Effects of enhanced productivity of resources shared by predators in a food-web module: Comparing results of a field experiment to predictions of mathematical models of intra-guild predation.

We compared the response to resource enhancement of a simple empirical model of intra-guild predation (IGP) to the predictions of published, simple mathematical models of asymmetric IGP (a generalist IG Predator that feeds both on a specialist IG Prey and a Resource that it shares with the IG Prey). The empirical model was a food-web module created by pooling species abundances across many families in a speciose community of soil micro-arthropods into three categories: IG Predator (large predatory mites), IG Prey (small predatory mites), and a shared Resource (fungivorous mites and springtails). By pooling abundances of species belonging to broadly defined functional groups, we tested the hypothesis that IGP is a dominant organizing principle in this community. Simple mathematical models of asymmetric IGP predict that increased input of nutrients and energy to the shared Resource will increase the equilibrium density of Resource and IG Predator, but will decrease that of IG Prey. In a field experiment, we observed how the three categories of the empirical model responded to two rates of addition of artificial detritus, which enhanced the food of fungivores, the Resource of the IGP module. By the experiment's end, fungivore densities had increased ~1.5× (ratio of pooled fungivore densities in the higher-input treatment to plots with no addition of detritus), and densities of IG Predators had increased ~4×. Contrary to the prediction of mathematical models, IG Prey had not decreased, but instead had increased ~1.5×. We discuss possible reasons for the failure of the empirical model to agree with IGP theory. We then explore analogies between the behavior of the empirical model and another mathematical model of trophic interactions as one way to gain insights into the trophic connections in this community. We also propose one way forward for reporting comparisons of simple empirical and mathematical models.

Evaluation of Tarsonemus bilobatus and Podosphaera xanthii as suitable resources for Proprioseiopsis mexicanus in cucurbit systems in the Southeast USA.

Tritrophic relationships involving tarsonemids and predatory phytoseiids are common in a variety of agroecosystems, but due to the wide range of diets in both families, it is necessary to understand what food resources they are consuming to determine potential impact on crops. We investigated a frequent association of cucurbit powdery mildew (Podosphaera xanthii), Tarsonemus bilobatus, and Proprioseiopsis mexicanus in watermelon and pumpkin fields to determine whether P. mexicanus is consuming either or both of the other organisms. We also examined developmental and reproductive capability of P. mexicanus on these diets. If P. mexicanus is an effective predator of T. bilobatus, it may also be useful in controlling pest tarsonemids, such as broad mites. Proprioseiopsis mexicanus either starved or escaped from arenas rather than consume P. xanthii. When consuming T. bilobatus, P. mexicanus females developed from larva to adult in ca. 3 days. On this diet, the preoviposition period was ca. 2 days and P. mexicanus laid 1.7 eggs/day. These results are comparable to some of the higher-quality non-prey resources investigated in the literature. Starved female P. mexicanus consumed 6.5 T. bilobatus of mixed stages in 1 h. This study provides support for further research into the importance of non-pest tarsonemids as a resource to maintain the presence of generalist predatory mites as an early-intervention natural enemy. Further work should examine the efficacy of P. mexicanus as a natural enemy of economically important pest tarsonemids.

Suitability of food resources for Proprioseiopsis mexicanus, a potentially important natural enemy in eastern USA agroecosystems.

