Host tree architecture mediates the effect of predators on herbivore survival

Abstract.  1. Vegetation structural complexity is an important factor influencing ecological interactions between different trophic levels. In order to investigate relationships between the architecture of trees, the presence of arthropod predators, and survival and parasitism of the autumnal moth Epirrita autumnata Borkhausen, two sets of experiments were conducted.
2. In one experiment, the architectural complexity of mountain birch was manipulated to separate the effects of plant structure and age. In the other experiment the trees were left intact, but chosen to represent varying degrees of natural complexity. Young autumnal moth larvae were placed on the trees and their survival was monitored during the larval period.
3. The larvae survived longer in more complex trees if predation by ants was prevented with a glue ring, whereas in control trees smaller canopy size improved survival times in one experiment. The density of ants observed in the trees was not affected by canopy size but spider density was higher on smaller trees. The effect of canopy structure on larval parasitism was weak; larger canopy size decreased parasitism only in one year. Until the fourth instar the larvae travelled shorter distances in trees with reduced branchiness than in trees with reduced foliage or control treatments. Canopy structure manipulation by pruning did not alter the quality of leaves as food for larvae.
4. The effect of canopy structure on herbivore survival may depend on natural enemy abundance and foraging strategy. In complex canopies herbivores are probably better able to escape predation by ambushing spiders but not by actively searching ants.     

Oviposition site preference and larval mortality in a leaf-mining moth

Abstract. 1. The univoltine leaf-mining moth, Lithocolletis quercus Ams., is endemic to Israel, where it spends its 10.5 month larval period feeding only in the leaves of Quercus calliprinos Webb.
2. We compared patterns of egg deposition and sources of larval mortality to test whether oviposition patterns and site preferences confer an enhanced likelihood of larval survival.
3. Dominant sources of larval mortality were premature leaf abscission and death from unknown causes, whereas predation, parasitism and intraspecific interference accounted for relatively little larval mortality.
4. Eggs, and thus mines, were aggregated among leaves of host trees even though premature leaf abscission was positively correlated with density of mines per leaf. Interference competition among larvae was the only other density-dependent mortality factor.
5. Oviposition patterns within leaves mitigated the probability of death from larval interference, and probably also from early leaf abscission.
6. Despite these density-dependent mortality factors, overall probability of larval survival to pupation was independent of initial density of mines on a leaf.
7. The long larval period allows synchrony between oviposition flights and times of predictable resource availability.     

Effects of the light environment on oviposition preference and survival of a leaf-mining moth, Cameraria hamadryadella (Lepidoptera: Gracillariidae), on Quercus alba L.

Abstract.  1. Oviposition preference and the survival and mortality of larvae of Cameraria hamadryadella (Lepidoptera) on the foliage of Quercus alba L. (Fagaceae) in sun and shade were examined. To separate the effects of the environment of foliation from that of rearing, C. hamadryadella were allowed to oviposit on oak saplings that foliated either in the sun or shade. Subsequently, trees were placed into the sun or shade and larval survival was monitored.
2. Trees that foliated in the shade had leaves that were 30% longer than those that foliated in the sun. However, the density of leaf mines did not differ between trees that foliated in the sun or shade.
3. Larval survival was higher in the shade than in the sun, but no difference in survival due to the site of foliation was detected. Differences in larval survival between the sun and shade environments could not be attributed to differences in natural enemy mortality. However, 'death by other causes' was higher for larvae reared in the sun than in the shade. In sunny environments photochemical processes may reduce foliage quality, which results in lower survival of C. hamadryadella larvae.
4. Ovipositing C. hamadryadella do not appear to discriminate between foliage produced in the sun or shade. This lack of discrimination, coupled with the fact that ovipositing C. hamadryadella are active after dark, accounts for the absence of a difference in abundance between sun and shade.
5. The results presented here, combined with other published studies, argue that different patterns of insect herbivory and abundance in relation to the light level experienced by host-plant foliage might be expected for species that oviposit either in the night or in the day.     

Host-specific variation in Ieaf miner population dynamics: effects on density, natural enemies and behaviour of Stilbosis quadricustatella (Lepidoptera: Cosmopterigidae)

Abstract. 1. We compared high and low density populations of a leaf miner ( Stilbosis quadricustatella (Cham.)) on two host oaks to ascertain mechanisms influencing abundance. High density miner populations occurred on sand live oak, Quercus geminata (Fagaceae), found in homogeneous stands at inland and coastal sites. Quercus nigra , water oak, a patchily distributed inland species, supported a low density leaf miner population.
2. Average foliar nitrogen of Q.geminata was significantly lower than that of Q.nigra , and lad mining period on Q.geminata was correspondingly longer than on Q.nigra .
3. The average leaf area of Q.nigra was significantly greater than that of Q.geminata .
4. Parasitism was significantly greater in Q.geminata miner populations and predation was significantly higher in Q.nigra populations. However, parasitism and predation rates were roughly reciprocal so that overall larval mortality did not differ significantly between hosts.
5. In a laboratory experiment, pupal overwintering survivorship did not differ significantly between moist and dry treatments of the sand and loam soil types that typify Qgeminata and Q.nipra habitats, respectively.
6. Within-leaf miner density on Q.geminata significantly affected larval survivorship, parasitism and predation. Leaf area and within-leaf miner density were positively correlated.
7. We detected no host-patch area or isolation effect on miner densities on coastal Qgeminata and there was no significant gradient of local coastal conditions affecting larval survivorship or natural enemies.
8. Coastal leaf miner densities were significantly higher on oak patch edges than in interiors.     

