidophagous predators in a Wisconsin Christmas tree plantation. Environ. Entomol. 26(5): 1393-1397.


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