Effects of feeding by mealybugs (Planococcus citri, homoptera: pseudococcidae) on the growth of Colliguaya odorifera seedlings

Abstract

Mediterranean-type ecosystems are characterized by their capacity to recover after extended droughts and wildfires. Much of this resilience is attributable to underground storage tissue combined with the capacity for crown sprouting from root burls (Specht 1981; Kummerow and Mangan 1981). Some of the dominant shrubs, however, are poor crown sprouters or obligate seeders. Declining soil moisture typically limits the growing season to spring and early summer months. Only those seedlings which develop a deeply penetrating root system in the season following germination will survive the first summer. First-year shrub seedling survivorship in the southern California chaparral has been measured at 3-10% (Ellis 1983). Thus, seedlings must pass through a critical and very vulnerable establishment phase after fire. The effect of herbivores during this phase may be of great importance. Mills (1983) found that approximately 50% of seedlings of Ceanothus greggii, an obligate-seeding shrub in the southern California chaparral, were lost to small-mammal herbivores during the first growing season after fire. The effects of chewing insects and mammals are relatively easy to observe and quantify (Fuentes et al. 1981, 1983). Louda (1983) has cautioned, however, that assessing damage from estimates of the percent of missing leaf area may seriously underestimate the total herbivore effect and recommends an experimental approach which compares plants with herbivores to plants kept free of herbivores. The effects of sucking insects, although much more difficult to measure, can be significant. About 10% of the one-yearold Ceanothus greggii seedlings were killed by a psyllid infection in a post-burn area in the southern California chaparral (Mills, unpublished data). If seedlings are not killed, however, the relative damage to the plant is difficult to estimate. It would be valuable to correlate a particular level of infection by sap feeders with the growth reduction caused by these insects. The effects of phloem-feeding insects differ from those of chewing herbivores in several respects. Chewing insects or mammals often damage or remove the apical meristems and release axillary buds from apical dominance, causing the sprouting of new shoots which represent competitive sinks for photosynthate and increase the demand for water. Thus, the relative magnitude of the belowground sink may be decreased and relatively less photosynthate would reach the root system. Because phloem feeders do not stimulate axillary growth, the relative magnitude of the belowground sink should remain high and the immediate effect on root growth should be less severe. In fact, reallocation of assimilate from belowground to aboveground meristems has been observed due to feeding of chewing insects (Ryle and Powell 1975; Youngner 1972). On the other hand, increased root or shoot growth, or both, may result from feeding by chewing or sucking herbivores (Bardner and Fletcher 1974; Harris 1973, 1974; Howe et al. 1982; Owen 1980; Owen and Wiegert 1981). A second difference is that phloem feeders do not damage the existing photosynthetic structure but remove photosynthate. Thus all feeding effects would be manifest in decreased growth rates of aboveground and/or belowground plant organs. Regardless of the cause, however, reduction of the flow of carbohydrate to the roots of seedlings in summer-dry ecosystems might prevent establishment of the deep tap root system needed to survive the first summer. The purpose of this study is to assess the damage sustained by seedlings of a sclerophyllous shrub species from a Mediterranean ecosystem under three levels of infection by a phloem-feeding insect. The relative growth of root and shoot systems will be compared. The organisms involved were chosen for convenience. My goal was not to define a specific interaction, but to provide a general model which may be tested by more specific applications. The results should also facilitate estimates of damage to seedlings in the field.