Opposing effects of spring defoliation on late season oak caterpillars

ABSTRACT. 1. The pedunculate oak, Quercus rohur L., suffers high annual levels of spring defoliation in Wytham Woods. near Oxford.
2. This spring defoliation affects late season caterpillars through a variety of damage-induced changes in the leaves.
3. Diurnea fagella (D. & S.), one of the commonest late season caterpillars, shows reduced larval survival and pupal weight on regrowth foliage when compared to undamaged primary foliage.
4. D. fagella also suffer higher larval mortality on naturally damaged primary foliage than they do on undamaged foliage.
5. Despite this, the three commonest late season caterpillar species are more abundant on damaged trees than undamaged ones. and their distributions are biased towards damaged leaves within the canopy.
6. Other factors that may be more important than leaf damage in determining the distribution and abundance of late season caterpillars are discussed. D.fagella larvae spin their larval refuges more rapidly on damaged than undamaged foliage, and this may reduce mortality by natural enemies, or ameliorate adverse effects of weather.     

Larval parasitoids and pathogens of the groundnut leaf miner,Aproaerema modicella (Lep.: Gelechiidae), in India

Natural enemies of the groundnut leaf miner,Aproaerema modicella (Deventer), were studied at the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) located near Hyderabad in peninsular India. Hymenopterous parasitoids attacking leaf miner larvae were the most important group of natural enemies. Nine primary and eight secondary parasitoids emerged from host larvae, and killed up to 50% of the leaf miner larvae sampled. The trophic relationships between primary and secondary parasitoids are incompletely understood. The influence of pathogens of this species is reported for the first time. These pathogens killed up to 30% of the leaf miner larvae. The combined effects of all mortality agents killed up to 95% of the leaf miner larvae per sample period. However, use of insecticides in sprayed plots reduced the efficacy of parasitoids. The impact of predators on larval populations was not studied and may explain underestimates of leaf miner mortality rates.      

Influence of host plant heterogeneity on the distribution of a birch aphid

Abstract.  1. The spatial and temporal abundance of the aphid Euceraphis betulae was investigated in relation to heterogeneity in host plant ( Betula pendula ) vigour and pathogenic stress. The performance of aphids feeding on vigorous and stressed foliage was also examined.
2. The plant stress and plant vigour hypotheses have been suggested as opposing ways in which foliage quality influences herbivore abundance. In many plants, however, vigorous growing foliage co-exists with stressed or damaged foliage.
3. There was a negative correlation between branch growth (vigour) and branch stress (leaf chlorosis), with the most vigorous branches displaying little or no stress, and the most stressed branches achieving poor growth. There was a similar negative correlation between vigour and stress at the level of individual trees, which themselves represented a continuum in quality.
4. At the beginning of the season, E. betulae were intermittently more abundant on vigorous branches than on branches destined to become stressed, but aphids became significantly more abundant on stressed branches later in the season, when symptoms of stress became apparent. Similar patterns of aphid abundance were seen on vigorous and stressed trees in the following year.
5. Euceraphis betulae performance was generally enhanced when feeding on naturally stressed B. pendula leaves, but there was some evidence for elevated potential reproduction when feeding on vigorous leaves too.
6. Overall, plant stress probably influences E. betulae distribution more than plant vigour, but the temporal and spatial variability in plant quality suggests that plant vigour could play a role in aphid distribution early in the season.     

Variation in rates of leaf abscission between plants may affect the distribution patterns of sessile insects

Summary At Sand Lake, Leon County, Florida, mines of Stilbosis quadricustatella, a leaf-mining moth, occur on sand live oak trees (Quercus geminata) over a broad range of densities. Some trees have fewer than 2% of their leaves mined (lightly infested), others up to 70% (heavily infested). Similar levels of infestation are maintained on the same trees year after year. There are no significant negative correlations of miner density per tree with any mortality factor that might explain miner preferences for certain trees. Nor is there a positive correlation with host leaf quality as measured by foliar nitrogen or by total or individual amino acid concentrations of host leaves. Egg-transfer experiments showed that larvae from eggs transferred to lightly infested trees were more likely to be killed by leaf abscission than were those that were transferred to and developed on heavily infested trees. This is the first demonstration that variation in rates of leaf abscission could be an important cause of the observed distribution pattern of sessile insects between conspecific host plants.     

