Richard A. Redak

Is it enemy-free space ? The evidence for terrestrial insects and freshwater arthropods

Abstract. 1 Enemy‐free space (EFS) was defined by Jeffries & Lawton (1984) as ‘ways of living that reduce or eliminate a species’ vulnerability to one or more species of natural enemies’. EFS has emerged in the literature as a significant niche‐moulding factor. However, the lack of consistency among the empirical studies as to how EFS should be defined, and what hypotheses should be tested in order to evaluate its relative importance, prompted us to review the literature and to propose a working definition that results in a general set of testable hypotheses. 2 To test the relative importance of EFS in structuring the communities of organisms, we propose a set of three falsifiable null hypotheses that must be tested sequentially and rejected. Ho1: The fitness of the organism in an original habit (e.g. on an original host plant) in the presence of natural enemies is equal to the fitness of the organism in that habit in the absence of natural enemies. Acceptance of the alternative hypothesis that the fitness of the organism in the presence of natural enemies is less than in the absence of natural enemies is necessary to demonstrate the importance of natural enemies. Ho2: The fitness of the organism in an alternative habit with natural enemies is equal to the fitness of the organism in the original habit with natural enemies. Acceptance of the alternative hypothesis that the fitness of the organism in the alternative habit with natural enemies is greater than that in the original habit with natural enemies is necessary to demonstrate that the alternative habit provides EFS. Ho3: The fitness of the organism in an alternative habit without natural enemies equals the fitness of the organism in the original habit without natural enemies. Acceptance of the alternative hypothesis that the fitness of the organism in an alternative habit without natural enemies is less than in the original habit without natural enemies is necessary to demonstrate the relative importance of EFS compared with other co‐occurring niche‐moulding factors such as competition or host nutritional quality. 3 We searched the literature and evaluated fifty‐three references (nineteen references to seventeen different terrestrial systems and thirty‐four references to twenty‐four different freshwater systems) to test our hypotheses. 4 Of the forty‐one systems examined, nineteen (46%) tested only for differences in vulnerability of the prey or host species between EFS and non‐EFS options (our Ho2); sixteen (39%) tested for the importance of natural enemies and the effectiveness of the alternative habit in providing EFS (our Ho1 and Ho2); and only ten systems (24%) tested for Ho1, Ho2 and the relative importance of EFS in the system as measured by fitness (our Ho3). 5 Of the systems that tested for EFS, sixteen of nineteen (84%), thirteen of sixteen (81%) and seven of ten (70%) showed evidence in support of the existence of EFS according to hypothesis Ho2 only, hypotheses Ho1 and Ho2, and our three working hypotheses, respectively. 6 These results indicate that very few studies have actually tested for the existence of EFS. Nevertheless, results from this limited number of natural systems suggest that EFS may be important in moulding the niches of arthropods. Because of the large number of claims for EFS in systems where none of the basic hypotheses were investigated, we suggest that authors test for EFS experimentally, be judicious in selecting articles to cite in support of EFS, and exert care in attributing it as a selective force in the evolution of arthropods in specific systems.

Further comments on analysis of covariance in insect dietary studies

Raubenheimer & Simpson (1992) recently discussed the advantages of using analysis of covariance in insect dietary studies as an alternative to the more conventional ratio‐based nutritional indices. We expand on some interpretations of Raubenheimer & Simpson and illustrate our points with examples from published and unpublished data sets. Specifically, we show that an ANCOVA on biomass gain incorporating initial biomass as a covariate provides information not immediately available using the analysis suggested by Raubenheimer & Simpson (ANCOVA on final biomass incorporating initial biomass as a covariate). Second, we show that dietary studies in which the covariate (food consumption) is affected by diet can provide information about the relative importance of preingestive effects (e.g., deterrence) and postingestive effects (e.g., antibiosis) on performance. Cautions about this latter use of ANCOVA are discussed.

Melon aphid performance on chrysanthemum as mediated by cultivar, and differential levels of fertilization and irrigation

Population parameters of the melon aphid, Aphis gossypii Glover, were compared for insects grown on six cultivars of chrysanthemum, Dendranthema grandiflora Tzvelev (‘Fontana’, ‘Iridon’, ‘Pink Lady’, ‘Splendor’, ‘White Diamond’, and ‘White View Time’) fertilized with 3 different levels of nitrogen (80, 160 and 240 mg N/l) and 2 levels of irrigation (high 300 ml and low 210 ml every other day). Fecundity, longevity, and survivorship of aphids were significantly affected by the cultivar treatment, but were not affected by irrigation or fertilizer treatments or by any combination of interactions among cultivar, fertilizer and irrigation. Significantly fewer aphids survived on the cultivar ‘Pink Lady’ (populations reduced by 20.8%) than any other cultivar examined. Intrinsic rates of increase (rm), finite rates of increase (R0), and population doubling times (DT) for aphids were different among fertilizer levels and cultivars, but were not different among irrigation levels. There were no significant fertilizer by irrigation interaction effects upon rm, R0, or DT. The highest rate of fertilizer applied (240 mg N/liter) adversely affected the intrinsic rate of increase of melon aphids; however, aphid intrinsic rate of increase was weakly correlated with foliar soluble protein levels (r= 0.989, P = 0.0954). Foliar soluble protein levels were not associated with fertilization treatment. Melon aphids perform consistently better on the cultivars ‘White Diamond’, ‘Fontana’, and ‘Splendor’, relative to those aphids growing on ‘Iridon’, ‘Pink Lady’, or ‘White View Time’. In general, melon aphids performed best on ‘White Diamond’ and poorest on ‘Pink Lady’.

