Cover Cropping Alters the Diet of Arthropods in a Banana Plantation: A Metabarcoding Approach

Plant diversification using cover crops may promote natural regulation of agricultural pests by supporting alternative prey that enable the increase of arthropod predator densities. However, the changes in the specific composition of predator diet induced by cover cropping are poorly understood. Here, we hypothesized that the cover crop can significantly alter the diet of predators in agroecosystems. The cover crop Brachiaria decumbens is increasingly used in banana plantations to control weeds and improve physical soil properties. In this paper, we used a DNA metabarcoding approach for the molecular analysis of the gut contents of predators (based on mini-COI) to identify 1) the DNA sequences of their prey, 2) the predators of Cosmopolites sordidus (a major pest of banana crops), and 3) the difference in the specific composition of predator diets between a bare soil plot (BSP) and a cover cropped plot (CCP) in a banana plantation. The earwig Euborellia caraibea, the carpenter ant Camponotus sexguttatus, and the fire ant Solenopsis geminata were found to contain C. sordidus DNA at frequencies ranging from 1 to 7%. While the frequencies of predators positive for C. sordidus DNA did not significantly differ between BSP and CCP, the frequency at which E. caraibea was positive for Diptera was 26% in BSP and 80% in CCP; the frequency at which C. sexguttatus was positive for Jalysus spinosus was 14% in BSP and 0% in CCP; and the frequency at which S. geminata was positive for Polytus mellerborgi was 21% in BSP and 3% in CCP. E. caraibea, C. sexguttatus and S. geminata were identified as possible biological agents for the regulation of C. sordidus. The detection of the diet changes of these predators when a cover crop is planted indicates the possible negative effects on pest regulation if predators switch to forage on alternative prey.     

Weeds mediate the level of intraguild predation in arthropod food webs

Intraguild predation, which is common for generalist predators, is a specific form of omnivory that may suppress the biological control of a pest. The dietary flexibility of a given organism depends on the choice of the C3 (banana crop) and the C4 (weeds) pathways they use and on the trophic level on which they feed. Understanding the conditions in which intraguild predation decreases biological control is a major issue in agroecosystems. We tested whether the contribution of different primary producer pathways in diets of generalist predators mediates the level of intraguild predation. We studied 10 agroecosystems in which banana plants (C3 metabolism) were diversely associated with weeds (C4 metabolism). Diversity in litter macrofauna was relatively low, with a mean between three and eight species per trap. Measurement of stable isotopes showed a significant decrease in the δ¹⁵N values of generalist predators when the C4 pathway contributed more than the C3 pathway to their diet. We rejected hypotheses that an increase in the abundance of prey and that a decrease in prey's δ¹⁵N values occur when the C4 pathway contributes more than the C3 pathway to their diet. The results are consistent with the diet modification hypothesis, that is, intraguild predation is lower when the C4 (weeds) pathway is preferred to the C3 pathway. Our results suggest that when the C4 pathway of weeds is more exploited by herbivores (or detritivores), generalist predators tend to consume these herbivores and thus neglect the intraguild prey. The diverse C4 plant community probably supports a diverse herbivore community that provides alternative prey. Our results provide evidence that increasing plant diversity in agroecosystems should decrease intraguild predation of generalist predators and should therefore improve pest regulation. In an applied perspective, plant diversity could be increased by establishing a more diverse cover‐crop community.     

Experimental evaluation of the impacts of two ant species on banana weevil in Uganda

The banana weevil, Cosmopolites sordidus (Germar), is an important pest of bananas. Predatory ants are increasingly being viewed as possible biological control agents of this pest because they are capable of entering banana plants and soil in search of prey. We studied ant predation on banana weevil in Uganda in crop residues and live plants in both laboratory and field experiments. Field studies with live plants used chemical ant exclusion in some plots and ant enhancement via colony transfer in others to measure effects of Pheidole sp. 2 and Odontomachus troglodytes Santschi on plant damage and densities of immature banana weevils.In crop residues, an important pest breeding site, twice as many larvae were removed from ant-enhanced plots as in control plots. In young (2 month) potted suckers held in shade houses, ant ability to reduce densities of banana weevil life stages varied with the weevil inoculation rate. At the lowest density (2 female weevils per pot), densities of eggs, larvae, and pupae were reduced by ants. At higher rates there was no effect. In older suckers (5–11 months) grown in larger containers, banana weevil densities were not affected by ants, but damage levels were reduced. In a field trial lasting a full crop cycle (30 months), we found that the ants tested reduced the density of banana weevil eggs in suckers during the crop, but did not affect larval densities in the sampled suckers. However, most larvae occur in the main banana plants, rather than associated suckers. Nevertheless, levels of damage in mature plants at harvest did not differ between Amdro-treated and ant-enhanced plots, suggesting the ant species studied were not able to provide economic control of banana weevil under our test conditions.     

