Effects of three different cultivars of cruciferous plants on the age‐stage,two‐sex life table traits of Plutella xylostella (L.) (Lepidoptera: Plutellidae)

Plutella xylostella is an important pest of cruciferous crops worldwide. However, information regarding the age‐stage, two‐sex life parameters of P. xylostella, which is vital for designing more effective control methods, is currently lacking. The present study reports age‐stage, two‐sex life table parameters for P. xylostella on napa cabbage (Brassica oleracea var. napa), white cabbage (B. oleracea var. capitata), and cauliflower (B. oleracea var. botrytis) under laboratory conditions at 25 ± 2°C, 50–60% relative humidity, and a 16‐h light : 8‐h dark photoperiod. The time for development from an egg to a male or female adult P. xylostella on white cabbage (mean [± SE] 41.15 ± 0.54 and 39.50 ± 0.54 days, respectively) was significantly longer than that on cauliflower and napa cabbage. Furthermore, P. xylostella fecundity on cauliflower (261.90 ± 4.53 eggs female) was significantly highest than on napa cabbage and white cabbage. Intrinsic rate of increase (r) and finite rate of increase (λ) were highest on cauliflower 0.182 day^?1 and 1.199 day^?1 respectively as comparison to napa cabbage and white cabbage. The highest gross reproductive rate (GRR) and net reproductive rates (R₀) of P. xylostella 65.87 and 52.58 respectively on cauliflower then those of other hosts. The findings of the present study indicate that cauliflower is the most suitable cultivar (host) for the development of P. xylostella. Based on these findings, crops like cauliflower can be used as trap crops when napa cabbage and white cabbage are the main crops.     

Relative influences of plant type and parasitoid initial density on host–parasitoid relationships in a tritrophic system

To explore the effects of bottom-up and top-down forces on the relationships between a host, Plutella xylostella (L.) (Lepidoptera, Plutellidae), and its parasitoid, Cotesia vestalis (Haliday) (Hymenoptera, Braconidae), a short-term field experiment was established as a factorial experiment using three different host plants (Brassica pekinensis cv. Yuki F1, Brassica oleracea var. capitata cv. Midorimaru F1 and B. oleracea var. botrytis cv. Snow Crown) in the presence of C. vestalis at two different levels (low and high initial release). The tritrophic interactions were monitored by census counts of live adults 20?days after parasitoid release. The mean numbers of P. xylostella and C. vestalis adults were compared using log-linear analysis of deviance. Also, differences in the levels of parasitism were analysed using logistic analysis of deviance. There was a significant effect of host plant type on the abundance of P. xylostella, the abundance of C. vestalis and the percentage parasitism of P. xylostella by C. vestalis. The mean number of P. xylostella adults per cage on common cabbage or cauliflower was significantly greater than that on Chinese cabbage. The mean number of C. vestalis adults and the proportion of hosts attacked by C. vestalis per cage were significantly greater on Chinese cabbage compared with common cabbage or cauliflower. Indeed, initial parasitoid release did not significantly affect the abundance of P. xylostella but there was a significant influence of initial parasitoid release on the abundance of C. vestalis and the levels of parasitism of P. xylostella by C. vestalis. The mean number of C. vestalis adults and the proportion of P. xylostella parasitised by C. vestalis per cage were greater in high level of parasitoid release compared with low level of parasitoid release. However, there were no significant interacting effect of the factors (plant type?×?parasitoid initial abundance) on the abundance of P. xylostella, the population size of C. vestalis and parasitism of P. xylostella by C. vestalis.     

