Impact of introduced native herbivores on a Pistia stratiotes infestation close to the Paraná Delta in Argentina

Pistia stratiotes L. (Araceae) is an important weed in many waterways worldwide. The South American weevil Neohydronomus affinis Hustache (Coleoptera: Curculionidae) and the planthopper Lepidelphax pistiae Remes Lenicov (Hemiptera: Delphacidae) are specific herbivores of P. stratiotes. Both species, plus the oligophagous moth Samea multiplicalis Guenée (Lepidoptera: Crambidae), were released in a P. stratiotes-infested lake in the suburbs of Buenos Aires, Argentina, where previous sampling indicated that specific herbivores were absent. The weevil and the planthopper established and covered the whole lake in under two months. The moth may have also established but its numbers were very low and erratic. Plant cover, biomass per square metre, and individual plant size decreased markedly within a year after the release of the insects. Concomitantly, other floating macrophytes appeared in the lake. Insect abundance was related to mean monthly temperature and light hours, but not the other insect species. However, insect damage levels and the state of the plant population suggest that herbivory was the main factor in lowering the P. stratiotes populations. N. affinis and L. pistiae could be specific, damaging and compatible as biocontrol agents for P. stratiotes.     

Biological control of water lettuce,Pistia stratiotes [Araceae] byNeohydronomus affinis [Coleoptera: Curculionidae]

Neohydronomus affinis was imported into Australia as a potential biological control agent for the floating aquatic weedPistia stratiotes. Quarantine studies demonstrated that this weevil was host specific toP. stratiotes and it was released into the field in 1982. Subsequently it was released as a control agent for this weed in several other countries. In AustraliaN. affinis reduced infestations ofP. stratiotes by 40% or more within 12–18 months. The results are discussed in relation to its effectiveness in other parts of the world. We concluded thatN. affinis effectively controlsP. stratiotes in tropical regions (to lat. 22°), but in cooler regions its effectiveness fluctuates with seasonal conditions.      

Host range of Lepidelphax pistiae (Hemiptera: Delphacidae) and its potential impact on Pistia stratiotes L. (Araceae)

Pistia stratiotes L. (Araceae) is a floating aquatic plant that has become invasive in Florida. It is primarily controlled with herbicides, but two biocontrol agents have previously been released to assist in management of this species. A new potential biocontrol agent from Argentina, Lepidelphax pistiae Remes Lenicov (Hemiptera: Delphacidae), has been evaluated comprehensively for specificity after initial host range studies done in its native range indicated that it is likely specific to P. stratiotes. Host range studies indicated that this insect is specific to P. stratiotes, with no survival or reproduction occurring on any of the 42 other plant species tested. Impact studies indicated that this insect can significantly damage P. stratiotes at medium and high population densities, which were comparable to those seen in its native range. Lepidelphax pistiae is sufficiently specific enough to warrant release and has a high probability of aiding management of P. stratiotes populations in Florida.     

Interactive effects of light environment and herbivory on growth and productivity of an invasive annual vine, Persicaria perfoliata

Plant populations often exist in spatially heterogeneous environments with varying light levels, which can affect plant growth directly through resource availability or indirectly by altering behavior or success of herbivores. The plant vigor hypothesis predicts that herbivores are more likely to attack vigorously growing plants than those that are suppressed, for example in more shaded conditions. Plant tolerance of herbivory can also vary under contrasting resource availability. Observations suggest that damage by Rhinoncomimus latipes Korotyaev (Coleoptera: Curculionidae), introduced into the United States in 2004 as a biological control agent for mile-a-minute weed (Persicaria perfoliata [L.] H. Gross), is greater in the sun than in shade. We compared weevil densities and plant growth in paired plots in full sun or under shade cloth; a second experiment included insecticide-treated plots in sun and shade, to assess the ability of the plant to compensate for herbivore damage. Greater density of weevils and more node damage (indicating internal larval feeding) were found on P. perfoliata plants growing in sun than on those in shade. Nodes were 14% thicker in the sun, which may have provided better larval habitat. Biomass produced by plants without weevils in the sun was about twice that produced in any other treatment. Herbivory had a greater effect on plant growth in the high-light environment than in the shade, apparently because of movement into the sun and increased feeding there by the monophagous herbivore, R. latipes. Results support the plant vigor hypothesis and suggest that high weevil densities in the sunny habitats favored by P. perfoliata can suppress plant growth, negating the resource advantage to plants growing in the sun.     

