Mobility of three generalist predators is greater on cabbage with glossy leaf wax than on cabbage with a wax bloom

Adult coccinellids Hippodamia convergens Guerrin-Menneville, adult minute pirate bugs Orius insidiosus (Say), and larval lacewings Chrysoperla carnea (Stephens) have been reported to more effectively reduce populations of Plutella xylostella (L.) (Plutellidae: Lepidoptera) larvae on a cabbage with glossy surface waxes than on a standard cabbage variety. To examine the mechanisms, the behavior of each predator species was quantified on the two cabbage types. All three predators spent less time walking on the standard variety and more time in other activities, including grooming and scrambling (ineffective forward locomotion). In addition, walking by each predator was distributed more evenly among the parts of the leaf on the glossy cabbage than on the standard variety. In small enclosures, C. carnea and O. insidiosus found and attacked individual first instar P. xylostella more quickly on the glossy cabbage. Scanning electron micrographs showed debris, probably wax, on the tarsae of C. carnea and O. insidiosus that had walked on the standard variety, but not on those that had walked on glossy cabbage. H. convergens tarsae did not accumulate debris on either cabbage type. Predator mobility and effectiveness were apparently impeded by wax crystals, which are present only on the standard variety. This mechanism can explain the greater effectiveness of these generalist predators against P. xylostella larvae on glossy vs normal-wax cabbage.     

Survival and behavior of Plutella xylostella larvae on cabbages with leaf waxes altered by treatment with S-ethyl dipropylthiocarbamate

Cabbages (Brassica oleracea L.) treated with S-ethyl dipropylthiocarbamate (EPTC) herbicide had reduced amounts of leaf surface waxes (40.6% of controls) and reduced densities of leaf surface wax crystallites (20.8% of controls). Leaf waxes of EPTC-treated plants chemically and morphologically resembled leaf waxes of genetically glossy cabbages resistant to the diamondback moth Plutella xylostella (L.) (Lepidoptera: Plutellidae). Survival of larvae was significantly reduced on EPTC-treated cabbage plants in three out of four experiments (62.0–15.3% of survival on controls). P. xylostella neonates also moved more rapidly on EPTC-treated plants than on untreated controls (1.84±0.16 cm/min on controls vs. 3.94±0.24 cm/min on treated plants; P=0.0001). These results support the hypotheses that reduction in leaf waxes is the basis of resistance to P. xylostella in genetically glossy plants and that reduced acceptance by larvae is associated with this resistance. Modification of leaf surface waxes with EPTC or similar compounds may have potential as an economic control for P. xylostella in Brassica crops.     

Predation, behavior, and attachment by Chrysoperla plorabunda larvae on Brassica oleracea with different surface waxblooms

Predation by Chrysoperla plorabunda (Fitch) (Chrysopidae) first instars on Plutella xylostella (L.) (Plutellidae) neonates was measured on nine Brassica oleracea L. types with different surface wax crystal morphology. Forty-eight-hour survival of P. xylostella caged on leaves was significantly reduced by C. plorabunda on plants expressing glossy mutations that reduce surface waxbloom, but not on plants with normal waxbloom. During 5-min observations, C. plorabunda spent more time walking and less time scrambling (moving the legs with no locomotion) on glossy types than on the normal-wax types. Adhesive force produced by C. plorabunda on plant surfaces was 20 to 200-fold greater on glossy types than on normal-wax types. All the glossy types provided an advantage to C. plorabunda over normal-wax types by increasing the force of attachment to the leaf suface by the larvae, which in turn increased time allocated to walking, leading to greater predation of P. xylostella larvae. Among glossy and normal wax types together, attachment force and time allocated to walking were significantly correlated with predation by C. plorabunda. Within glossy or normal-wax types, however, these correlations were not significant. Neither time allocated to walking, nor attachment to the leaf surface was a predictor of predation by C. plorabunda within glossy or normal-wax types. Although diverse mechanisms therefore contribute to differences in predation, the results show that reduction in waxbloom can substantially affect the mobility and effectiveness of this generalist predator.     