Characteristic of parasitism of diamondback moth by two larval parasites

Laboratory and greenhouse studies were conducted to investigate the suitability of 2 hymenopterous parasites,Diadegma eucerophaga Horstmann andApanteles plutellae Kurdjumov for introduction to control diamondback moth (DBM),Plutella xylostella (L.), a destructive pest of crucifers in tropical to subtropical Southeast Asia. Parasitism byD. eucerophaga was high at temperature range of 15°C to 25°C and that ofA. plutellae, at 20°C to 35°C. Both parasites were active in searching for host and oviposited only during photophase. No parasitism was observed during darkness. WhereasA. plutellae could parasitize all instars of DBM larvae,D. eucerophaga parasitized only the first 3 instars and failed to parasitize the 4^th. Parasitism byD. eucerophaga was greater when DBM larvae were feeding on common cabbage (Brassica oleracea var.capitata L.), than on cauliflower (B. oleracea var.italica L.), broccoli (B. oleracea var.botrytis L.) or Chinese cabbage [Brassica campestris L. ssp.pekinensis (Lour) Olsson].A. plutellae parasitism was greater when DBM larvac were feeding on Chinese cabbage than on common cabbage, cauliflower or broccoli. Storage of pupae at 0°C and 4°C to 6°C for up to 2 weeks reduced emergence ofD. eucerophaga adults more than that ofA. plutellae. A non-selective insecticide, deltamethrin, was toxic to adults of both parasites but selective ones such asBacillus thuringiensis, teflubenzuron, and pirimicarb were not. Pupae were more tolerant than adults to insecticides. The insecticide-resistant Luchu strain and susceptible laboratory strain of DBM suffered an equal level of parasitism by both parasites.      

Impact of a glossy collard trap crop on diamondback moth adult movement, oviposition, and larval survival

One component of developing a systematic approach for deployment of trap crops is to understand how the trap crop modifies pest behavior. Glossy‐leafed collards, Brassica oleracea L. var. acephala (Brassicaceae), were evaluated as a potential trap crop for diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), because they are attractive to P. xylostella adults and are a poor host for P. xylostella larvae compared to cabbage, Brassica oleracea L. var. capitata. We used large field plots to measure the changes in adult, egg, and larval P. xylostella densities in cabbage when the trap crop was planted in the field. Furthermore, we planted the trap crop in dispersed and concentrated spatial arrangements to determine the impact of trap crop arrangement on the behavior of P. xylostella. In 2002, results showed that the presence of collards within a cabbage field reduced larval density on cabbage. In 2003, neither trap crop arrangement had a significant impact on P. xylostella larval density on cabbage. Adult moths aggregated in proximity to collards in 2002, but not in 2003. Egg and larval data in both years in all treatments showed that total oviposition was highest near a central release point, indicating that females lay many eggs before dispersing very far when suitable host plants are available. The mean direction of P. xylostella movement and oviposition from a central release point was not consistent or correlated to wind direction. Plant size of the trap crop in relation to the main crop and environmental factors may have been responsible for the inconsistent effectiveness of the trap crop.     

Interactions of Bacillus thuringiensis bioinsecticides and the predatory stink bug Podisus nigrispinus to control Plutella xylostella

The diamondback moth (DBM), Plutella xylostella (L.), is a major pest of brassica crops worldwide. Control of this pest is difficult because it rapidly develops resistance to synthetic and biological insecticides and because of the effects of insecticides on its natural enemies. Podisus nigrispinus (Dallas) is a predator that feeds on its prey, as well as on the host plants of its prey, and is an important biological control agent of DBMs. The aim of this study was to determine the susceptibility of P. xylostella larvae to two bioinsecticides: the HD1 strain of Bacillus thuringiensis (B. thuringiensis var. kurstaki) and the commercial product Agree^® (B. thuringiensis var. aizawai CG 91). In addition, the impact of these bioinsecticides on the P. nigrispinus consumption of DBM larvae and phytophagy was evaluated. Both the HD1 strain and Agree^® caused 100% mortality in P. xylostella larvae. P. nigrispinus nymphs fed only with kale leaves (Brassica oleracea var. acephala) sprayed with water, the HD1 strain, or Agree^® did not complete their nymphal development. When prey was also available, P. nigiripinus fed on kale leaves to obtain water. Both nymphs and adults of P. nigrispinus consumed greater numbers of DBM larvae, and fed less on kale leaves, when sprayed with the HD1 strain or Agree^®. These results suggest a positive interaction of B. thuringiensis‐based products and the predator P. nigrispinus in the control of P. xylostella larvae.     

