Ecology, 68(3), 1987, pp. 547-557 ? 1987 by the Ecological Society of America VARIATION IN INSECT DENSITIES ON DESERT CREOSOTEBUSH: IS NITROGEN A FACTOR?1 David C. Lightfoot and Walter G. Whitford Department of Biology, Box 3AF, New Mexico State University, Las Cruces, New Mexico 88003 USA Abstract. A field experiment was conducted to assess the effects of nitrogen and water on the densities and taxonomic and trophic compositions of arthropods on the foliage of desert creosotebush (Larrea tridentata). Nitrogen and water were manipulated through a series of fertilizer and irrigation treatments applied to naturally growing creosotebush shrubs at a site in the northern Chihuahuan Desert. Water with nitrogen, and nitrogen fertilization alone, significantly increased creosotebush foliage production and foliar nitrogen contents. Water alone had less effect on foliage production and no effect on foliar nitrogen contents. Foliage production, foliar nitrogen contents, and numbers of foliage arthropods were all higher in the late spring than in the mid or late summer. Numbers of foliage arthropods increased significantly on fertilized plants in the late spring. Water treatments alone had no effect on numbers of foliage arthropods, but a positive water-fertilizer interaction effect on numbers of foliage arthropods was evident in the late spring. Overall, there were sig? nificant positive correlations between foliage production, foliar nitrogen contents, and foliage arthropod densities. Phytophagous sap-sucking insects accounted for the majority of arthropods on creosotebush, and their densities varied most in relation to foliage pro? duction and foliar nitrogen contents. Results of our study demonstrated that sap-sucking phytophagous insects are more responsive than leaf-chewing insects to increased nitrogen content of creosotebush foliage, and that much ofthe between-plant variation in densities of phytophagous insects within a stand of creosotebush may be due to sap-sucking insects tracking variable foliar nitrogen. Key words: creosotebush; desert; fertilization; foliage; phytophagous insects; Larrea tridentata; leaf- chewing; nitrogen; sap-sucking; variable; water. Introduction and Szarek 1981). Both water and nitrogen may be The dynamic relationships between phytophagous available for brief periods in ample quantities to sup? insects and their host plants are largely port consequences pulses of productivity (Noy-Mier 1973, Crawford of various host plant characteristics such and as Gosz architec- 1982). As water and nitrogen availability to ture, abundance, chemical defenses, nutrient the plants content, varies, so does nitrogen availability to her? and spatial and temporal patterns and interactions bivores due to changes of in plant growth rates and con? those characteristics (Denno and McClure centrations 1983, Strongof nutrients, water, and chemical defensive et al. 1984). The nature of plant chemical defenses compounds in the plant tissues (see Mattson 1980 and largely determines which types of insects feed on par? references therein). Little is known about how desert ticular plants (Rhoades and Cates 1976, Rosenthal and herbivores respond to temporal or spatial fluctuations Janzen 1979), and variations in nutrient content and in resource availability or quality. chemical defenses interact to influence the population McNeill and South wood (1978) proposed several hy- dynamics of phytophagous insects (Reese 1979, potheses to explain how phytophagous insects have Rhoades 1983). Nitrogen availability is particularly adapted to varying nitrogen availability in their host important in regulating populations of insects on plants plants. Variation in foliage nutrient quality among (McNeill and South wood 1978, White 1978, Mattson plants may partially account for spatial and temporal 1980). Theoretically, insect herbivores have evolved variation in the densities of phytophagous insects. Pop? various adaptations to maximize nitrogen consump? ulations of arthropods on desert plants exhibit consid? tion from their host plants, either by directly exploiting erable spatial and temporal variability (Hsiao and plants or plant parts with high nitrogen contents, or by Kirkland 1973, Orians et al. 1977, Crawford 1981), counteradapting to plant chemical defenses that render possibly reflecting variation in host plant nutrient sta- plant nitrogen unavailable (Mattson 1980, Rhoades tus. We attempted to determine how foliage arthropods 1983). on desert shrubs respond to spatially and temporally In desert ecosystems water and nitrogen are limiting variable foliage production and nitrogen content, and resources for plant and herbivore productivity (Hadley whether or not such responses may explain variation in numbers of foliage arthropods