Observations on Selective Feeding by Root Weevils
R.L. Ticknor, Canby, OR
Preferential feeding (leaf notching) by weevils is a widely observed occurrence in rhododendron plantings. In many gardens, there are very few plants which have one or more common parents except for the Catawbiense hybrids grown in colder areas. A visit to Dr. Richard Wellman's garden in Tarrytown, N.Y., during October 1983 provided an opportunity to see whether there was an apparent relationship between the parentage of plants and weevil feeding on the foliage.
Dr. Wellman has been collecting and hybridizing for a number of years and has both mature and young seedling plants in his 1/3 acre garden. Dr. David Nielsen of Ohio State University has found that adult Strawberry root weevils are capable of walking several hundred feet so a garden of this size does not present a problem for uniform distribution of weevils. A total of 379 plants were rated as no, light, medium, or heavy feeding injury. While visible and easy to record, most leaf feeding is only a cosmetic problem; however, root feeding by the grub stage can kill plants. Leaf notching can be an early warning of a serious problem, so that treatment can be done in time to save plants. No adult weevils were observed, so the species responsible for the leaf injury in this garden is not known.
Ratings for named varieties and species are shown in Table 1. In the species - R. adenopodum , R. diaprepes , and R. yakushimanum - had no injury. R. metternichii from two sources had no injury, but from a third source were heavily injured. Three plants of R. yakushimanum ancestry 'Coral Velvet', 'Koichiro Wada', and 'Mist Maiden' did not have weevil injury. Also on the no feeding list were 'Cornell Pink', 'Montchanin', and P.J.M. which is not too surprising since Doss (1) has reported that naturally occurring chemicals in many lepidotes repel weevils. 'Mrs. AT. de la Mare', 'Scintillation' and 'Virginia Richards' were elepidote hybrids not showing weevil injury in this garden.
Of the ten yellow or salmon colored elepidote varieties listed in the table, six were heavily notched by weevils. Only one, 'Virginia Richards', did not have evidence of weevil feeding in this garden. Any relationship between parentage and weevil feeding is of interest to hybridizers and to people who do not wish to spray to prevent injury. When the parentage of the seedlings and named hybrids in the garden (Table 2) was compared to the amount of weevil feeding, some interesting relationships appeared. There were 78 plants which had either 100 or 75 or 50 or 25 percent R. yakushimanum , in their pedigree. The percentage of plants with no weevil injury was 83, 50, 42, and 25 percent. If the percentages for no and light injury are added together the results are 100, 75, 73, and 56 for the 100, 75, 50 and 25 percent R. yakushimanum parentage. In contrast 75 percent of the seedlings of 'Scintillation', which had no injury itself, were classed as medium or heavy injury.
Some other parents which had relatively little weevil injury in their crosses were R. decorum , 'David Gable', 'Fabia' and 'Golden Star'. The record of 'Hello Dolly' (Fabia X R. smirnowii ) is not as good, since 57% of the seedlings had medium or heavy injury. This is surprising since Dr. David Nielsen of Ohio State University has reported that many adult Black Vine weevils die when fed a diet of R. smirnowii leaves.
It is unfortunate that seedlings of several plants used as sources of hardiness 'Catalgla', 'Caroline', Nova Zembla' and R. maximum were very attractive to weevils. 'Catawbiense Album' appeared to be a better source of hardiness if resistance to weevil feeding is part of the objective as 57 percent of the plants had only light feeding.
A number of plants were crosses of Dexter hybrids or unregistered plants for which the parentage is not known, so are not included in Table 2. Also a number of plants were represented in too few crosses to include in the table.
These observations are only for one year in one garden, but I hope they will stimulate other people to observe and report their findings on weevil injury. This should provide guidance for future hybridizing efforts to produce plants less attractive to weevils.
1. Doss, Robert P. 1983. Root weevil feeding on Rhododendron: A review J. Environ. Hort. 1(3):67-71.
|Table 1. Ratings of weevil injury to leaves of Rhododendron cultivars at Tarrytown, N.Y. October 1983.|
|Ratings of Leaf Injury|
Mrs. AT. de la Mare
Duke of York
R. metternichii (Mt. Kayaf)
R. metternichii (Omoshiroyama)
R. makinoi (Mt. Arisan)
R. vernicosum #2
R. fortune' LuShan
R. makinoi (source unknown)
R. metternichii (Larson)
|Table 2. Ratings of weevil injury to leaves of Rhododendron seedlings of different parentages at Tarrytown, N.Y. October 1983.|
|No. of||Percent of Plants in a Category|
|R. yakushimanum 100%||6||83.3||16.7|
|R. yakushimanum 75%||4||50.0||25.0||25.0|
|R. yakushimanum 50%||52||42.3||30.8||13.5||13.5|
|R. yakushimanum 25%||16||25.0||31.2||18.8||25.0|