Diamondback moth oviposition: effects of host plant and herbivory

The oviposition behaviour of Plutella xylostella L. (Lepidoptera: Plutellidae) on Chinese cabbage (Brassica rapa L. Pekinensis, cv. Wombok), canola (Brassica napus L. cv. Thunder TT), and cabbage (Brassica oleracea L. Capitata, cv. sugarloaf) (Brassicaceae) was studied in the laboratory. In no‐choice experiments moths laid most eggs on the stems and lower three leaves of cabbage plants, the lower three leaves of canola plants, but on the upper three leaves of Chinese cabbage plants. The effects of conspecific herbivore damage to foliage could be replicated by mechanical damage. When foliage was damaged, injured cabbage and canola plants were preferred for oviposition over intact conspecifics, whereas injured Chinese cabbage plants were less preferred than intact conspecifics. However, when root tissue was damaged, intact cabbage and canola plants were preferred over injured conspecifics, whereas moths did not discriminate between root‐damaged and intact Chinese cabbage plants. Injury to upper leaves significantly affected the intra‐plant distribution of eggs. In cabbage and canola plants, injury to leaf 6 significantly increased the number of eggs laid on this leaf, resulting in a significant decrease in the number of eggs laid on the lower foliage/stem of plants, whereas in Chinese cabbage plants it significantly decreased the number of eggs laid on leaf 6. Following oviposition on intact plants, neonate larvae established the vast majority of feeding sites on leaves 5–8 in all three host plants, indicating that larvae moved a considerable distance from preferred oviposition sites in cabbage and canola plants. The growth rate of neonates fed on leaf‐6 tissue was significantly greater than that of those fed on leaf‐1 tissue; >90% of larvae completed development when fed exclusively on leaf‐6 tissue but no larvae completed development when fed exclusively on leaf‐1 tissue. The study demonstrates the complex and unpredictable interactions between P. xylostella and its host plants and provides a basis from which we can begin to understand observed distributions of the pest in Brassica crops.     

The performance of Oomyzus sokolowskii (Hymenoptera: Eulophidae) parasitizing Plutella xylostella (Lepidoptera: Plutellidae) on different host plants

Oomyzus sokolowskii Kurdjumov is a larval endoparasitoid of the diamondback moth, Plutella xylostella L., which is one of the most destructive pests of cruciferous vegetables worldwide. The performance of O. sokolowskii parasitizing P. xylostella on cauliflower (Brassica oleracea var. botrytis), Chinese cabbage (Brassica pekinensis), pakchoi (Brassica chinensis), cabbage (Brassica oleracea var. capitata), and broccoli (Brassica oleracea var. italica) was determined under laboratory conditions. Our results showed that O. sokolowskii performed best on cauliflower compared with the others, with the greatest net reproductive rate (R _o = 34.99), intrinsic rate of increase (r _m = 0.196), offspring emerged/host larva (9.0), percentage of parasitized host larvae with adults emerged (67.6 %), adults produced (43.6/female), and the shortest developmental time from egg to adult (15.8 days). Chinese cabbage-, pakchoi-, cabbage-, and cauliflower-adapted O. sokolowskii performed the best in percentage of parasitized host larvae with adults emerged, offspring adults produced per female, and offspring produced per host larva when parasitizing fourth instar P. xylostella larvae on cauliflower. The percentage of host-adapted female O. sokolowskii parasitizing different host-adapted fourth instar P. xylostella larvae was similar. Based on our results, we suggest that O. sokolowskii had performed best on cauliflower, which might be the best host plant to rear O. sokolowskii.     