The challenge of biological control of Cosmopolites sordidus Germar (Col. Curculionidae): A review

The banana weevil (also known as banana root borer) Cosmopolites sordidus Germar (Coleoptera Curculionidae) is the major pest of banana and plantain. Because banana ranks 2nd in fruit production worldwide, this pest has substantial social and economic importance. The biological control of the C. sordidus remains challenging because of its behaviour and resistance to parasitism and predation. The last review concerning the biological control of C. sordidus was published two decades ago, and relevant knowledge and methods have developed in the interim. The present paper provides an update of that knowledge and summarizes past and current challenges as well as providing perspectives on achieving sustainable control of C. sordidus. We first discuss studies on the classical biological control of C. sordidus, underlining the limits of classical biological control methods such as the importation of predators, parasitoids or pathogens. Next, we consider conservation biological control of C. sordidus, with a focus on ants. We also highlight an ‘arthropod bias’ that has led to a lack of information on the role of vertebrates in the regulation of C. sordidus.     

Contrasting effects of low‐level phosphorus and nitrogen enrichment on growth of the mat‐forming alga Didymosphenia geminata in an oligotrophic river

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Effect of fallows on population dynamics of Cosmopolites sordidus: toward integrated management of banana fields with pheromone mass trapping

• 1 The banana weevil Cosmopolites sordidus Germar (Coleoptera: Curculionidae) is the most serious pest of banana and plantain in most production areas, including the West Indies.
• 2 During a period of 2 years, we assessed the efficacy of a pheromone mass‐trapping control strategy of C. sordidus in field conditions in Guadeloupe at different cropping stages, both in sanitation fallows and in different ratoon banana crops.
• 3 In the fallows, catches peaked 3 months after beginning trapping and then decreased to zero after 9 months. By contrast, for the new plantations, the catches of C. sordidus increased after the 11th month and, in the older banana fields, there was no decrease in C. sordidus catches. The C. sordidus catches increased in the neighbouring banana plots, whereas they decreased in the fallows, and these catches decreased with the distance from fallow.
• 4 In conclusion, mass trapping with synergized pheromone traps within fallows should allow better sanitation of banana plantations. Yet, within the farms, fallows must not be located next to new plantations to avoid massive damage to the young plants. More generally, the present study emphasizes that the control of this insect should be managed at the farm scale and not at the field scale, with special attention being paid to the location of fallows.

     

Arthropod food webs in organic and conventional wheat farming systems of an agricultural long‐term experiment: a stable isotope approach

• 1 Agricultural intensification not only alters the structure of arthropod communities, but also may affect biotic interactions by altering the availability of basal resources. We analyzed variations in stable isotope ratios (¹⁵N/¹⁴N and ¹³C/¹²C) of fertilizers, plants, prey and generalist predators in organic and conventional farming systems in a long‐term agricultural experiment [DOK trial (bioDynamic, bioOrganic, Konventionell)]. Two basal resources with pronounced differences in carbon isotope signatures, wheat litter (C₃ plant) and maize litter (C₄ plant), were used to uncover differences in food web properties between the two farming systems (conventional versus organic).
• 2 Predators incorporated significantly higher proportions of carbon from wheat sources in organically managed fields, suggesting that they were more closely linked to wheat‐consuming prey in this system. The δ¹⁵N values of three predaceous species were more than 2‰ greater in summer than in spring.
• 3 The results obtained suggest that generalist predators consumed higher proportions of herbivore prey in the organic system and that starvation and intraguild predation rates increased in some predator species with time.
• 4 Because the effects of farming system and sampling date on predators were species‐specific, conserving a diverse natural enemy community including species with different phenologies and sensitivities to management practices may, in the long term, be a good strategy for maintaining high pest suppression throughout the growing season.