Managing lepidopteran pests in cabbage with herbicide-induced resistance, in combination with a pyrethroid insecticide

S-ethyldipropylthiocarbamate (EPTC) applied as a soil treatment or over-the-top spray on cabbage plants (Brassica oleracea L.) caused the leaves to turn ‘glossy’ for as long as 30 days. EPTC-induced glossy plants were damaged significantly less than untreated plants by diamondback moth,Plutella xylostella (L.), imported cabbage worm,Pieris rapae (L.), and cabbage looper,Trichoplusia ni (Hbn.). Reductions in damage were equivalent to those obtained from treatment with permethrin. When used in combination with permethrin, EPTC provided additive control of damage by these pests. Our calculations show EPTC-induced resistance to be cost-effective. This use of EPTC has several limitations, however. Younger plants (<9 leaves) were killed or injured by the herbicide. The growth of older plants was not affected, but plants did not become glossy for ca. 10 days after they were treated with EPTC. The crop must be protected with insecticides until the plants are mature enough to treat with EPTC, and until treated plants become glossy. In addition, since the glossy trait is only effective against first instar larvae, populations of later instars on glossy plants must be reduced with an application of insecticide. Finally, EPTC formulations are water-soluble and can be washed away from the plants by heavy rains and irrigation, which may make this use of EPTC impractical in some situations. Where its use is practical, and the indicated precautions are taken, EPTC-induced resistance could reduce dependence on chemical insecticides and reduce selection for insecticide resistance in diamondback moth.     

Evaluation of factors influencing augmentative releases of Chrysoperla carnea for control of Scirtothrips perseae in California avocado orchards

Weekly releases of Chrysoperla carnea for control of Scirtothrips perseae were evaluated in replicated field plots in two commercial avocado orchards in southern California, USA. Two release techniques and rates commonly employed by commercial pest control advisors who routinely use this generalist predator for S. perseae control were assessed. Release technique one utilized C. carnea eggs glued to paper squares that were stapled to leaves of experimental trees at a rate of 41,000 eggs per ha. Release technique two used a motorized backpack sprayer to apply a dry mixture of lacewing eggs and larvae to trees at a rate of 514,501 per ha. Pest populations were monitored by making bi-weekly population counts of S. perseae larvae and adults on leaves, and adult densities were simultaneously monitored in each experimental plot with yellow sticky cards. In the laboratory, degree-day accumulation until death of immature C. carnea was determined at temperatures representative of field conditions when predators were provisioned with varying amounts of food or different food types. Preference for S. perseae instars by first, second, and third instar C. carnea was assessed in the laboratory, and intraguild predation towards larvae and adult females of a co-occurring generalist predatory thrips, Franklinothrips orizabensis, was investigated along with intraspecific predation rates. Both release strategies failed to significantly reduce S. perseae populations in comparison to non-treated control plots. Approximately 35–96% of C. carnea eggs and larvae applied with the motorized sprayer landed on the ground. C. carnea larvae lived for approximately 1–2 days when provisioned with either no food, an avocado leaf or avocado pollen. Longevity was extended to 14–15 days when prey was provided. C. carnea larvae showed no preference for first or second instar S. perseae, all predator instars attacked first instar F. orizabensis, but only second and third instar C. carnea managed attacks on second instar F. orizabensis larvae. No adult female F. orizabensis were attacked and no attacks by F. orizabensis on C. carnea were recorded. Second instar C. carnea engaged in the highest levels of intraspecific predation.     

Interference and competitive behaviour of the aphid predators,Chrysoperla carnea andCoccinella septempunctata in the laboratory

Interference and competition betweenChrysoperla carnea (Stephens) andCoccinella septempunctata L. was investigated in the absence and presence of aphid prey. When larvae of similar vigour encountered each other,C. carnea larvae were superior toC. septempunctata. Otherwise the larger of 2 individuals always killed the smaller. In the absence of preyC. carnea adults were attacked by their own 2nd and 3rd instar larvae as well as by 3rd instar larvae ofC. septempunctata. In all encounters the adults ofC. septempunctata were superior. Since the eggs ofC. carnea are protected to some extent by being on egg stalks, they were less susceptible to cannibalism and predation thanC. septempunctata eggs which are deposited in batches directly on plant leaves. In the presence of prey cannibalism and predation were reduced, especially between larvae. Only eggs and 1st instar larvae were endangered. In the present experimentsC. carnea showed a slight superiority overc. septempunctata. Paper presented at the 17th International Congress of Entomology held in August 20–26 1984 in Hamburg, F.R. Germany.     