Biogeographical comparison of the arthropod herbivore communities associated with Lepidium draba in its native, expanded and introduced ranges

Aim To examine the composition and structure of the arthropod community on the invasive weed Lepidium draba in its native, expanded and introduced ranges, in order to elucidate the lack of a biotic constraint that may facilitate invasion. Location Europe and western North America. Methods Identical sampling protocols were used to collect data from a total of 35 populations of L. draba in its native (Eastern European), expanded (Western European) and introduced (western US) ranges. A bootstrapping analysis was used to compare herbivore richness, diversity and evenness among the regions. Core species groups (monophages, oligophages and polyphages) on the plant were defined and their abundances and host utilization patterns described. Results Species richness was greatest in the native range, while species diversity and evenness were similar in the native and expanded range, but significantly greater than in the introduced range of L. draba. Specialist herbivore abundance was greater in the native and expanded compared with the introduced range. Oligophagous Brassicaceae‐feeders were equally abundant in all three ranges, and polyphagous herbivore abundance was significantly greater in the introduced range. Overall herbivore abundance was greater in the introduced range. Host utilization was more complete in the two European ranges due to monophagous herbivores that do not exist in the introduced range. Root feeders and gall formers were completely absent from the introduced range, which was dominated by generalist sap‐sucking herbivores. However, one indigenous stem‐mining weevil, Ceutorhynchus americanus, occurred on L. draba in the introduced range. Main conclusions This is, to our knowledge, the first study documenting greater herbivore abundance on an invasive weed in its introduced, compared with its native, range. However, greater abundance does not necessarily translate to greater impact. We argue that, despite the greater total herbivore abundance in the introduced range, differences in the herbivore community structure (specialist vs. generalist herbivory) may contribute to the invasion success of L. draba in the western USA.     

Host specificity and biology of the weevilNeohydronomus affinis [Coleoptera: Curculionidae] a biological control agent ofPistia stratiotes

Waterlettuce,Pistia stratiotes L., has been successfully controlled in Australia with the weevil,Neohydronomus affinis Hustache. In this study, the weevil was tested in quarantine against 34 plant species in 27 families. In non-replicated no-choice tests, it fed and oviposited on 3 species of duckweeds [(Lemna minor L.,Spirodela punctata (Meyer) Thomps.,S. polyrhiza (L.) Schleid.], frogbit [(Limnobium spongia (Bosc.) Steud.], mosquitofern (Azolla caroliniana Willd) and waterfern (Salvinia mimima Baker). Feeding also occurred on golden club (Orontium aquaticum L.) and one egg was laid on the terrestrial panda plant (Kalanchoe tomentosa Baker). Plants oviposited and fed upon were retested in choice tests. The weevils then fed and oviposited exclusively on waterlettuce. Various aspects of weevil biology, including larval feeding and mining, plant response to weevil attack, and adult behavior are discussed. Florida Agric. Expt. J. series No 8580.     

Zooplankton characteristics in monitoring of Lake Ladoga

Water lettuce (Pistia stratiotes L. (Araceae)) is alien to Africa and a declared weed in South Africa. In large perennial rivers it is effectively controlled by its biological control agent, Neohydronomus affinis (Coleoptera: Curculionidae). On those shallow isolated water bodies which are regularly subjected to alternate wet and dry regimes that become infested with water lettuce, chemical control is necessary to prevent further spread of the weed and to facilitate access to water. This paper discusses the short-term chemical control and the long-term biological control of water lettuce. The need for further research is outlined.     

Herbivore-herbivore interactions complicate links between soil fertility and pest resistance

Soil fertility is tightly linked with herbivore pressure because it affects the nutritional status of host plants as well as the production of anti-herbivore defenses. This in turn can influence whether herbivores in different feeding guilds render plants more or less susceptible to one another. Thus, growers’ fertility management choices may impact herbivores through a variety of indirect channels. We examined relationships between soil fertility and interactions between phloem-feeding and leaf-chewing herbivores on broccoli (Brassica oleracea) plants in the greenhouse, taking advantage of natural variation in nitrogen (N) and phosphorus (P) in soils from 20 working organic vegetable farms. Next, we experimentally fertilized soil in a field trial with N and/or P to examine the consequences of these nutrients for growth of and interactions between specialist and generalist herbivores. Soils on our cooperating farms varied widely in P and N concentrations, with 40% exceeding recommended pre-plant N concentrations and 90% exceeding P recommendations. In single-herbivore infestations, augmenting N in the soil increased caterpillar (Pieris rapae) growth, augmented N and P additively enhanced generalist green peach aphid (Myzus persicae) colonization, and augmented P (but not N) increased specialist cabbage aphid (Brevicoryne brassicae) growth. In dual-guild herbivore infestations, caterpillars facilitated specialist cabbage aphid growth in the absence of fertilizer, but this pattern disappeared under augmented N, and reversed under augmented P. We found that a complex web of indirect effects linked soil fertility to herbivore performance, depending on the identity of the nutrients being altered, the ecological roles of responding herbivore species (i.e., specialist versus generalist), and indirect interactions between chewing and sucking herbivores. More generally, we highlight that successful use of fertility management to improve pest resistance requires careful consideration of herbivore feeding niches and herbivore-herbivore interactions.     