Mediation of host selection and oviposition behavior in the diamondback moth Plutella xylostella and its predator Chrysoperla carnea by chemical cues from cole crops

Host plant-mediated orientation and oviposition by diamondback moth (DBM) Plutella xylostella (L.) (Lepidoptera: Yponomeutidae) and its predator Chrysoperla carnea Stephens (Neuroptera: Chrysopidae) were studied in response to four different brassica host plants: cabbage, (Brassica oleracea L. subsp. capitata), cauliflower (B. oleracea L. subsp. botrytis), kohlrabi (B. oleracea L. subsp. gongylodes), and broccoli (B. oleracea L. subsp. italica). Results from laboratory wind tunnel studies indicated that orientation of female DBM and C. carnea females towards cabbage and cauliflower was significantly greater than towards either broccoli or kohlrabi plants. However, DBM and C. carnea males did not orient towards any of the host plants. In no-choice tests, oviposition by DBM did not differ significantly among the test plants, while C. carnea layed significantly more eggs on cabbage, cauliflower, and broccoli than on kohlrabi. However, in free-choice tests, oviposition by DBM was significantly greater on cabbage, followed by cauliflower, broccoli, and kohlrabi, while C. carnea preferred to oviposit on cabbage and cauliflower, followed by broccoli and kohlrabi. The predation rates of DBM by C. carnea on kohlrabi and broccoli were not significantly different from one another, but were significantly higher than that on cabbage and cauliflower. When two types of plant, intact and injured, were available to adult DBM, female oviposition was significantly greater on injured plant leaves than on intact plants leaves. Similarly, C. carnea oviposition was significantly greater on injured plant leaves than on intact leaves. Implications regarding the possible role of green leaf volatiles in host selection/preference, as well as in tritropic interactions, are discussed.     

Pythium root rot of crucifers

A destructive root disease of cabbage (Brassica oleracea L. var. capitata) and cauliflower (Brassica oleracea L. var. botrytis) incited by a species of Pythium Pringsheim is described as occurring in Varanasi, U.P. The pathogen was isolated on potato dextrose and corn meal agar. Pathogenicity and host range of the disease were studied. Cultural characters, morphology and developmental stages and life cycle of the fungus indicated its identity with Pythium middletonii Sparrow.     

Interactions between the diamondback moth, Plutella xylostella L. and glasshouse and outdoor-grown cabbage cultivars

Interactions between Brassica cultivars (cvs) and adult and larval stages of Plutella xylostella L. were examined. In six-way choice oviposition experiments, glasshouse-grown Chinese cabbage cv. Tip Top was preferred to five outdoor-grown temperate cabbage cultivars but was similar to four out of five tropical cultivars. Bionomic studies with neonate larvae on temperate cabbage cultivars showed that mean larval duration was longer and percentage survival less on older compared with younger plants but that plant age or cultivar had little effect on pupal weight. The fecundity of P. xylostella reared on outdoor-grown cabbage cultivars varied ten-fold between the poorest and the best cultivar but plant age had little effect on P. xylostella reared on Chinese cabbage. In a glasshouse experiment, survival of P. xylostella on Chinese cabbage was significantly greater compared with field-grown glossy-leaved, normal bloom green cabbage and red cabbage. In bioassays with neonate larvae, when the leaves were placed vertically instead of horizontally a significant level of intrinsic resistance was revealed for glasshouse and field-grown glossy-leaved cabbage but not for Chinese cabbage. Observed differences in host status to P. xylostella are discussed in relation to the potential for partial plant resistance in control programmes.     

Diamondback moth egg susceptibility to rainfall: effects of host plant and oviposition behavior

Oviposition patterns of the diamondback moth (DBM), Plutella xylostella L. (Lepidoptera: Plutellidae), differ between common cabbage (Brassica oleracea L. var. capitata) and Chinese cabbage (Brassica rapa L. var. pekinensis) (Brassicaceae) host plants. This study shows that the moth prefers to oviposit on adaxial rather than abaxial leaf surfaces and petioles of both host plants. More eggs were laid in leaf veins than on leaf laminas of both host plants, especially in Chinese cabbage, where 94.6% of eggs were laid in veins. On Chinese cabbage, very few eggs were laid in clusters (≥2 eggs), whereas on common cabbage approximately 30% of eggs were laid in groups of 2 or more eggs. Removal of wax from common cabbage leaves dramatically increased the number of eggs laid singly on the leaf lamina of treated plants, suggesting that leaf waxes affect how eggs are distributed by ovipositing DBM. Eggs were most susceptible to removal by rainfall from the plant surface immediately (<1 h) after oviposition and when close to hatching (>72h old) whereas they were least susceptible 24 h after oviposition. Eggs laid on common cabbage plants were more susceptible to simulated rainfall than eggs laid on Chinese cabbage plants. On common cabbage plants, egg susceptibility to rainfall on different plant parts ranked adaxial leaf surfaces>petioles = abaxial leaf surfaces>stem, but there was no difference in egg susceptibility to rainfall on the various plant parts of Chinese cabbage. Furthermore, on common cabbage plants, eggs laid on both adaxial and abaxial leaf surfaces were afforded significant protection from the effects of rainfall by leaves higher in the plant canopy. On common cabbage plants, oviposition patterns reduce the potential impact of rainfall on eggs, possibly reducing the effect of this important abiotic mortality factor in the field.     

