Daniel A. Herms, Bradley Proper, and Harry A. J. Hoitink
This study investigated the effects of three types of container media on the root and shoot growth of Taxus, as well as the growth, survival, and host impact of black vine weevil larvae. The three media, which differed in organic matter composition, consisted of: (1) a mix of composted pine bark (60%), composted municipal sludge (20%), rice hulls (15%), and sand (5%); (2) a mix of composted pine bark (60%), composted hardwood bark (20%), and peat (20%); and (3) a mix of peat (50%), perlite (37.5%), and sand (12.5%). Root growth and total plant biomass of noninfested plants were greater in the medium dominated by composted pine bark and municipal sludge than in the other two media. Survival and growth of black vine weevil larvae were also highest in this medium. Consequently, so was root loss due to larval feeding. However, when standardized for the number of larvae present, black vine weevil had a much smaller impact on the root biomass of plants grown in the pine bark/sludge medium, probably because of more rapid growth of new roots. These results indicate that composition of container media not only affects plant growth, but also the survival, growth, and host impact of black vine weevil. There may be a potential for manipulating container media to contribute to integrated management of root-feeding insect pests.
As a result of the Food Quality Protection Act of 1996, the registrations of many pesticides currently used to control nursery pests may be cancelled, creating the need for alternative management strategies. Root-rot pathogens such as Phytophthora and Pythium are successfully managed in container production through the use of media having disease-suppressive properties (Quarles and Grossman, 1995; Hoitink et al., 1996). However, the effects of container media on the growth, survival, and host impact of soil inhabiting insects have received little attention.
Black vine weevil, Otiorhynchus sulcatus, is one of the most important pests of nursery crops, causing considerable economic damage due to root feeding by larvae, as well as restricted sales of infested plants (Moorhouse et al., 1992; Cross et al., 1995). Larvae damage many species of plants by feeding on the roots and crown, often girdling and killing the plant. Taxus, rhododendron, and hemlock rank among the most preferred hosts (Nielsen and Dunlap 1981). The objectives of this study were to determine how different container media affect the root and shoot growth of Taxus, as well as the growth, survival, and host impact of black vine weevil larvae.
Container Media
We evaluated three types of container media differing in organic matter composition: (1) a mix of composted pine bark (60%), composted municipal sludge (20%), rice hulls (15%), and sand (5%); (2) a mix of composted pine bark (60%), composted hardwood bark (20%), and peat (20%); and (3) a mix of peat (50%), perlite (37.5%), and sand (12.5%) to which dolomite lime, CaCO3, KNO3, treble super phosphate, and gypsum were added. These mixes are subsequently referred to as bark/sludge, bark/peat, and peat/perlite, respectively.
Plant Material
This greenhouse study was initiated on May 15, 1997, when rooted Taxus x media ‘Hicksii’ cuttings were transplanted to four-inch pots containing the peat/perlite mix. After being allowed to establish for three weeks, 192 plants were selected for uniformity and then randomly divided into three groups. Plants were subsequently transplanted to one-gallon pots containing the experimental media, with each group of 64 plants randomly assigned to one of the three mixes. Within each of the three media treatments, plants were divided into four blocks of 16 plants based on similarity in size. Over the course of the experiment, plants were irrigated two to three times per week (all plants were watered at each irrigation event), fertilized weekly with 150 ppm 20-20-20 (Peter’s Professional General Purpose Water Soluble Fertilizer), and monthly with micronutrients (100 ppm Peter’s Professional Water Soluble Trace Elements).
Black Vine Weevil Infestation
After being allowed to establish for nearly two months, half of the plants from each medium were selected randomly to be inoculated with black vine weevil eggs, which were added to pots on July 27, 1997. Black vine weevil eggs were obtained from a laboratory culture established from adults collected in a nursery Taxus field in late June 1997, and subsequently reared in the laboratory on Taxus foliage. Each pot was inoculated by placing 100 eggs in a 2.5 cm deep circular trench formed in the medium around the base of each plant. Larvae were allowed to feed until early to mid-October, at which time all plants (infested and non-infested) were harvested over a two-week period (all plants from a block were harvested on the same day). Larvae were collected by sifting the medium of each plant, and the number of larvae recovered from each pot was recorded. Larvae were then frozen, oven dried at 60°C for 48 hrs, and weighed.
Following harvest, the root system of each Taxus plant was severed at the crown and carefully rinsed to remove the growing medium. To quantify above and below ground biomass, roots and shoots (stems and foliage) were dried at 60°C for four hours and weighed separately. Total plant biomass was calculated for each plant as the sum of above- and below-ground biomass.