Delineating the effects of a plant trait on interactions among associated insects

Plant traits can affect ecological interactions between plants, herbivores, and predators. Our study tests whether reduced leaf wax in peas alters the interaction between the pea aphid ( Acyrthosiphon pisum), a foliar-foraging predator (a lady beetle, Hippodamia convergens) and a ground-foraging predator (a ground beetle, Poecilus scitulus). We performed a 2×2×2 factorial experiment in which wax level, presence of H. convergens, and presence of P. scitulus were manipulated. Experimental arenas consisted of a cage surrounding three pea plants. One plant in each cage was stocked with 15 pea aphids. In greenhouse and field cage experiments, we assessed the effect of each factor and their interactions on aphid density. As in previous studies, H. convergens foraged for aphids more effectively on reduced wax peas than on normal peas. Other interactions among H. convergens, P. scitulus , and A. pisum were the same on both types of peas. We consider how aphid movement, plant growth, and a high frequency of predation by P. scitulus on H. convergens influenced pea aphid density.     

Effects of wax bloom variation in Brassica oleracea on foraging by a vespid wasp

Foraging by individual Polistes dominulus (Christ) (Hymenoptera: Vespidae) was observed and compared on Brassica oleracea L. plants that differed in surface wax bloom. Twenty-six wasps, previously trained to forage on plants for 4th instar Trichoplusia ni (Walker), were presented in the greenhouse with mixtures of plants with normal wax bloom or genetically reduced wax bloom, and on which T. ni caterpillars had been placed. During foraging, the wasps were observed to slip significantly more frequently from the leaf surfaces of normal-wax bloom plants than from reduced-wax bloom plants (129 vs 63 occurrences) and to retrieve significantly fewer pieces of caterpillar after attack on normal-wax bloom plants (151 vs 223 pieces). Altogether the wasps retrieved 333 caterpillar pieces from reduced-wax bloom plants and 248 pieces from normal-wax bloom plants. Despite these differences, the number of caterpillars attacked and killed did not differ between the two wax bloom types (116 vs 121), nor did handling time for individual attacks (time from contact with prey until prey piece was carried to the nest) (170±12.5 s vs 180±10.5 s). Thus, in contrast with previous reports for smaller predators, wax bloom variation in B. oleracea did not influence the effectiveness of P. dominulus as a predator of T. ni.     

Effects of Brassica oleracea waxblooms on predation and attachment by Hippodamia convergens

Four mutations that reduce waxbloom in Brassica oleracea L. were examined for their effects on predation, mobility, and adhesion to the plant surface by the general predator Hippodamia convergens (Guérin-Menéville) (Coleoptera: Coccinellidae). The mutations reduce waxbloom to different degrees, but all produce a glossy phenotype. Plants tested were inbred lines, near isogenic lines, or segregating F₂ populations, depending on the mutation. In an experiment on caged leaves, predation of Plutella xylostella L. larvae by H. convergens adult females was significantly greater on glossy types as compared with normal-wax or wild-type counterparts. Although the trend was the same for each mutation, individual comparisons between glossy and normal-wax lines or segregants were only significant for two of them, those producing mutant alleles gl_a and gl_d. Individual H. convergens were observed to spend more time walking on leaf edges and less time walking on leaf surfaces of normal-wax plants than glossy plants. Hippodamia convergens also obtained better adhesion to the surfaces of glossy plants than to normal-wax plants when tested using a centrifugal device. Two of the mutations produced similarly strong effects on predation, behaviour, and adhesion by H. convergens. These two are the same previously determined by us to provide the strongest similar effects on another generalist predator, Chrysoperla plorabunda (Fitch). The results indicate that waxbloom variation in nature could affect herbivore populations through its effects on generalist predators.     