The effect of two biological control agents, the weevil Neochetina eichhorniae and the mirid Eccritotarsus catarinensis on water hyacinth, Eichhornia crassipes , grown in culture with water lettuce, Pistia stratiotes

We assessed the effect of two biological control agents, the mirid Eccritotarsus catarinensis (Carvalho) and the weevil Neochetina eichhorniae (Warner), singly or in combination, on the competitive ability of their host plant, water hyacinth, Eichhornia crassipes (Mart.) Solms-Laub., grown in a screen house, in competition with another aquatic plant (Pistia stratiotes L.). Water hyacinth plant growth characteristics measured included fresh weight, leaf and petiole lengths, number of inflorescences produced, and new shoots. Without herbivory, water hyacinth was 18 times more competitive than water lettuce (across all experimental combinations of initial plant densities), as estimated from fresh weights. Both insect species, singly or in combination, reduced water hyacinth plant growth characteristics. E. catarinensis alone was less damaging than the weevil and under normal conditions, i.e., floating water hyacinth, is not expected to increase control of water hyacinth beyond that of the weevil. When combined with the weevil, half the inoculum of weevils and half the inoculum of mirids produced the same growth reduction as the full inoculum of the weevil. Under conditions where the weevils are not effective because water hyacinths are seasonally rooted in mud, the mirid, which lives entirely on leaves, should become a useful additional biological control agent. Handling Editor: John Scott.     

Impact of the biological control agent Rhinoncomimus latipes (Coleoptera: Curculionidae) on mile-a-minute weed,Persicaria perfoliata,in field cages

A host-specific Asian weevil, Rhinoncomimus latipes Korotyaev, was approved in 2004 for release in North America for control of mile-a-minute weed, Persicaria perfoliata (L.) H. Gross (formerly Polygonum perfoliatum L.), an invasive annual vine from Asia. The impact of R. latipes feeding on P. perfoliata was studied in field cages over a 2-year period. In 2006, 20 weevils introduced into cages with single plants in May (when weevils first emerge from overwintering) suppressed seed production for about 9 weeks, while weevils introduced in June (when the first summer generation of adults emerge) did not affect seed phenology. Plants in all cages produced substantial numbers of seeds late in the year, but the average seed (achene) weight was reduced for plants with 20 weevils per plant introduced in May. In 2007, plants grown with some competition from other plants within field cages showed substantial mortality, with 63% of plants with 10 or 20 weevils and 75% of plants with 40 weevils per plant dead by mid-August, compared with 12.5% of control plants. Reproduction was delayed by more than a month in surviving plants with 10 or 20 weevils, and by more than 2 months in the few survivors with 40 weevils. Surviving plants with 40 weevils per plant showed loss of apical dominance, which can allow plants to compensate for herbivore damage, but in the case of a light-adapted vine like P. perfoliata may prevent the plants from achieving needed sun exposure. These results suggest that R. latipes feeding on P. perfoliata has the potential to impact plant growth and reproduction, and can put affected plants at a substantial competitive disadvantage.     