Effects of Container Media on Taxus Growth
Container media significantly affected the above- and below-ground growth of Taxus. The root growth of noninfested Taxus plants was greater in the bark/sludge medium than in the other two media (Figure 1). Shoot growth of Taxus was greater in the bark/sludge and the peat/perlite medium than in the bark/peat mix (Figure 2). As a result of effects on root and shoot growth, total biomass of non-infested plants was highest in the bark/sludge mix and lowest in the bark/peat mix (Figure 3).
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| Figure 1. Effect of container media and black vine weevil larvae on
root growth of Taxus (data expressed as mean ± standard error; means with
different letters are significantly different). |
Figure 2. Effect of container media and black vine weevil larvae on
shoot growth of Taxus (data expressed as mean ± standard error; means
with different letters are significantly different). |
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| Figure 3. Effect of container media and black vine weevil larvae on total biomass of Taxus (data expressed as mean ± standard error; means with different letters are significantly different). |
Effects of Container Media on Black Vine Weevil Larvae
The container media also affected the growth and survival of black vine weevil. The greatest number of larvae was recovered from the bark/sludge medium than from the other two media (Figure 4). Media also affected larval growth, with larvae from the bark/sludge medium being larger than larvae from the other two media (Figure 4).
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| Figure 4. Effect of container media and black vine weevil survival and larvae growth (data expressed as mean ± standard error; means with different letters are significantly different). |
Effects of Container Media on Host Impact of Black Vine Weevil
Root feeding by black vine weevil larvae dramatically decreased the root biomass of Taxus (Figure 1), but had minimal impact on shoot biomass (Figure 2). The net loss of root biomass due to larval feeding (calculated as difference between root biomass in the infested plants and the non-infested control plants) is a function of both the rate at which roots are consumed by larvae, and the rate of new root growth. Net root loss was greatest in the bark/sludge mix, probably because this medium also produced the most and biggest larvae (Figure 5). However, when the amount of root mass lost was standardized for the average number and size of larvae present in each container mix (calculated as net root loss/total larval biomass per pot), net root loss in the bark/sludge medium was much less than in the other two media (Figure 5).
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| Figure 5. Effect of container media on net root loss of Taxus as a result of larval feeding by black vine weevil, and root loss per mg of larvae biomass (data expressed as mean ± standard error; means with different letters are significantly different). |
This study indicates that composition of container media not only affects plant growth, but also the survival and performance of black vine weevil, as well as the impact that root feeding has on plant growth. Both root and total plant biomass of noninfested plants was greatest in the bark/sludge mix. However, black vine weevil growth and survival was also highest in this medium, as was the amount of roots lost as a result of larval feeding.
It is not clear why plants and larvae grew fastest in the bark/sludge mix. Perhaps the slow release of nutrients from decomposition of the composted municipal sludge improved plant nutrition, which may have also made the plants a more suitable host for larvae. The effect of media on the number of black vine weevil recovered from each pot could be due to effects on survival of eggs and/or larvae. These questions warrant further research.
The ability of Taxus to tolerate root feeding by black vine weevil larvae was much higher in the bark/sludge mix than in the other two media. On a pound-for-pound basis, larvae had a much smaller impact on the root biomass of plants grown in the bark/sludge medium, probably because of more rapid growth of new roots.
The Food Quality Protection Act is anticipated to decrease the availability of pesticides for use in commercial nurseries. As alternative approaches to managing insect pests become important, it will be necessary to understand the impact of cultural practices on pest populations and host-plant resistance. The role of container media in managing root-rot pathogens is already widely appreciated. This study suggests that components of container media can also be manipulated to impact insect pests.
Ridge Manor Nurseries, Inc., donated the Taxus cuttings used in this study, and Cottage Gardens, Inc., and Kurtz Brothers, Inc., provided container media. We are very grateful for their support of this project.
Cross, J. V., J. H Buxton, R. Jacobson, and D. M. Richardson. 1995. Chemical control of vine weevil larvae on container-grown hardy ornamental nursery stock, 1986-1989. Ann. Appl. Biol. 127: 533-542.
Hoitink, H. A. J., Madden, L. V., and Boehm, M. J. 1996. Relationships among organic matter decomposition level, microbial species diversity, and soilborne disease severity. Pages 237-249. In: Strategies for Managing Soilborne Plant Pathogens. R. Hall, Ed. APS Press, St. Paul, Minn. 330 p.
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Nielsen, D. G. and M. J. Dunlap. 1981. Black vine weevil: reproductive potential on selected plants. Ann. Entomol. Soc. Amer. 74:60-65.
Quarles, W. and J. Grossman. 1995. Alternatives to methyl bromide in nurseries – disease suppressive media. The IPM Practitioner 17:1-13.