Parasitoid Preference for Host-Infested Plants Is Affected by the Risk of Intraguild Predation

Flowering Rorippa indicaplants are attended by ants that collect nectar and, at the same time, prey on herbivorous insects, including larvae of the diamondback moth, Plutella xylostella.Here, we showed that P. xylostellalarvae suffered higher predation on R. indicawhose flowers were accessible by ants than on plants those whose flowers were inaccessible. Ants showed equal predation preference between unparasitized and larvae parasitized by Cotesia plutellae,a dominant specialist parasitic wasp of P. xylostellalarvae. C. plutellaepreferred non-flowering, host-infested R. indicato flowering, host infested R. indica.Based on these results, we infer that the preference of C. plutellaefor non-flowering, host-infested plants is in part explained by the avoidance of intraguild predation by attending ants.     

Impact of intraguild predation by adult Harmonia axyridis (Coleoptera: Coccinellidae) on Aphis glycines (Hemiptera: Aphididae) biological control in cage studies

The soybean aphid, Aphis glycines Matsumura, has become a principal arthropod pest of soybean in the U.S. since its first detection in 2000. This species threatens soybean production through direct feeding damage and virus transmission. A diverse guild of insect predators feeds on soybean aphid in Michigan including the exotic coccinellid Harmonia axyridis, the native gall midge Aphidoletes aphidimyza and the native lacewing Chrysoperla carnea. In addition to feeding on A. glycines some members of this guild may also engage in intraguild predation. These interactions may produce positive, negative, or neutral impacts on A. glycines biological control. We explored the impact of intraguild predation on soybean aphid population dynamics by comparing aphid populations in microcosms with either A. aphidimyza larvae or C. carnea larvae alone, with both a H. axyridis adult and either A. aphidimyza or C. carnea larvae, and without predators. When H. axyridis was present with larval A. aphidimyza or C. carnea, the lady beetle acted as an intraguild predator. However, intraguild feeding did not result in a release of aphid populations compared with microcosms containing only the intraguild and aphid prey. A similar result was found in field cages. Cages allowing large predators had reduced numbers of A. aphidimyza and C. carnea larvae but also significantly fewer aphids compared with predator exclusion cages. Thus, in both lab and field studies the direct impact of H. axyridis on A. glycines overcame its negative impact as an intraguild predator. Together, these studies indicate that while the exotic H. axyridis does act as an intraguild predator and may contribute to local declines in A. aphidimyza and C. carnea, it is also currently important in overall biological control of A. glycines.     

Tri-trophic consequences of UV-B exposure: plants,herbivores and parasitoids

In this paper we demonstrate a UV-B-mediated link between host plants, herbivores and their parasitoids, using a model system consisting of a host plant Brassica oleracea, a herbivore Plutella xylostella and its parasitoid Cotesia plutellae. Ultraviolet-B radiation (UV-B) is a potent elicitor of a variety of changes in the chemistry, morphology and physiology of plants and animals. Recent studies have demonstrated that common signals, such as jasmonic acid (JA), play important roles in the mechanisms by which plants respond to UV-B and to damage by herbivores. Plant responses elicited by UV-B radiation can affect the choices of ovipositing female insects and the fitness of their offspring. This leads to the prediction that, in plants, the changes induced as a consequence of UV damage will be similar to those elicited in response to insect damage, including knock-on effects upon the next trophic level, predators. In our trials female P. xylostella oviposited preferentially on host plants grown in depleted UV-B conditions, while their larvae preferred to feed on tissues from UV-depleted regimes over those from UV-supplemented ones. Larval feeding patterns on UV-supplemented tissues met the predictions of models which propose that induced defences in plants should disperse herbivory; feeding scars were significantly smaller and more numerous – though not significantly so – than those on host plant leaves grown in UV-depleted conditions. Most importantly, female parasitoids also showed a clear pattern of preference when given the choice between host plants and attendant larvae from the different UV regimes; however, in the case of the female parasitoids, the choice was in favour of potential hosts foraging on UV-supplemented tissues. This study demonstrates the potential for UV-B to elicit a variety of interactions between trophic levels, most likely mediated through effects upon host plant chemistry.     