福寿螺对稻田水生植物群落实验的影响

福寿螺是在我国南方地区危害水稻生产的一种外来入侵软体动物,已被世界自然保护联盟入侵生物专家组列为100种危害最为严重的外来入侵物种之一。在水域生态系统中福寿螺主要以摄食各种水生植物而生存。以水稻、大薸、水葫芦、粉绿狐尾藻、水花生和鸭舌草构成的稻田水生植物群落为对象,研究了低密度（4 株/m²）、中密度（8 株/m²）和高密度（12 株/m²）福寿螺处理对稻田水生植物群落的影响,同时结合福寿螺的食物选择性实验分析其相关行为特性。结果表明：1）低密度、中密度和高密度福寿螺处理1周内水稻密度均急剧下降,高密度福寿螺造成水稻密度下降超过70%,2周后福寿螺危害水稻秧苗的程度减轻;2）各密度处理福寿螺均抑制了水花生和鸭舌草扩散,第8周时高密度福寿螺处理中水花生和鸭舌草的密度仅为对照的30%和25%。同时,低密度福寿螺处理中的水花生和鸭舌草的密度显著高于中密度和高密度处理。不同密度福寿螺处理对水葫芦、大薸和粉绿狐尾藻影响不明显;3）水稻的日均密度增长率为负值,低密度、中密度和高密度福寿螺处理间存在明显差异,且均显著低于对照。水花生和鸭舌草的日均密度增长率虽为正值,但显著低于对照,水葫芦、大薸和粉绿狐尾藻的日均密度增长率和对照无差异。福寿螺日均密度增长迅速,各密度处理无显著差异。福寿螺对水稻、水花生、鸭舌草的选择性显著高于水葫芦、粉绿狐尾藻和大薸;4） 福寿螺引入8周后稻田水生植物群落物种多样性Simpson指数、Shannon指数和均匀度指数均显著降低。Simpson指数的大小顺序为：对照 > 低密度 > 中密度、高密度,Shannon指数和均匀度指数的大小顺序为：对照 > 低密度 > 中密度>高密度。说明福寿螺对供试水生植物有明显的选择性,其入侵会严重危害稻田水生植物群落的物种多样性。     

Mile-a-minute weed (Persicaria perfoliata) and weevil (Rhinoncomimus latipes) response to varying moisture and temperature conditions

The combined effects of herbivory and water stress on growth and reproduction of mile-a-minute weed (Persicaria perfoliata (L.) H. Gross) were investigated in greenhouse trials over two years, with well-watered or water-limited plants either exposed or not exposed to herbivory by the mile-a-minute weevil (Rhinoncomimus latipes Korotyaev). Moisture limitation and weevil herbivory significantly reduced the number of seeds produced by P. perfoliata, with the fewest seeds produced when both factors were present. Seed weight was reduced by moisture limitation and weevil herbivory the second year, and seed viability was reduced by herbivory both years. Plant biomass was lower both years under conditions of water limitation, with an additional effect of herbivory the second year. Well-watered plants the second year also produced substantially more weevils than water-limited plants by the end of the season. Results are consistent with field observations suggesting that years of high rainfall allow resurgence of P. perfoliata plant populations that were previously suppressed by R. latipes. An additional environmental chamber trial assessed the interaction between the weed and weevil at two different temperatures. Here, plant mortality occurred only at the higher temperature with weevil herbivory, suggesting that herbivory has a greater negative effect on P. perfoliata under warm conditions. Additional studies on temperature effects are needed for a more complete understanding of interactions between P. perfoliata and R. latipes under different abiotic conditions.     

Impact of herbivory on mile-a-minute weed (Persicaria perfoliata) seed production and viability

The monophagous weevil Rhinoncomimus latipes Korotyaev (Coleoptera: Curculionidae) has been introduced into North America as a biological control agent for the invasive vine mile-a-minute weed, Persicaria perfoliata (L.) H. Gross. This weevil has been shown to reduce the percent cover of mile-a-minute and the number of seed clusters produced, as well as altering the phenology of seed production. However, prior work has not examined the potential impact of the weevil on seed viability and numbers of seeds per cluster. When both adult and larval weevils fed on whole plants in the greenhouse, seed production and fruit maturation were delayed and both the total number of seeds and number of seeds per cluster were reduced. Overall, weevils in greenhouse cages reduced the reproductive potential of P. perfoliata by 35%. When adult weevils were confined on developing seed clusters in the field, the number of seeds produced per cluster, the weight of seeds produced, and seed viability all decreased. Overall, the number of viable seeds per cluster was reduced by 37% in the presence of adult weevils. Taken together, these findings demonstrate that this biological control agent can have important impacts on the reproductive potential of its target weed through mechanisms that have not previously been shown.     