Arthropod dynamics on sagebrush (Artemisia tridentata): effects of plant chemistry and avian predation

We conducted a field experiment to assess interrelationships between leaf- tissue secondary chemistry, avian predation, and the abundance and diversity of arthropods occurring on sagebrush (Artemisia tridentata) in central Oregon. Arthropods were removed from individual shrubs, some of which were then caged to exclude birds. Secondary chem- istry and arthropods were sampled at intervals up to 56 wk following the defaunation/ caging treatment. Recovery rates differed among arthropod taxa and functional groups. Several sap- sucking homopterans and hemipterans reached control levels within 2-4 wk of the treat- ment, whereas abundances ofparasitoids and predators recovered to match control numbers only 6 wk after defaunation. Abundances of several herbivorous leaf-chewing taxa (pri- marily lepidopteran larvae) remained significantly depressed even 56 wk after the treatment. Fungivores (oribatid mites) reached greater abundances on defaunated than on control shrubs by the end of the experiment. There were also significant changes in the concen- trations or frequency of occurrence of several chemical compounds following the defauna- tion treatment. Several hydrocarbons, sesquiterpenes, and monoterpenes that were present in most shrubs exhibited sharply reduced concentrations in leaf tissues within 4 wk of the treatment, whereas some alcohols and ketones (linalool, borneol, thujone), which occurred at relatively low frequencies among control plants, increased dramatically in their fre- quencies of occurrence following arthropod removal. Both changes persisted for the du- ration of the experiment. We found several significant associations between the abundance or occurrence of arthropod taxa or groups and concentrations or frequencies of secondary compounds, but these were most prevalent among the leaf-chewing lepidopterans. We suggest that the shrubs responded to the removal of herbivorous, leaf-chewing arthropods by altering chemical allocation patterns; the changes in chemistry persisted for over a year because recolonization of the defaunated plants by these herbivores was slow. Effects of the caging treatment were much less obvious. The recovery of the diversity of arthropods known to constitute prey for birds in this system was slower on shrubs

Seasonal flight activity of two Homalodisca species (Homoptera: Cicadellidae) that spread Xylella fastidiosa in southern California.

Abstract Homalodisca coagulata (Say) and Homalodisca lacerta (Fowler) are vectors of a new bacterial disease of oleander in California known as oleander leaf scorch, induced by the bacterium Xylella fastidiosa. H. coagulata also has been implicated in the spread of the strain of X. fastidiosa that induces Pierce’s disease of grapevines in California. We monitored the flight activity of H. coagulata and H. lacerta in oleander and citrus by using yellow sticky cards at three southern California locations where outbreaks of oleander leaf scorch have been documented, and where vector compliments are different. Areas sampled included a mesic coastal area (Irvine, CA) that supports predominantly H. coagulata and few H. lacerta, a dry inland location (Palm Desert, CA) that supports predominantly H. lacerta and few H. coagulata, and an intermediate area (Riverside, CA) supporting both Homalodisca species. From November 1996 to October 1999 peak catches of both Homalodisca species occurred during the midsummer at all locations. H. coagulata was trapped in greater numbers in citrus than in oleander at both the Riverside and the Irvine sites. Likewise, H. lacerta in Riverside was more associated with citrus than oleander, yet H. lacerta in Palm Desert was trapped in greater numbers in oleander than citrus.

Foraging ecology of the California gnatcatcher deduced from fecal samples

Abstract The California gnatcatcher is a threatened species essentially restricted to coastal sage scrub habitat in southern California. Its distribution and population dynamics have been studied intensely, but little is known about its diet. We identified arthropod fragments in 33 fecal samples of the California gnatcatcher to gain insight into its foraging ecology and diet. Fecal samples were collected from adult males, adult females, fledglings, and nestlings. Leaf- and planthoppers (Homoptera) and spiders (Araneae) predominated numerically in samples. Spider prey was most diverse, with eight families represented. True bugs (Hemiptera) and wasps, bees, and ants (Hymenoptera) were only minor components of the gnatcatcher diet. Gnatcatcher adults selected prey to feed their young that was larger than expected given the distribution of arthropod size available in their environment, and chicks were provisioned with larger prey items and significantly more grasshoppers and crickets (Orthoptera) and spiders than adults consumed themselves. Both adults and young consumed more sessile than active prey. Further studies are needed to determine whether arthropods sampled in coastal sage scrub that are common in fecal samples are good indicators of California gnatcatcher habitat.