Influence of intraguild predation among generalist insect predators on the suppression of an herbivore population

We evaluated the influence of intraguild predation among generalist insect predators on the suppression of an herbivore, the aphid Aphis gossypii, to test the appropriateness of the simple three trophic level model proposed by Hairston, Smith, and Slobodkin (1960). We manipulated components of the predator community, including three hemipteran predators and larvae of the predatory green lacewing Chrysoperla carnea, in field enclosure/exclosure experiments to address four questions: (1) Do generalist hemipteran predators feed on C. carnea? (2) Does intraguild predation (IGP) represent a substantial source of mortality for C. carnea? (3) Do predator species act in an independent, additive manner, or do significant interactions occur? (4) Can the experimental addition of some predators result in increased densities of aphids through a trophic cascade effect? Direct observations of predation in the field demonstrated that several generalist predators consume C. carnea and other carnivorous arthropods. Severely reduced survivorship of lacewing larvae in the presence of other predators showed that IGP was a major source of mortality. Decreased survival of lacewing larvae was primarily a result of predation rather than competition. IGP created significant interactions between the influences of lacewings and either Zelus renardii or Nabis predators on aphid population suppression. Despite the fact that the trophic web was too complex to delineate distinct trophic levels within the predatory arthropod community, some trophic links were sufficiently strong to produce cascades from higher-order carnivores to the level of herbivore population dynamics: experimental addition of either Z. renardii or Nabis predators generated sufficient lacewing larval mortality in one experiment to release aphid populations from regulation by lacewing predators. We conclude that intraguild predation in this system is wide-spread and has potentially important influences on the population dynamics of a key herbivore.     

Nutritional suitability and ecological relevance of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> and <Emphasis Type="Italic">Brassica oleracea</Emphasis> as foodplants for the cabbage butterfly, <Emphasis Type="Italic">Pieris rapae</Emphasis>

The thale cress, Arabidopsis thaliana, is considered to be an important model species in studying a suite of evolutionary processes. However, the species has been criticized on the basis of its comparatively small size at maturity (and consequent limitations in the amount of available biomass for herbivores) and on the duration and timing of its life cycle in nature. In the laboratory, we studied interactions between A. thaliana and the cabbage butterfly, Pieris rapae, in order to determine if plants are able to support the complete development of the herbivore. Plants were grown in pots from seedlings in densities of one, two, or four per pot. In each treatment, one, two, or five newly hatched larvae of P. rapae were placed on fully developed rosettes of A. thaliana. In a separate experiment, the same densities of P. rapae larvae were reared from hatching on single mature cabbage (Brassica oleracea) plants. Pupal fresh mass and survival of P. rapae declined with larval density when reared on A. thaliana but not on B. oleracea. However, irrespective of larval density and plant number, some P. rapae were always able to complete development on A. thaliana plants. A comparison of the dry mass of plants in different treatments with controls (= no larvae) revealed that A. thaliana partially compensated for plant damage when larval densities of P. rapae were low. By contrast, single cress plants with 5 larvae generally suffered extensive damage, whereas damage to B. oleracea plants was negligible. Rosettes of plants that were monitored in spring, when A. thaliana naturally grows, were not attacked by any insect herbivores, but there was often extensive damage from pulmonates (slugs and snails). Heavily damaged plants flowered less successfully than lightly damaged plants. Small numbers of generalist plant-parasitic nematodes were also recovered in roots and root soil. By contrast, plants monitored in a sewn summer plot were heavily attacked by insect herbivores, primarily flea beetles (Phyllotreta spp.). These results reveal that, in natural populations of A. thaliana, there is a strong phenological mismatch between the plant and most of its potential specialist insect herbivores (and their natural enemies). However, as the plant is clearly susceptible to attack from non-insect generalist invertebrate herbivores early in the season, these may be much more suitable for studies on direct defense strategies in A. thaliana.     