Predators mediate host plant resistance to a phytophagous pest in cabbage with glossy leaf wax

This study examined the role of generalist predators in producing higher mortality ofPlutella xylostella L. (Plutellidae) larvae on glossy vs. normal-wax cabbage,Brassica oleracea var.capitata L. To test this, survival and feeding ofP. xylostella were measured on individually caged glossy and normal-wax plants with and without each of three generalist predators,Chrysoperla carnea (Stephens) (Chrysopidae),Orius insidiosus (Say) (Anthocoridae), andHippodamia convergens Guerin-Meneville (Coccinellidae). In the greenhouse, predators always significantly reduced survival ofP. xylostella larvae on glossy plants, but never on normal-wax plants. In the field, predators significantly reducedP. xylostella survival on glossy plants, but onlyC. carnea was effective on normal-wax plants. In similar experiments with excised leaves,O. insidiosus andC. carnea were more effective predators on the glossy leaves, whileH. convergens was equally effective on both kinds of leaves. Patterns for feeding were similar, but significance levels differed from those forP. xylostella survival. The greater effectiveness of predators on glossy plants is apparently due to the reported improved mobility of these animals on glossy leaf surfaces. The data also suggest that reduced mining byP. xylostella exposes the larvae to more predation on glossy plants and contributes some to the resistance. Regardless of the mechanism, resistance toP. xylostella on glossyB. oleracea appears to depend on the action of generalist predators for its full expression. This dependence on predation must be considered in the development and deployment of glossy insect-resistantB. oleracea.     

Temporal changes in olfactory and oviposition responses of the diamondback moth to herbivore‐induced host plants

Temporal changes in the pre‐ and post‐alighting responses of mated female diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), to two species of Brassica (Brassicaceae) host plants induced by larval feeding were studied using olfactometer and oviposition assays. Females displayed strong olfactory and oviposition preferences for herbivore‐induced common cabbage (Brassica oleracea var. capitata L. cv. sugarloaf) plants over intact plants; these preferences decreased with time and disappeared by the 7th day after induction. In herbivore‐induced common cabbage plants, eggs were clustered near feeding damage on the younger leaves (leaves 5–7), whereas in intact plants, eggs were clustered on the stem and lower leaves (leaves 1–4) . However, as the time interval between larval feeding and oviposition increased, more eggs were laid on the lower leaves of induced plants. This demonstrates a change in egg distribution from the pattern associated with induced plants to that associated with intact plants. In contrast, females displayed strong olfactory and oviposition preferences for intact Chinese cabbage [Brassica rapa ssp. pekinensis (Lour.) Hanelt cv. Wombok] plants over induced plants; these preferences decreased with time and disappeared by the 5th day after induction. More eggs were laid on the upper leaves (leaves 4–6) than on the lower leaves (leaves 1–3) of intact Chinese cabbage plants at first, but the distribution changed over time until there were no significant differences in the egg count between upper and lower leaves by the 4th day post induction. For both host plant species, pre‐alighting responses of moths were reliable indicators of post‐alighting responses on the first 2 days post induction. The results suggest that temporal changes in a plant's profile (chemical or otherwise) following herbivory may influence attractiveness to an insect herbivore and be accompanied by changes in olfactory and oviposition preferences.     

