The Life History of the Obscure Strawberry Root Weevil
Harry T. Bell, Corvallis, Oregon
From a paper given at the 1977 Breeders Round table at Eugene, Oregon
Good morning. I certainly appreciate the opportunity to be here. My talk is about the Obscure Strawberry Root Weevil, which has become a pest of Rhododendron. This subject is a timely one, because once again the weevils are active, and now is the time to begin control measures. Because my work on the preferred Rhododendron hosts of this insect is still in progress, I will devote most of this time to a discussion of its life cycle. The idea being, that by knowing more about the insect, we'll be better able to control it.
We can think of this insect in terms of three stages. These stages are: the over wintered adult, meaning a weevil which emerged last summer and has survived the winter; the "new", or first year adult; and finally, the larvae or grub stage.
Let's begin with the over wintered adult, which is presently active in your gardens. It is impossible to visually distinguish between this adult and a "new", or first year adult. They look alike. Unless, of course, you mark an individual with paint, for example, to keep track of her from year to year. These insects are about the size of a house fly, gray in color, and with a jagged dark band across the back near the "tail" end. Throughout the winter they survive in cracks and crevices, wherever they can find protection from the cold. They do not return into the soil, but can be found on the soil surface in the duff, as well as on the plants. On rhododendrons and azalea they find protection beneath shaggy bark, in twig crotches, and most often in spent flowers. his is another reason why it is very important to dead-head rhododendrons during fall cleanup, if you haven't already done so, and dispose of the spent flowers. All the adults attempt to over winter, but I suspect no more than 10% make it.
This can still leave you with a relatively large population. And it is my opinion that this is the mechanism whereby these infestations grow so rapidly. A single adult can live up to 2 years. As the weather warms, these over wintered adults resume activity and begin feeding, so that by March or early April they are active and moving about. Once feeding begins egg laying follows within four to five weeks. This means that by mid-April the weevils are again feeding and depositing eggs. But, because the new foliage has yet to appear they feed less and lay fewer eggs. In addition the resulting larvae are less vigorous, and fewer survive, relative to what occurs during the summer months. As summer approaches, this activity increases in intensity.
This brings us to the second stage, or first year adult. By mid-June these new adults have begun to emerge, and things are in full swing by the Fourth of July. And as you well know, this business continues through November. In a mild winter, such as the one just passed, you may incur damage into December. As I said, these "new" adults look just like those already a year old.
The eggs are small, oval, and cream colored. About thirty laid end to end would cover only one inch. You don't n have to worry about recognizing them however, because I doubt you'll ever see one. They are laid in clusters and the adults have the peculiar habit of folding a bit of leaf tissue over each cluster, and cementing it closed. The typical rhododendron leaf is a bit too stiff for them to handle. Consequently, most eggs are laid in spent flower bracts, and pieces of leaf tissue remaining from their feeding. Such material usually falls to the ground allowing the larvae easy access to the soil. Larvae hatching on the plant easily drop to the soil anyway. Complete development from larva to adult takes about six to eight months. In general, the larvae are small, about one-fourth of an inch in length, cream colored with a brown head. Typically they assume a C-shape position.
During the summer and early fall the larvae live on the small feeder roots in the upper four inches of soil and do not cause girdling. I repeat, this weevil does not cause girdling. During the coldest months they burrow up to a foot deep into the soil and rest there until conditions are right to complete development, at which time they move closer to the surface, pupate, and are ready to emerge by July.
This graph shows a relationship between the number of adults present on rhododendrons throughout the spring, summer, and early fall. You can see that the greatest numbers are present from July till October, and you can assume that feeding damage is the greatest during this period.
This next slide illustrates typical obscure strawberry root weevil feeding. Notice that it begins on the leaf margin and channels in toward the mid-rib. Now, here is typical Black Vine Weevil damage. Feeding is usually limited to a notch on the leaf margin. More often, it is impossible to distinguish between the two. Let's take a quick look at some damaged foliage. Damage such as this is relatively mild. All the leaf notches are separate. This next shot is more severe. A greater portion of the leaf margin is notched and the notches tend to run together. Here we see some of the most extensive damage. The leaf margin is all but obliterated.
Suggested Control Application Intervals
For the Obscure Strawberry Root Weevil
; Heavy Infestations One Application During To Control May Over wintered adults July First year adults Aug. First year adults Sept. First year adults Oct.* First year adults Light Infestations One Application During To Control July First year adults Aug. First year adults Sept.* *This application may not be necessary - weather conditions and previous control success may eliminate its need.
Obscure Strawberry Root Weevil Non-Preferred
R. ferrugineum R. occidentale R. hippophaeoides R. roseum R. impeditum R. schlippenbachii R. hanceanum nanum R. vaseyi R. yakushimanum R. auriculatum R. auriculatum R. calophytum R. rigidum R. molle R. pemakoense R. morii R. chryseum R. litiense R. metternichii R. lutescens R. leucaspis R. brachycarpum R. trichostomum R. kaempferi R. lepidotum R. calendulaceum R. scintillans R. carolinianum R. decorum R. adenopodum R. hemsleyanum R. wightii R. campylogynum
My objectives in the host preference study were to determine if, in fact, a host preference does occur and if any rhododendron species are completely resistant to foliar feeding by this weevil. To meet the first objective, I collected new foliage from 107 rhododendron species to represent the 43 series. These were placed in cages, such as you see here along with leaves from 'Purple Splendour,' which served as a control. The species exposed simultaneously to the weevils were selected randomly to avoid any bias on my part. There were 28 leaves and 94 weevils in each cage. In this manner, I determined that the adults do feed selectively and that complete resistance is a possibility. To test for resistance I isolated adult weevils with species which were not fed upon in the first series of experiments in cages similar to the one you see here.
I don't consider this work conclusive and will conduct additional experimentation this June because I've had feedback indicating that some species which appeared resistant during the tests are attacked in the field. However, I will release to you now a partial list of preferred and non-preferred species. Also I'll pass around a chart indicating when best to apply Orthene as a foliar spray to control the adults.