Presence of predatory wasps and stinkbugs alters foraging behavior of cryptic and non-cryptic caterpillars on plantain (Plantago lanceolata)

We examined the foraging patterns of two species of caterpillar (Junonia coenia: Nymphalidae and Spilosoma congrua: Arctiidae) that contrast in feeding specialization and crypticity on plantain (Plantago lanceolata) in the absence and presence of two different insect predators [stinkbugs, Podisus maculiventris (Pentatomidae) and wasps, Polistes fuscatus (Vespidae)]. Junonia larvae were quite apparent to human observers, feeding on upper leaf surfaces during daylight, whereas Spilosoma larvae were relatively cryptic, often hiding under leaves and in soil crevices during daylight. In the presence of either predator species, the non-cryptic Junonia caterpillars more quickly left the plant on which they were initially placed and were less apparent than Junonia larvae not exposed to predators. The presence of predators had no detectable influence on where the caterpillars occurred on the plants (new, intermediate-aged or mature leaves, or reproductive stalks). Surprisingly, the predators influenced the behavior of the inherently cryptic Spilosoma: the apparency of these larvae at night increased when wasps had access to the plots during the day. Survivorship of the non-cryptic Junonia was less than 12% when stinkbugs were present compared to 60% in their absence. Although the presence of wasps resulted in a lower relative growth rate for the non-cryptic Junonia larvae, the indirect effect of predators on reduction in survivorship due to alterations in prey growth rate through behavioral changes was less than 3%. After taking into account the decline in caterpillars per plot through predation, we found that both the amount of leaves eaten and the proportion of plants eaten were altered on plots with predators present, which suggests that the caterpillars' increased consumption countered increased maintenance costs due to the presence of predators. Overall, our results indicate that hostplant size, level of predation and type of predator can influence the degree to which these caterpillars react to the presence of insect predators. In contrast, degree of inherent feeding specialization and cryptic behavior seemed to have little effect on the expression of reactive behaviors of these caterpillars to predators.     

Assessing the utilization of a carbohydrate food source and the impact of insecticidal proteins on larvae of the green lacewing, Chrysoperla carnea

A concern with the widespread use of insecticidal transgenic crops is their potential to adversely affect non-target organisms, including biological control agents such as larvae of the green lacewing, Chrysoperla carnea (Neuroptera: Chrysopidae). Since the insecticidal proteins expressed by the current transgenic plants are active only after ingestion, dietary bioassays are required to test direct effects on non-target organisms. After showing that C. carnea larvae utilize carbohydrate foods, we exposed them to insecticidal proteins dissolved in a sucrose solution. Feeding on snowdrop lectin (Galanthus nivalis agglutinin, GNA) as a model compound, the larvae were negatively affected in a number of life-table parameters. Interestingly, GNA caused a prolongation in first instar development, but had no effect on subsequent utilization of prey resulting in an increased weight of second instars. Comparable studies with avidin, a biotin-binding protein, revealed strong effects on C. carnea survival at the concentration tested. Despite the fact that the proteolytic digestion of C. carnea larvae is reported to be dominated by serine proteases, ingestion of soybean trypsin inhibitor (SBTI) did not cause any detrimental effects. Similarly, two Cry proteins derived from Bacillus thuringiensis (Cry1Ac and Cry1Ab) did not cause negative effects on C. carnea, what is consistent with earlier studies. The here presented bioassay provides a valuable tool to assess direct impacts of insecticidal proteins to C. carnea larvae and other predators that are known to feed on carbohydrate solutions.     

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.     

Predation by Podisus maculiventris (Hemiptera: Pentatomidae) on Plutella xylostella (Lepidoptera: Plutellidae) larvae parasitized by Cotesia plutellae (Hymenoptera: Braconidae) and its impact on cabbage

Biological control offers potentially effective suppression of the diamondback moth (DBM), Plutella xylostella, a serious pest of Brassica crops. Little is known of whether multiple natural enemies have additive, antagonistic, or synergistic effects on DBM populations. No-choice and choice tests were conducted to assess predation by Podisus maculiventris on DBM larvae parasitized by Cotesia plutellae and unparasitized larvae. In no-choice tests, P. maculiventris preyed on greater numbers of parasitized than unparasitized larvae and greater numbers of young larvae than old larvae. In choice tests with early third instar DBM, there was no difference in predation between parasitized or unparasitized larvae. However, in choice tests with older prey, P. maculiventris preyed on more parasitized than unparasitized larvae. Two field studies were conducted to test if this predator and parasitoid have additive, antagonistic or synergistic effects on DBM populations and plant damage in cabbage (Brassica oleracea var. capitata). In 2002, DBM populations were significantly lower in the presence of C. plutellae but not in the presence of P. maculiventris. There was not a significant interaction between the natural enemies. Plant damage was reduced only with C. plutellae. In 2003, DBM populations were significantly lower in the presence of C. plutellae and P. maculiventris, although the combination of natural enemies did not lead to a non-additive interaction. Plant damage was unaffected by the presence of either natural enemy. Because of its greater predation on parasitized larvae, P. maculiventris could be an intraguild predator of C. plutellae. Yet, their overall combined effect in the field was additive rather than antagonistic.     