Effect of temperature on life table parameters of Plutella xylostella (Lepidoptera: Plutellidae) on two brassicaceous host plants

Life table parameters of diamondback moth, Plutella xylostella (L.), were studied at seven constant temperatures (10, 15, 20, 25, 28, 30, and 35 °C) on two brassicaceous host plants, cauliflower (Brassica oleracea var. botrytis) and cabbage (Brassica oleracea var. capitata). Survival, longevity and reproduction were examined and used to construct a life table. The survival at immature stages varied from 53.0 to 84.1% on cauliflower and from 58.3 to 86.2% on cabbage at 10–30 °C. P. xylostella did not survive at 35 °C. The female adult longevity ranged from 12.9 days at 30 °C to 30.4 days at 10 °C on cauliflower and 9.7 days at 30 °C to 40.0 days at 15 °C on cabbage. The net reproductive rate (R₀) increased with increasing temperature, while generation time (T) decreased. This caused the intrinsic rate of increase (r_m) to increase from 0.038 to 0.340 on cauliflower and 0.033 to 0.315 on cabbage from 10 to 28 °C. The significant decrease in R₀ caused a decrease in r_m at 30 °C. The r_m values on cauliflower were significantly higher than cabbage at 15, 20, 28 and 30 °C.     

Herbivore-Specific,Density-Dependent Induction of Plant Volatiles: Honest or “Cry Wolf” Signals?

Plants release volatile chemicals upon attack by herbivorous arthropods. They do so commonly in a dose-dependent manner: the more herbivores, the more volatiles released. The volatiles attract predatory arthropods and the amount determines the probability of predator response. We show that seedlings of a cabbage variety (Brassica oleracea var. capitata, cv Shikidori) also show such a response to the density of cabbage white (Pieris rapae) larvae and attract more (naive) parasitoids (Cotesia glomerata) when there are more herbivores on the plant. However, when attacked by diamondback moth (Plutella xylostella) larvae, seedlings of the same variety (cv Shikidori) release volatiles, the total amount of which is high and constant and thus independent of caterpillar density, and naive parasitoids (Cotesia vestalis) of diamondback moth larvae fail to discriminate herbivore-rich from herbivore-poor plants. In contrast, seedlings of another cabbage variety of B. oleracea (var. acephala: kale) respond in a dose-dependent manner to the density of diamondback moth larvae and attract more parasitoids when there are more herbivores. Assuming these responses of the cabbage cultivars reflect behaviour of at least some genotypes of wild plants, we provide arguments why the behaviour of kale (B. oleracea var acephala) is best interpreted as an honest signaling strategy and that of cabbage cv Shikidori (B. oleracea var capitata) as a “cry wolf” signaling strategy, implying a conflict of interest between the plant and the enemies of its herbivores: the plant profits from being visited by the herbivore''s enemies, but the latter would be better off by visiting other plants with more herbivores. If so, evolutionary theory on alarm signaling predicts consequences of major interest to students of plant protection, tritrophic systems and communication alike.     

Contrary effects of jasmonate treatment of two closely related plant species on attraction of and oviposition by a specialist herbivore

Elevated jasmonic acid (JA) concentrations in response to herbivory can induce wounded plants to produce defences against herbivores. In laboratory and field experiments we compared the effects of exogenous JA treatment to two closely related cabbage species on the host‐searching and oviposition preference of the diamondback moth (DBM), Plutella xylostella. JA‐treated Chinese cabbage (Brassica campestris) was less attractive than untreated Chinese cabbage to ovipositing DBM, while JA‐treatment of common cabbage (B. oleracea) made plants more attractive than untreated controls for oviposition by this insect. Similar effects were observed when plants of the two species were damaged by DBM larvae. In the absence of insect‐feeding, or JA application, Chinese cabbage is much more attractive to DBM than common cabbage. Inducible resistance therefore appears to occur in a more susceptible plant and induced susceptibility appears to occur in a more resistant plant, suggesting a possible balance mechanism between constitutive and inducible defences to a specialist herbivore.     

