The Woods Weevil - A New Pest of Rhododendrons and Azaleas
E. P. Breaker, Entomologist
Washington Agricultural Experiment Station, Puyallup, Washington
Until quite recently, the woods weevil, Nemocestes incomptus (Horn.), was so seldom encountered as to be of interest primarily as a collector's item. Presently, it is a hazard to the production of such crops as strawberries, raspberries, blueberries, azaleas, rhododendrons, and primulas throughout western Washington. It has often been found in pure populations where soil treatments had eliminated the Brachyrhinus weevils. Aldrin, dieldrin, heptachlor and chlordane, have no apparent effect on either the grubs or adults of the woods weevil. Occasionally, it has been found associated with another, though smaller, native species, Scioptes obscurus (Horn.), the obscure root weevil. By eliminating the woods weevil's chief competitors, we have enabled it to move into and occupy the areas once held by the Brchyrhinus weevils.
According to Essig (1946), N. incomptus (Horn.) was taken on strawberries in Washington by M. J. Forsell in 1923. Hanson and Webster (1938) listed it as a pest of raspberries and called it the raspberry bud weevil because it was found feeding on the buds of red raspberry in early spring. However. both Paiiscopus torpidus (Le C.) and Dyslobus granicollis (Le C.) do the same, so a more appropriate common name was needed. Eide (1959-B) named it the 'woods weevil', "since the strawberry fields attacked are nearly always located in brushy areas." The name is appropriate, also, for the known natural distribution of the insect is limited to the milder and more humid portions of western boreal North America.
The adults of N. incomptus (Horn.) are voracious feeders and can seriously injure the buds and blossoms of strawberries as well as raspberries. The larvae or grubs also seriously injure such plants as strawberries and primulas by feeding on the roots and crowns. They not only destroy the roots of azaleas, rhododendrons, and blueberries, but girdle the plants by eating the bark from the stem immediately below the surface of the soil and often include the bark of the larger roots. Both adults and larvae of the woods weevil are much more active than those of the Brachyrhinus weevils and readily move about. Doubtless the larvae are capable of moving from plant to plant, providing the distance is not too great.
The Brachyrhinus root weevils are known to have fairly well defined life history patterns, even in western Washington where the climate seems to mitigate against an insect maintaining the same predictable development it would have in an area where the seasons are better defined. The woods weevil seems to have lost any life history pattern presumably it may once have had. Development is continuous throughout the year. When sifting the soil under azaleas in southwest Washington, in December, 1961, larvae of all sizes were found together with pupae and recently formed adults. Some of the larvae were not much larger than the eggs. Many adults were found when sifting the mulch of leaves under and around the plants. As soon as they were brought into the insectary, some of them began laying eggs.
Eggs have been found in neat rows near the margins on the undersides of rhododendron leaves where they had been placed side by side and firmly cemented to the leaf surface. The eggs were white, kidney shaped, and about twice as long as their greatest diameter. When thinner leaves were available, such as strawberry leaves, the eggs were found firmly cemented in a fold near the margin made by turning the edge of the leaf over the eggs, much as a hem is made in the edge of a napkin. This habit is quite different from that of the Brachyrhinus weevils which scatter their eggs indiscriminately through the soil and debris under the plants.
Aldrin, dieldrin, heptachlor, and chlorodane have no effect on the woods weevil when incorporated in the soil for weevil control. Lindane and DDT, each has shown promise when tested experimentally. Adults were completely destroyed by single applications of a weevil bait containing sodium fluosilicate and dried apple pomace. Since the emergence of the adults from the soil is practically continuous throughout the year, control with poisoned baits would be very expensive and unsatisfactory.
Such considerations as these prompted Eide (1959-A) to explore the possibility of developing a method for killing the grubs in the soil under the infested strawberry plants with a soil fumigant. Ethylene dibromide, EDB, proved to be the fumigant of choice and a method of making practical and effective use of this material was developed. Unfortunately, EDB proved to be highly phytotoxic to other plants. In fact, strawberries seem to be unique in their ability to tolerate EDB. Azaleas seem to be among those least tolerant of the fumigant.