Evidence for spider community resilience to invasion by non-native spiders

The negative impacts of non-native species are well documented; however, the ecological outcomes of invasions can vary widely. In order to determine the resilience of local communities to invasion by non-native spiders, we compared spider assemblages from areas with varying numbers of non-native spiders in California coastal sage scrub. Spiders were collected from pitfall traps over 2 years. Productive lowland coastal sites contained both the highest proportion of non-natives and the greatest number of spiders overall. We detected no negative associations between native and non-native spiders and therefore suggest that non-native spiders are not presently impacting local ground-dwelling spiders. Strong positive correlations between abundances of some natives and non-natives may be the result of similar habitat preferences or of facilitation between species. We propose that the effects of non-native species depend on resource availability and site productivity, which, in turn, affect community resilience. Our results support the contention that both invasibility and resilience are higher in diverse, highly linked communities with high resource availability rather than the classical view that species poor communities are more invasible.

Effect of Selected Insecticides on Homalodisca coagulata (Homoptera: Cicadellidae) and Transmission of Oleander Leaf Scorch in a Greenhouse Study

Abstract Homalodisca coagulata (Say) is a recent introduction to California. It is known to spread a strain of the bacterium Xylella fastidiosa Wells, Raju, Hung, Weisberg, Mandelco-Paul & Brenner that induces oleander leaf scorch disease in oleander, Nerium oleander L. Oleander leaf scorch is lethal to oleander and threatens to decimate one of the most important landscape shrubs in California. Towards developing a management strategy for H. coagulata-spread oleander leaf scorch, we documented the affects of selected insecticides on H. coagulata mortality, feeding behavior, and disease transmission in a greenhouse study. Oleanders treated with fenpropathrin, fenpropathrin + acephate, and imidacloprid caused significant mortality to caged H. coagulata within 4 h of exposure. Within 24 h, these pesticides caused nearly 100% mortality 3 wk after treatment. In other experiments, acetamiprid and fenpropathrin treatments reduced time spent feeding and total time on plants. H. coagulata on fenpropathrin-, acetamiprid-, and imidacloprid-treated oleander died in less than 13 min on average. Oleander leaf scorch transmission by H. coagulata was blocked by applications of foliar-applied acetamiprid, and soil-applied imidacloprid and thiamethoxam.

Feasibility of Tracking Within-Field Movements of Homalodisca coagulata (Hemiptera: Cicadellidae) and Estimating Its Densities Using Fluorescent Dusts in Mark–Release–Recapture Experiments

A mark-release-recapture technique was developed and tested for use in tracking the field movements of adult glassy-winged sharpshooters, Homalodisca coagulata (Say) (Hemiptera: Cicadellidae), in various agricultural and urban plantings. Greenhouse experiments in which adult H. coagulata were marked with one of five colored fluorescent dusts (Aurora Pink-All, Horizon Blue-A19, Blaze Orange-A15N, Saturn Yellow-A17, and Corona Magenta-A21) and released into cages with citrus seedlings showed that their mortality rates during a 30-d period were statistically similar to that of the undusted controls. Adults marked with a sixth dust color (Signal Green-A18N) suffered higher rates of mortality than did the undusted controls and thus were eliminated from further consideration. Adult H. coagulata marked with one of the five accepted colors of fluorescent dust were able to fly beyond 100 m in a field devoid of vegetation within minutes of their release, and the marking did not affect overall flight behavior significantly compared with that of the undusted controls. However, at wind speeds above 5 m/s, percentage recapture was significantly reduced, which indicates that both dusted and undusted adults were unable to orient their flight. In total, 41,124 marked and unmarked adults were released in the three field experiments in southern California (Riverside and Kern counties) during 2000 and 2001 to evaluate flight dispersal and estimate densities of H. coagulata. The mark-release-recapture and feral data obtained during the June, July, and August 2001 studies, when coupled with the Lincoln index, yielded estimates of adult H. coagulata of 1.2 and 2.2 million per ha, respectively, at a San Joaquin Valley (Kern Co.) and a southern California (Riverside Co.) citrus grove. The use of colored dusts to mark H. coagulata proved to be reliable, cost-effective, and time-efficient for mark-release-recapture studies with this insect within a citrus grove, but they are less likely to be useful for studies of adult H. coagulata movements among plantings.

Impact of acidic deposition onEncelia farinosa gray (Compositae: Asteraceae) and feeding preferences ofTrirhabda geminata horn (Coleoptera: Chrysomelidae)

Container grownEncelia farinosa were exposed to three 3-hr episodes of acidic fog (pH 2.5) typical of events in southern California. Adults and larvae of the specialist leaf-feeding herbivore,Trirhabda geminata, preferred to feed on the acidic-treated foliage compared to control fogged (pH 6.3–6.5) foliage. Previous feeding damage on the plants did not affect feeding preference. The acidic-fogged foliage was significantly higher in total nitrogen and soluble protein but not different from control-treated tissue in water content. Stress on native populations of this drought-deciduous shrub caused by atmospheric pollutants may also result in altered feeding ecology of the beetle.