A behavioural study to help clarify how undersowing with clover affects host-plant selection by pest insects of brassica crops

Laboratory and field-cage tests were done to determine how undersowing brassica plants (Brassica oleraceae L. and B. rapa L.) (Cruciferae) with subterranean clover (Trifolium subterraneum L.) (Papilionaceae) affected host-plant selection by eight pest insect species of brassica crops. The pest species tested were Pieris rapae (Lepidoptera: Pieridae) (the small white butterfly), Pieris brassicae (Lepidoptera: Pieridae) (the large white butterfly), Delia radicum (Diptera: Anthomyiidae) (the cabbage root fly), Phaedon cochleariae (Coleoptera: Chrysomelidae) (the mustard beetle), Plutella xylostella (Lepidoptera: Yponomeutidae) (the diamond-back moth), Evergestis forficalis (Lepidoptera: Pyralidae) (the garden-pebble moth), Mamestra brassicae (Lepidoptera: Noctuidae) (the cabbage moth) and Brevicoryne brassicae (Hemiptera: Aphididae) (the cabbage aphid). In all tests, except two in which the brassica plants were about three times as high as the clover background, 39%–100% fewer of the pest insect stage monitored were found on host plants presented in clover than on those presented in bare soil. Contrary to claims supporting the ‘enemies hypothesis’, differences in colonization alone appeared sufficient to account for the lower numbers of insects found when host plants are undersown with clover. To be effective in reducing plant colonization, the clover must cover 50%, and preferably more, of the vertical profile of the crop plants. As clover used as an undersown crop often has to be cut to make it less competitive with the main brassica crop, temporal aspects of the condition of the clover during critical periods of pest activity need to be recorded carefully before concluding that undersowing does not produce the effect desired against certain pest species under field conditions. The effective clover barrier is like any other treatment, if it is not present at the appropriate time it cannot be expected to reduce pest insect numbers.     

Infochemically mediated tritrophic interaction webs on cabbage plants

In response to damage by herbivores, plants are known to emit infochemicals that enhance the effectiveness of insect parasitoids. Studies on plant–parasitoid interactions mediated by such infochemicals have focused on the tritrophic systems in which plants are infested by a single herbivore species. In natural ecosystems, however, plants are often simultaneously infested by several herbivorous species. The present study focuses on two herbivorous species that simultaneously attack crucifer plants and their respective parasitic wasps. We first show the specific responses of the two specialist parasitic wasps [Cotesia plutellae and C. glomerata (Hymenoptera: Braconidae)] to infochemicals originating from cabbage plants (Brassica oleracea cv. Sikidori) infested by each of their respective host larvae [Plutella xylostella (Lepidoptera: Yponomeutidae) and Pieris rapae (Lepidoptera: Pieridae)]. We then coupled the two tritrophic systems on the same cabbage plants. These experiments demonstrated the presence of indirect interactions between the two species of herbivores. Overall, the results indicate the presence of infochemically mediated tritrophic interaction webs on a single plant. Received: September 1, 2000 / Accepted: February 8, 2001     

Resource concentration hypothesis: effect of host plant patch size on density of herbivorous insects

The resource concentration hypothesis (Root 1973) predicts that specialist herbivorous insects should be more abundant in large patches of host plants, because the insects are more likely to find and stay longer in those patches. Between August 1989 and January 1990 we experimentally tested Root's hypothesis by analyzing the numerical response of four species of herbivorous insects associated with patches of 4, 16, 64 and 225 cabbage plants, Brassica oleracea var. capitata. In addition, we studied the colonization of patches by adults of Plutella xylostella (L.) (Lepidoptera: Plutellidae), and the migration of their larvae in patches of different sizes. No herbivorous insect densities differed significantly with patch size. Adults of P. xylostella colonized all kind of patches equally. Larvae did not migrate between patches, and their disappearance rate did not differ between patches. The resource concentration hypothesis is organism-dependent, being a function of the adult and juvenile herbivore dispersal behavior in relation to the spatial scale of patchiness.