Predation byIridomyrmex humilis [Hym.: Formicidae] on eggs ofChrysoperla carnea [Neu.: Chrysopidae] released for inundative control ofIllinoia liriodendri [Hom.: Aphididae] infestingLiriodendron tulipifera

In an effort to suppress the tuliptree aphidIllinoia liriodendri (Monell), approximately 2,000 eggs ofChrysoperla carnea (Stephens) from a commercial insectary were released 4 times on each of 8 tuliptreesLiriodendron tulipifera L. in Berkeley, California, during the spring of 1984. On trees foraged by the Argentine antIridomyrmex humilis (Mayr), 98% of the eggs ofC. carnea were removed from the egg release tapes by the ants. A total of about 1,250 larvae per tree eclosed from the 8,000 eggs released on each tree without ants. Fifty percent of the larvae that did eclose died due to cannibalism or entrapment in the sticky egg release tapes and approximately 625 first instar larvae on each tree were free to forage for aphids. Inundative lacewing releases ofC. carnea did not suppress populations ofI. liriodendri due to ant predation, the low viability of commercial eggs (0–73% eclosion),

---

Résumé Dans le but de limiter les populations du puceron du tulipierIllinoia liriodendri (Monell), 4 lachers d'environ 2.000 œufs de provenance commerciale deChrysoperla carnea (Stephens) ont été réalisés au cours du printemps 1984 sur 8 tulipiersLiriodendron tulipifera L., à Berkeley en Californie. cannibalism by emerged larvae, and inadequate release technology. Sur les arbres visités par la fourmi d'ArgentineIridomyrmex humilis (Mayr), 98% des œufs deC. carnea ont été enlevés du support artificiel par les fourmis. Sur les arbres exempts de fourmis, la mortalité de 50% des larves est due au cannibalisme ou à leur engluement sur le support de lacher. A partir de 8.000 œufs déposés sur chaque arbre sans fourmi, on aboutit à environ 625 larves de 1^er stade susceptibles de rechercher des pucerons. Les lachers inondatifs deC. carnea n'ont pas limité les populations d'I. liriodendri. Les raisons en sont: la consommation par les fourmis, une faible viabilité de la plus grande partie des œufs commercialisés (0–73% d'éclosions), une technique inadaptée pour le lacher des œufs et le cannibalisme par les larves elles-mêmes deC. carnea.

     

Influence of crucifer cropping system on the parasitism ofPlutella xylostella (Lep., Yponomeutidae) byCotesia plutellae (Hym., Braconidae) andDiadegma semiclausum (Hym., Ichneumonidae)

Field experiments were conducted to study the influence of cabbage monoculture and mixed cropping on the parasitism of diamondback moth,Plutella xylostella (L.), a destructive pest of all crucifers, by 2 larval parasites,Diadegma semiclausum Hellén andCotesia plutellae Kurdjumov. There was no significant difference in parasitism by either species whether cabbage was planted in insecticide-free monoculture or in mixed cropping with 8 noncrucifers which were sprayed twice a week with chemical insecticides mevinphos, methamidophos and permethrin. Population ofP. xylostella increased as the cabbage plants grew older. Parasitism byC. plutellae was higher soon after cabbage transplanting but decreased as the plants grew older. Parasitism byD. semiclausum was very low soon after cabbage planting but increased as the plants grew older. A significant negative correlation was found betwen parasitism byC. plutellae andD. semiclausum. In a caged field study where only one parasite species was used in an individual cage, parasitism ofP. xylostella by both species decreased as theP. xylostella population increased. This is believed to be due to the absence of competition between the two parasites inside the cage. There was no relationship between host-plant age and parasitism ofP. xylostella larvae by either parasite species.