     

Living in an urban pod: Seed predation and parasitism of bruchid beetles in a native tree species

1. Urbanisation is one of the main drivers of insect species loss worldwide. However, its impacts on ecological interactions involving insects still deserve further research, especially seed predation and parasitism of seed predators.
2. Here, we evaluated the seed predation rate by the specialist bruchid beetle Pseudopachymerina spinipes and its parasitism rate in the native tree Vachellia caven (Fabaceae) along an urbanisation gradient in Cordoba (Argentina). Since resource availability can influence these ecological interactions, we also investigated whether seed and prey availability could affect seed predation and parasitism rates, respectively.
3. We sampled trees in 10 sampling sites along an urbanisation gradient estimated by the Normalised Difference Vegetation Index (NDVI) within a 100 m radius. In our system, sites with low NDVI, beyond representing the low amount of vegetation cover, also indicate high surface temperature and low availability of host trees.
4. Seed predation in V. caven and the parasitism rate of P. spinipes were significantly reduced with increasing urbanisation. Notably, seed availability at the pod level did not affect seed predation rate, while prey availability was negatively correlated with parasitism rate.
5. These findings suggest a deleterious effect of urbanisation on the studied antagonistic interactions, giving no support to the idea of resource limitation effects.

     

Migration potential of the banana weevil Cosmopolites sordidus

• 1 The banana weevil Cosmopolites sordidus Germar (Coleoptera: Curculionidae) is a major pest in banana (Musa spp.) growing areas. The weevil is known to be relatively sedentary and closely associated with its host plant but little is known about the species' ability to migrate between banana fields and in nonhost habitats.
• 2 Mark–recapture experiments were conducted to assess the weevils' migration potential, possible differences between the sexes, and the relative attractiveness of pseudostem and pheromone baits.
• 3 One thousand two hundred marked weevils were released in the nonhost habitat at five distances (5, 10, 20, 40 and 70 m) from fresh pseudostem, and from pitfall traps baited with 45 mg of sordidin.
• 4 Two hundred males and 200 females were marked and released at five distances (5, 10, 20, 40 and 70 m) from the pheromone traps.
• 5 Distance and distance/bait interactions had a significant effect on recaptured weevils (binary logistic regression). The two baits were almost equally attractive to weevils in the range 0–10 m, whereas the pheromone was more attractive in the range 10–100 m.
• 6 Distance, bait and distance/bait interactions had a significant effect on the time elapsed from release to recapture (regression with life data) but the pattern observed was not consistent.
• 7 There was no significant difference between males and females with respect to distance or time elapsed from release to recapture.
• 8 The results obtained in the present study show that the migration potential of the banana weevil is greater than previously reported. This should be taken into account when new banana fields are established with clean planting material.

     

Entrapped carrion increases indirect plant resistance and intra‐guild predation on a sticky tarweed

Many plants employ indirect defenses against herbivores; often plants provide a shelter or nutritional resource to predators, increasing predator abundance, and lessening herbivory to the plant. Often, predators on the same plant represent different life stages and different species. In these situations intraguild predation (IGP) may occur and may decrease the efficacy of that defense. Recently, several sticky plants have been found to increase indirect defense by provisioning predatory insects with entrapped insects (hereafter: carrion). We conducted observational studies and feeding trials with herbivores and predators on two sticky, insect‐entrapping asters, Hemizonia congesta and Madia elegans, to construct food webs for these species and determine the prevalence of IGP in these carrion‐provisioning systems. In both systems, intraguild predation was the most common interaction observed. To determine whether IGP was driven by resource abundance, whether it reduced efficacy of this indirect defense and whether stickiness or predator attraction was induced by damage, we performed field manipulations on H. congesta. Carrion supplementation led to an increase in predator abundance and IGP. IGP was asymmetric within the predator guild: assassin bugs and spiders preyed on small stilt bugs but not vice versa. Despite increased IGP, carrion provisions decreased the abundance of the two most common herbivores (a weevil and a mealybug). Overall seed set was driven by plant size, but number of seeds produced per fruit significantly increased with increasing carrion, likely because of the reduction in the density of a seed‐feeding weevil. Observationally and experimentally, we found that carrion‐mediated indirect defense of tarweeds led to much intraguild predation, though predators effectively reduced herbivore abundance despite the increase in IGP.     