Interactions between herbivorous fish guilds and their influence on algal succession on a coastal coral reef

Herbivory is an important mechanism affecting algal succession, particularly on coral reefs where the relationship between algae and corals is largely controlled by herbivores. However, different functional groups of herbivores may have contrasting effects on succession, which may explain different trajectories of coral reef recovery after disturbance. Here, the effects of different herbivore groups (roving herbivores = foragers and territorial damselfish = farmers) were isolated by a multi-factorial experiment carried out on a coastal coral reef with high macroalgal cover, high farmer densities and relatively low forager abundance. The effects of foragers and farmers were distinguished by monitoring algal succession on settlement tiles placed inside and outside exclusion cages, with orthogonal treatments established inside and outside damselfish territories (with appropriate cage controls). Within 12 months, algal assemblages on ungrazed tiles inside exclusion cages proceeded rapidly from fine filamentous turfs, to corticated algae, to tough erect (e.g. Amphiroa spp.) and foliose (e.g. Peyssonnellidae) calcified algae. Farmers had a dramatic impact on succession, essentially arresting the development of the algal community at a point where it was dominated by palatable filamentous algae of the genus Polysiphonia. Fleshy macroalgae such as Sargassum spp. were excluded from farmer territories. In contrast, foragers did not suppress fleshy macroalgae, but rather, appeared to decelerate succession and promote a relatively diverse assemblage. In contrast to forager-dominated reefs, farmer territories did not appear to function solely as forager exclusion areas or promote algal diversity as a result of intermediate grazing pressure. The relatively strong effects of farmers observed here may represent a future scenario for coral reefs that are increasingly subject to overfishing of large grazing fishes.     

Competition induces allelopathy but suppresses growth and anti-herbivore defence in a chemically rich seaweed

Many seaweeds and terrestrial plants induce chemical defences in response to herbivory, but whether they induce chemical defences against competitors (allelopathy) remains poorly understood. We evaluated whether two tropical seaweeds induce allelopathy in response to competition with a reef-building coral. We also assessed the effects of competition on seaweed growth and seaweed chemical defence against herbivores. Following 8 days of competition with the coral Porites cylindrica, the chemically rich seaweed Galaxaura filamentosa induced increased allelochemicals and became nearly twice as damaging to the coral. However, it also experienced significantly reduced growth and increased palatability to herbivores (because of reduced chemical defences). Under the same conditions, the seaweed Sargassum polycystum did not induce allelopathy and did not experience a change in growth or palatability. This is the first demonstration of induced allelopathy in a seaweed, or of competitors reducing seaweed chemical defences against herbivores. Our results suggest that the chemical ecology of coral–seaweed–herbivore interactions can be complex and nuanced, highlighting the need to incorporate greater ecological complexity into the study of chemical defence.     

Herbivores differentially limit the seedling growth and sapling recruitment of two dominant rain forest trees

Resource heterogeneity may influence how plants are attacked and respond to consumers in multiple ways. Perhaps a better understanding of how this interaction might limit sapling recruitment in tree populations may be achieved by examining species’ functional responses to herbivores on a continuum of resource availability. Here, we experimentally reduced herbivore pressure on newly established seedlings of two dominant masting trees in 40 canopy gaps, across c. 80 ha of tropical rain forest in central Africa (Korup, Cameroon). Mesh cages were built to protect individual seedlings, and their leaf production and changes in height were followed for 22 months. With more light, herbivores increasingly prevented the less shade-tolerant Microberlinia bisulcata from growing as tall as it could and producing more leaves, indicating an undercompensation. The more shade-tolerant Tetraberlinia bifoliolata was much less affected by herbivores, showing instead near to full compensation for leaf numbers, and a negligible to weak impact of herbivores on its height growth. A stage-matrix model that compared control and caged populations lent evidence for a stronger impact of herbivores on the long-term population dynamics of M. bisulcata than T. bifoliolata. Our results suggest that insect herbivores can contribute to the local coexistence of two abundant tree species at Korup by disproportionately suppressing sapling recruitment of the faster-growing dominant via undercompensation across the light gradient created by canopy disturbances. The functional patterns we have documented here are consistent with current theory, and, because gap formations are integral to forest regeneration, they may be more widely applicable in other tropical forest communities. If so, the interaction between life-history and herbivore impact across light gradients may play a substantial role in tree species coexistence.     

Reciprocal feeding facilitation between above- and below-ground herbivores

Interspecific interactions between insect herbivores predominantly involve asymmetric competition. By contrast, facilitation, whereby herbivory by one insect benefits another via induced plant susceptibility, is uncommon. Positive reciprocal interactions between insect herbivores are even rarer. Here, we reveal a novel case of reciprocal feeding facilitation between above-ground aphids (Amphorophora idaei) and root-feeding vine weevil larvae (Otiorhynchus sulcatus), attacking red raspberry (Rubus idaeus). Using two raspberry cultivars with varying resistance to these herbivores, we further demonstrate that feeding facilitation occurred regardless of host plant resistance. This positive reciprocal interaction operates via an, as yet, unreported mechanism. Specifically, the aphid induces compensatory growth, possibly as a prelude to greater resistance/tolerance, whereas the root herbivore causes the plant to abandon this strategy. Both herbivores may ultimately benefit from this facilitative interaction.     