An infestation of the woods weevil in deciduous azaleas in S.W. Washington was investigated on Oct. 22 and 23. 1964. Many adults were found on the surface of the soil under the loose mulch around the plants. Many of these were still light colored and some were quite soft indicating recent emergence from the soil. For a time, it appeared we were witnessing the maturing of a distinct brood or generation. Later, when the soil under several of these plants had been screened for grubs, pupae, and adults, grubs of all sizes were found. Pupae and adults were also found indicating that adults would continue to emerge throughout the winter. This infestation seemed to be limited to N. iucomptus (Horn.). Two specimens of the black vine weevil were found but no larvae of this species were encountered.
Experimental Methods and Results
Plots were established in this planting of azaleas on November 4, 1964. Each plot was 7 plants wide by 7 plants long. Each replication was 1 plot of approximately 49 plants (some plants were missing). Each treatment was replicated 3 times, and the replications were located at random through the infested area.
On November 4, 1964, the following materials were applied over the feeding areas of the plants:
Cygon 2E 4 ounces in 5 gallons water Aldrin 4E 2½ ounces in 5 gallons water Endrin (1.6 lbs./gal.) 2 ounces in 5 gallons water Dieldrin (1.5 lbs./gal.) 2 ounces in 5 gallons water DDT (50r/, W.P.) 2 ounces in 5 gallons water Parathion 2E 5 ounces in 5 gallons water Diazinon A G 500 2½ ounces in 5 gallons water
Each was applied with a sprinkling can until the mulch of leaves and other organic debris under the azaleas was wet (if bare ground, until it was wet). Rubber gloves and boots were worn during the entire application as a safety measure. The weather was ideal for making the applications.
November 19, 1964
The mulch under and around the azalea plants was collected, sifted through a ¼-inch screen and the siftings searched for root weevils. Rubber gloves, boots and rain suit were worn for protection and safety. The data indicate the effectiveness of the treatments applied on November 4, 1964. With the exception of one black vine weevil found in the Diazinon treated plot, all weevils found were N. incomptus (Horn.). All living and moribund weevils were placed in cages, fed and held for observation. All moribund weevils from the plot treated with DDT died within 24 hours, those from the Aldrin treated plot recovered.
December 8 and 9, 1964
Data were taken from the azalea plots using the method outlined under the date of November 19, 1961. Again, protective clothing was worn. The weather was wet with frequent showers of rain. An electric lamp that operated off the car battery enabled us to work longer hours in the field. The data again indicate the effectiveness of the treatments applied on November 4, 1964. All weevils found were N. incomptus (Horn.). All living and moribund weevils were placed in cages, fed and held for observation. All moribund weevils died within 24 hours.
December 10, 1964
The soil under several azalea plants in plots treated with parathion and DDT was sifted and searched for weevil adults, pupae, and grubs. Many grubs of all sizes were found, several pupae, and a number of adults. None of these from beneath the soil surface had been affected by the insecticide in the mulch and on the soil surface.
DDT, parathion, and diazinon each gave excellent kill of the adults of the woods weevil, Nemocestes incomptus (Horn.), when applied as drenching sprays to the mulch and soil surface under infested azalea plants. Aldrin, endrin, dieldrin, and cygon each gave no kill whatever when applied in the same way. DDT was slower in action than parathion or Diazinon. However, DDT has a long residual life, is inexpensive and relatively safer to use. Since larvae were present in the soil in large numbers and of all ages, together with pupae and adults, it is reasonable to assume adults will continue to emerge for many months. No method for killing the larvae in the soil without killing the azaleas is known at present. An insecticide that will kill the adults and has a long residual life may hold the solution to this problem.
Hanson, A. J. and R. L. Webster, 1938, Insects of Blackberry, Raspberry, Strawberry, Currant and Gooseberry, Wash. State Coll. Popular Bul., 155:21-29.
Essig, E. O., 1946, Insects of Western North America, The MacMillan Co., N.Y.C., p. 494.
Fide, P. M., 1952, Soil Treatments for Strawberry Weevil Control, Proceedings of the Western Washington Horticultural Association, Jan., 1952, 43:7.
Fide, P. M., 1959-A, Soil Fumigation to Control Weevil Grubs in Strawberries, Jour. Econ. Ent., 52:3-5.
Eide, P. M., 1959-B, Ethylene Dibromide for Strawberry Weevil Control, Down to Earth. Fall, 1959 Issue, The Dow Chemical Company.