Identifying inter‐ and intra‐guild feeding activity of an arthropod predator assemblage

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Predation risk and habitat complexity modify intermediate predator feeding rates and energetic efficiencies in a tri‐trophic system

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Intraspecies mixture exerted contrasting effects on nontarget arthropods of Bacillus thuringiensis cotton in northern China

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The influence of moss on grazers in high‐altitude streams: food,refuge or both?

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Importance of predation by Coleomegilla maculata larvae in the natural control of the corn earworm in sweet corn

The corn earworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) is a serious pest of many crops in North America, particularly sweet corn. The larvae of Coleomegilla maculata (DeGeer) (Coleoptera: Coccinellidae) are the most frequently observed predators of H. zea eggs in sweet corn. Companion cropping strategies have been developed to augment densities of C. maculata in sweet corn plantings. In this study, the importance of predation by C. maculata larvae was evaluated in experiments when larval abundance was manipulated using hand removal and physical exclusion. In 2003 and 2004, sentinel H. zea eggs were exposed on corn for 48 h. Egg mortality was 13.8% greater in the treatment where C. maculata larvae were present. In 2004 and 2005 eggs were exposed on corn under the same hand removal and physical exclusion techniques and monitored at 3-h intervals for 24 h to directly observe predation on sentinel eggs. Coleomegilla maculata larvae were the most frequently observed predators of the eggs, accounting for 45.9% of 85 total observed predation events in the control. Whereas in the treatment where C. maculata larvae were reduced, egg survival was 26.13% greater and only 37 total predation events were observed. All pest eggs were eliminated from individual ears 22.1% more often when C. maculata larvae were present at natural densities indicating that predation by this predator stage lessens crop damage by H. zea. The results of this study suggest that C. maculata larvae are an irreplaceable source of natural mortality for H. zea eggs on sweet corn.     

Cover crop mulches influence biological control of the imported cabbageworm (Pieris rapae L., Lepidoptera: Pieridae) in cabbage

Increasing structural complexity within crop fields can provide a way to manipulate pest abundance and biological control in agroecosystems. Here, we examine the effect of cover crop mulches in cabbage on the structure and function of an insect food web, investigating the role of cover crop species, structure, and volatile cues on important interactions. We focused on the imported cabbageworm (Pieris rapae L., Lepidoptera: Pieridae), and three of its natural enemies, the spined soldier bug (Podisus maculiventris (Say), Hemiptera: Pentatomidae), the convergent lady beetle (Hippodamia convergens (Guerin), Coleoptera: Coccinellidae), and the parasitoid, Cotesia rubecula Marshall (Hymenoptera: Braconidae). We measured the abundance of these insects in a field experiment and conducted a natural enemy exclusion cage study to determine the level of biological control of the imported cabbageworm in the field. Our field experiments indicated that cover crop species, but not structure, influenced insect abundance, with significantly more imported cabbageworm and C. rubecula in rye cover crop mulch plots compared to vetch mulch or bare soil plots. In the Y-tube assays we found some evidence that the increased parasitoid abundance did not result in increased parasitism because of interference of the mulch with short-range host finding odor cues. The natural abundance of the two predators was not different among our field plots with different cover crop treatments. Mortality and parasitism of sentinel imported cabbageworm larvae was not different in field cages among the different cover crop mulch treatments, but there was a significant difference among cage types indicating that small natural enemies play an important role in the biological control of this cabbage pest.     

Linking habitat complexity with predation of pests through molecular gut-content analyses

Improving the diversity of farm systems or landscapes can lead to more effective biological control by providing refuge and alternative resources for colonising natural enemies. Within an experimental cabbage agroecosystem, we examined the effects of habitat management (i.e. herbicide use and cover crops) on pest populations and predator community structure, and report one of the first studies on the trophic links in this system using molecular gut-content analysis. In response to herbicide and cover crop management treatments designed to create different levels of habitat diversity, we quantified the abundance of two pests, Plutella xylostella Linnaeus (Lepidoptera: Plutellidae) and Pieris rapae Linnaeus (Lepidoptera: Pieridae), and predators. We designed species-specific primers to detect prey DNA in predators' guts. Pieris rapae were significantly more abundant in plots where cover crops were killed early in the season, and habitat management generated unique predator communities in response to weed management treatments. Thirty-five per cent of predators tested positive for prey DNA, and habitat management had interactive effects on predation of P. xylostella. Combined we found that habitat management has variable effects on natural enemy–pest interactions.     