Sequential effects of root and foliar herbivory on aboveground and belowground induced plant defense responses and insect performance

Plants are often simultaneously or sequentially attacked by multiple herbivores and changes in host plants induced by one herbivore can influence the performance of other herbivores. We examined how sequential feeding on the plant Plantago lanceolata by the aboveground herbivore Spodoptera exigua and the belowground herbivore Agriotes lineatus influences plant defense and the performance of both insects. Belowground herbivory caused a reduction in the food consumption by the aboveground herbivore independent of whether it was initiated before, at the same time, or after that of the aboveground herbivore. By contrast, aboveground herbivory did not significantly affect belowground herbivore performance, but significantly reduced the performance of later arriving aboveground conspecifics. Interestingly, belowground herbivores negated negative effects of aboveground herbivores on consumption efficiency of their later arriving conspecifics, but only if the belowground herbivores were introduced simultaneously with the early arriving aboveground herbivores. Aboveground–belowground interactions could only partly be explained by induced changes in an important class of defense compounds, iridoid glycosides (IGs). Belowground herbivory caused a reduction in IGs in roots without affecting shoot levels, while aboveground herbivory increased IG levels in roots in the short term (4 days) but only in the shoots in the longer term (17 days). We conclude that the sequence of aboveground and belowground herbivory is important in interactions between aboveground and belowground herbivores and that knowledge on the timing of exposure is essential to predict outcomes of aboveground–belowground interactions.     

Phenological synchrony affects interaction strength of an exotic weevil with Platte thistle,a native host plant

Phenological synchrony of a consumer population with its resource populations is expected to affect interaction intensity. We quantified phenological variation and synchrony of populations of an invasive Eurasian flower head weevil, Rhinocyllus conicus, that consumes florets, ovules, and seeds of developing flower heads of a native North American thistle, Cirsium canescens, in Sand Hills prairie in Nebraska, USA. Variation in timing of adult activity among weevil populations was larger than variation in timing of flower head development among C. canescens populations, and it drove the observed variation in the phenological synchrony between weevil and host plant populations. Furthermore, the degree of phenological synchrony between populations was significant in explaining variation in weevil egg load on the newly acquired host plant. Because population growth of C. canescens is limited by predispersal seed losses to floral herbivores, variation in the synchrony of herbivore and plant flowering will affect the density of the plant population. These results provide strong quantitative support for the hypothesis that the synchrony of insect activity with plant resources can determine the magnitude of impact of floral herbivores on their host plant populations.     

The impact of weed diversity on insect population dynamics and crop yield in collards, Brassica oleraceae (Brassicaceae)

Vegetational diversity within agricultural fields is often suggested as a means to reduce insect herbivore populations and to increase their natural enemies. In this paper we compare population densities of herbivores, predators, and parasitoids on collards in monocultures and on collards interplanted with two different groups of weeds, one with weed species from the same plant family as the collards (Brassicaceae) and one with weed species from unrelated plant families (non-Brassicaceae). The collards in the Brassicaceae weed polyculture had higher densities (number of herbivores/mean leaf area (cm²) per plant) of specialist herbivores than collards in the non-Brassicaceae weed polyculture and in collard monoculture. The “resource concentration” hypothesis is supported by the observation of higher populations of Phyllotreta spp., acting as facultative polyphages, in the Brassicaceae weed polyculture than in the non-Brassicaceae weed polyculture where Phyllotreta spp. are facultative monophages. Population densities of natural enemies (mostly coccinellids, carabids, and staphylinids) were higher in the polycultures than in the monoculture: carabid and staphylinid predators may be responsible for larval mortality in the imported cabbage worm, Pieris␣rapae, and in the diamondback larvae, Plutella xylostella. In spite of differences in densities of specialist herbivores across treatments, crop yield, leaf area (cm²), the proportion of leaf area damaged, and the number of leaves undamaged did not differ. These findings suggest that plant competition may interfere with attempts to reduce herbivore damage. We conclude that the use of weedy cultures can provide effective means of reducing herbivores if the crop and weed species are not related and plant competition is prevented. Received: 25 December 1995 / Accepted: 24 February 1997