on desert shrubs. We 1 Manuscript received 15 October 1985; revised 30 June examined populations of arthropods on the desert shrub, 1986; accepted 16 October 1986. creosotebush (Larrea tridentata [DC] Cov.) in response This content downloaded from 128.123.176.43 on Thu, 21 Nov 2019 15:38:31 UTC All use subject to https://about.jstor.org/terms 548 DAVID C. LIGHTFOOT AND WALTER G WHITFORD Ecology, Vol. 68, No. 3 to varying productivity and nitrogen Foliage content arthropods of their were sampled from one branch host plants, resulting from water and on nitrogen each of the fertilizer tagged shrubs at three times during the amendments. Most insects on creosotebush are host spring and summer of 1983: early June, mid-July, and specific (Hurd and Linsley 1975, Shultz et al. 1977), late September. We attempted to sample similar-sized providing us with a relatively simple plant-herbivore branches from all shrubs. Each branch was placed into system to manipulate. a 40 cm diameter insect sweep net and shaken vigor- ously to dislodge arthropods from the branch into the Methods net. Initial testing of sampling techniques for creo- sotebush foliage arthropods revealed that shaking Study site branches into sweep nets was more effective and con? Our investigation was conducted in the northern sistent than sweeping the foliage or using a mechanical Chihuahuan Desert at the Jornada Long-Term Eco? vacuum device. Insecticide fumigations of entire shrubs logical Research site near Las Cruces, New Mexico. revealed that branch shaking adequately sampled the The study site was situated in an extensive stand of variety of arthropod taxa present on creosotebush creosotebush (Larrea tridentata) on the eastern slope plants. Branch shaking is appropriate for creosotebush ofthe Dona Ana Mountains. Other less abundant plants because individual shrubs consist of series of similar- included the shrubs snakeweed (Xanthocephalum sa- sized branches with foliage largely limited to the ends rothrae) and mormon tea (Ephedra trifurca), the of the branches. Only one branch per shrub was sam? succulents prickly pear cactus (Opuntia violacea) and pled at each date to minimize the effects on samples soap-tree yucca (Yucca elata), the perennial grasses fluff- taken at later dates. All plots were sampled at sunrise grass (Erioneuron pulchellum) and bush-muhly (Muh- when arthropod activity was lowest and flying insects lenbergia porteri), and numerous annual herbs. were not likely to escape. The net contents from each branch were emptied into individual zip-lock plastic Sampling techniques storage bags, and taken to the lab where the arthropods To assess the effects of variable water and nitrogen were sorted to taxa and trophic groups. on creosotebush plants and foliage arthropods, a series Data analysis of nine plots was constructed and amended with water and nitrogen in a split-plot factorial experimental de? Data for creosotebush branch growth increments, sign. Each ofthe plots measured 5 x 10 m and received nitrogen contents of leaves, and foliage arthropod num? the following treatments: (1) simulated rainfall: three bers were analyzed by SAS GLM factorial analysis of treatments randomly assigned to plots, each replicated variance models (SAS Institute 1982). Normality of three times, consisting of three plots as controls, three data was verified by using PROC UNIVARIATE (SAS plots receiving 6 mm of water once a week, and three Institute 1982). With these models, we could test for plots each receiving 25 mm of water once every 4 wk; response differences between the various treatments (2) one-half of each of the plots was treated with am? and sample dates, and test for interaction effects. In all monium nitrate fertilizer, with the total nitrogen equiv- analyses, plots were used as replicates, and the mean alent of 100 kg/ha, by one application in February values of growth increments, nitrogen contents, or 1983. Simulated rainfall was applied through a series numbers of arthropods (arthropod data were log trans? of sprinklers that were above the shrub canopy. Water formed) were used as response variables. Differences was obtained from a well-fed concrete storage pond between the levels of class variables were tested by prior to transport to the plots. The combination of using orthogonal contrasts and Duncan's multiple-range simulated rainfall and nitrogen fertilizer treatments re? tests for main effect means. sulted in a total of five treatments and one absolute Analysis of variance procedures were performed on control, each replicated three times. All plots were branch growth increments, numbers of foliage arthro? fenced with chicken wire to exclude rabbits. pods for each of the three sample dates separately, and Five similar-sized shrubs in each of the