Masting behaviour in a Mediterranean pine tree alters seed predator selection on reproductive output

• Context‐dependency in species interactions is widespread and can produce concomitant patterns of context‐dependent selection. Masting (synchronous production of large seed crops at irregular intervals by a plant population) has been shown to reduce seed predation through satiation (reduction in rates of seed predation with increasing seed cone output) and thus represents an important source of context‐dependency in plant‐animal interactions. However, the evolutionary consequences of such dynamics are not well understood.
• Here we describe masting behaviour in a Mediterranean model pine species (Pinus pinaster) and present a test of the effects of masting on selection by seed predators on reproductive output. We predicted that masting, by enhancing seed predator satiation, could in turn strengthen positive selection by seed predators for larger cone output. For this we collected six‐year data (spanning one mast year and five non‐mast years) on seed cone production and seed cone predation rates in a forest genetic trial composed by 116 P. pinaster genotypes.
• Following our prediction, we found stronger seed predator satiation during the masting year, which in turn led to stronger seed predator selection for increased cone production relative to non‐masting years.
• These findings provide evidence that masting can alter the evolutionary outcome of plant‐seed predator interactions. More broadly, our findings highlight that changes in consumer responses to resource abundance represent a widespread mechanism for predicting and understanding context dependency in plant‐consumer evolutionary dynamics.

     

Disentangling the multiple effects of stream drying and riparian canopy cover on the trophic ecology of a highly threatened fish

1. Understanding risks to aquatic systems posed by changing drought regimes is particularly important for the conservation of already threatened taxa. However, little is known about how local environmental conditions, especially those in heavily human‐influenced situations, interact with regional shifts such as droughts to alter realised impacts on aquatic communities, including threatened top predators.
2. Here, we investigated the combined effects of stream drying intensity and riparian canopy cover on the trophic interactions of critically endangered kōwaro, or Canterbury mudfish (Neochanna burrowsius) in an agricultural area of New Zealand. Fish populations and their potential prey, both terrestrial and aquatic, as well as environmental variables, including riparian canopy cover and drying measured with stage loggers, were sampled over eight visits to 24 sites spanning orthogonal drying and canopy gradients. Stable isotope ratios, ¹³C/¹²C and ¹⁵N/¹⁴N, were used to investigate trophic links between mudfish and their terrestrial and aquatic prey across these gradients.
3. When non‐native willows (predominantly Salix fragilis) dominated the riparian canopy, increased tree cover led to elevated drying intensity, probably driven by their relatively high water demands compared to other trees. However, in the absence of willows, canopy cover had no effect on drying intensity. Although this was the only direct link between these two environmental factors, they had opposing effects on kōwaro populations, which will be important for management under drought.
4. Increased drying intensity contributed to elevated abundance of microcrustacea and aquatic Diptera larvae, and an increase in the relative abundance of kōwaro juveniles. However, drying‐affected kōwaro populations also had fewer large reproductive adults and elevated δ¹⁵N values, probably driven by physiological limitations and an increase in kōwaro cannibalism, respectively.
5. By comparison, increased canopy cover enhanced input of terrestrial invertebrates, a food resource for larger kōwaro, leading to elevated kōwaro δ¹³C values, no effects on δ¹⁵N values, and higher relative abundance of large kōwaro in shaded streams compared to unshaded streams. Thus, the riparian canopy cover was able to offset some of the effects of drying.
6. Overall, we found no interactions between drying intensity and canopy cover affecting kōwaro. However, their opposing effect highlights the important role local conditions such as riparian canopies play on aquatic communities and their potential role as a restoration tool to mitigate the effects of large‐scale shifts such as drought.

     

Sowing of margin strips rich in floral resources improves herbivore control in adjacent crop fields

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