The phytoseiid Proprioseiopsis mexicanus has been collected from a wide range of plants in the western hemisphere, including many cucurbit agroecosystems in South Carolina, USA. Our aim was to characterize the lifestyle of P. mexicanus and its potential as a natural enemy of Tetranychus urticae, a common pest in cucurbits. We determined developmental time, pre-oviposition time, and fecundity of females on pollen-only diets from a commercial Typha spp. supplement source, Citrullus lanatus, Cucurbita maxima, Cucurbita moschata, Cucurbita pepo, Delosperma cooperi, Trifolium incarnatum, and on T. urticae and a combination of T. urticae and Typha spp. pollen. Female development time differed based on diet-development was fastest on C. lanatus and D. cooperi diets (ca. 3 days) and slowest on Typha pollen diet (ca. 5 days). Pre-oviposition time was shorter for females fed C. lanatus and T. incarnatum (1.6 days) and longest when fed Typha pollen (3.1 days). Citrullus lanatus, T. incarnatum, and D. cooperi pollen diets resulted in more eggs/day compared to other diets. Cucurbita moschata pollen resulted in the lowest oviposition rate (0.69 eggs/day). Because these pollens varied in size, we examined pollen size as a factor in developmental and reproductive success. With the exception of Typha pollen, small-pollen diets (C. lanatus, D. cooperi, and T. incarnatum) resulted in faster development, shorter pre-oviposition time, and higher fecundity than large-pollen diets (Cucurbita spp.). We concluded that P. mexicanus is a generalist predator that may require pollen for survival and reproduction, but alone this species may not be an effective predator of T. urticae.

Residual Activity of Acaricides for Controlling Spider Mites in Watermelon and Their Impacts on Resident Predatory Mites.

Twospotted spider mite, Tetranychus urticae Koch (Trombidiformes: Tetranychidae), is an important, worldwide pest of watermelon, Citrullus lanatus L. (Thunb.) Matsum. & Nakai (Cucurbitales: Cucurbitaceae). Feeding results in chlorotic spots and leaf necrosis, which can substantially reduce yields. In watermelon, T. urticae is managed solely with acaricides. Issues with acaricide resistance and pesticide label restrictions on number of applications per season require research-based recommendations on products with effective, long-lasting residues. To improve recommendations for T. urticae management in watermelon and to measure possible effects on non-target beneficial mites, we conducted acaricide efficacy trials in two locations in South Carolina, United States. The adulticidal products abamectin, bifenazate, fenpyroximate, and tolfenpyrad and the ovicidal products spiromesifen and etoxazole were tested. We also conducted two bioassays to better determine duration of acaricide residues. In the field trials, all acaricides except tolfenpyrad reduced T. urticae abundance, but all acaricides also reduced abundance of the most common predatory mite, Neoseiulus fallacis (Garman) (Mesostigmata: Phytoseiidae). In the bioassays, abamectin and bifenazate residues caused high adult T. urticae mortality at up to 21 d after treatment, performing better than fenpyroximate and tolfenpyrad. Etoxazole and spiromesifen were longer lasting, with <1 offspring per treated female in the etoxazole treatment at 28 d after treatment. Based on efficacy, abamectin or bifenazate should be rotated with etoxazole for fast knockdown of active stages while reducing reproduction, respectively. However, development and registration of more selective acaricides in watermelon is needed to preserve biological control of T. urticae by predatory mites.

Invasive plants have different effects on trophic structure of green and brown food webs in terrestrial ecosystems: a meta-analysis.

Although invasive plants are a major source of terrestrial ecosystem degradation worldwide, it remains unclear which trophic levels above the base of the food web are most vulnerable to plant invasions. We performed a meta-analysis of 38 independent studies from 32 papers to examine how invasive plants alter major groupings of primary and secondary consumers in three globally distributed ecosystems: wetlands, woodlands and grasslands. Within each ecosystem we examined if green (grazing) food webs are more sensitive to plant invasions compared to brown (detrital) food webs. Invasive plants have strong negative effects on primary consumers (detritivores, bacterivores, fungivores, and/or herbivores) in woodlands and wetlands, which become less abundant in both green and brown food webs in woodlands and green webs in wetlands. Plant invasions increased abundances of secondary consumers (predators and/or parasitoids) only in woodland brown food webs and green webs in wetlands. Effects of invasive plants on grazing and detrital food webs clearly differed between ecosystems. Overall, invasive plants had the most pronounced effects on the trophic structure of wetlands and woodlands, but caused no detectable changes to grassland trophic structure.