Factors underlying the late seasonal appearance of the lepidopterous leaf-mining guild on oak

Abstract. 1. Most lepidopterous leaf mining species found on the oak Quercus robur in Britain develop in summer. At this time of year, externally feeding caterpillars remove little leaf area since most of these free living folivores are predominantly spring feeders.
2. I forced Phyllonorycter harrisella (L.) miners to oviposit in spring, then exposed developing larvae to a wide range of leaf damage levels.
3. Leaf miner survivorship and mean female pupal weight were significantly greater in the experimental spring generation on undamaged oak leaves, this being when oak foliage is of the highest nutritional quality.
4. Leaf miner survivorship in all generations is correlated with natural leaf damage levels. Experimental leaf damage also reduces miner survivorship.
5. Leaf damage reduces miner survivorship by increasing the probability of larval death due to wound induced responses.
6. The role of asymmetrical competitive interactions between caterpillars and leaf-miners in determining the late seasonal appearance of miners is discussed. It is suggested that wound induced responses may play an important part in the structuring of phytophagous insect communities.     

Consequences for a host–parasitoid interaction of host-plant aggregation, isolation, and phenology

Abstract.  1. Spatial habitat structure can influence the likelihood of patch colonisation by dispersing individuals, and this likelihood may differ according to trophic position, potentially leading to a refuge from parasitism for hosts.
2. Whether habitat patch size, isolation, and host-plant heterogeneity differentially affected host and parasitoid abundance, and parasitism rates was tested using a tri-trophic thistle–herbivore–parasitoid system.
3.  Cirsium palustre thistles ( n = 240) were transplanted in 24 blocks replicated in two sites, creating a range of habitat patch sizes at increasing distance from a pre-existing source population. Plant architecture and phenological stage were measured for each plant and the numbers of the herbivore Tephritis conura and parasitoid Pteromalus elevatus recorded.
4. Mean herbivore numbers per plant increased with host-plant density per patch, but parasitoid numbers and parasitism rates were unaffected. Patch distance from the source population did not influence insect abundance or parasitism rates. Parasitoid abundance was positively correlated with host insect number, and parasitism rates were negatively density dependent. Host-plant phenological stage was positively correlated with herbivore and parasitoid abundance, and parasitism rates at both patch and host-plant scales.
5. The differential response between herbivore and parasitoid to host-plant density did not lead to a spatial refuge but may have contributed to the observed parasitism rates being negatively density dependent. Heterogeneity in patch quality, mediated by variation in host-plant phenology, was more important than spatial habitat structure for both the herbivore and parasitoid populations, and for parasitism rates.     

Larval performance in relation to oviposition site preference in olive kernel moth (Prays oleae Bern., Yponomeutidae,Praydina)

1 The tri‐voltine moth Prays oleae Bern. spends its larval stages on the native olive tree (Olea europaea L. var. sylvestris Brot. and five cultivars, Oleaceae) mining the leaves, the flowers and the fruits in each generation; it seldom switches to other native or introduced confamilial plant species. 2 In this study the pattern of oviposition of the olive moth was examined in olive fields and natural vegetation, in relation to in situ recruitment as an outcome of processes such as density dependence or risk spreading. 3 Larval body size (width of epicranial sclerites) was also examined and compared between host substrates, years and morphological, physiological, ecological and nutritional attributes of the host. 4 The factors influencing the oviposition pattern of the olive moth such as the carbon/nitrogen ratio, number of flowers, branch length and previous leaf damage were ranked differently in different cultivars. 5 ‘Hot spots’, i.e. olive trees or parts of trees receiving a high egg load, were found to be the result of in situ recruitment. 6 Physiological mortality among first instar larvae was significantly negatively correlated with the number of oviposited upon leaves; suggesting that the adult selects for oviposition the best available substrate. 7 As adult moths selected leaves with minimal probability of abscission for oviposition, leaf abscission was not a major mortality factor, although first instar larvae reduced leaf longevity. 8 Host quality did not affect all larval stages in the same way. 9 The more nutritionally poor the substrate, the longer the duration of the larval stage feeding on it. The phenological timing of the insect life stages very closely tracks the phenological phases of its host plant, primarily focusing on the most nutritious host stage in terms of larval performance.     

Two methods of assessing the mortality factors affecting the larvae and pupae of Cameraria ohridella in the leaves of Aesculus hippocastanum in Switzerland and Bulgaria

The horse-chestnut leaf miner, Cameraria ohridella, is an invasive alien species defoliating horse-chestnut, a popular ornamental tree in Europe. This paper presents quantitative data on mortality factors affecting larvae and pupae of the leaf miner in Switzerland and Bulgaria, both in urban and forest environments. Two sampling methods were used and compared: a cohort method, consisting of the surveying of pre-selected mines throughout their development, and a grab sampling method, consisting of single sets of leaves collected and dissected at regular intervals. The total mortality per generation varied between 14 and 99%. Mortality was caused by a variety of factors, including parasitism, host feeding, predation by birds and arthropods, plant defence reaction, leaf senescence, intra-specific competition and inter-specific competition with a fungal disease. Significant interactions were found between mortality factors and sampling methods, countries, environments and generation. No mortality factor was dominant throughout the sites, generations and methods tested. Plant defence reactions constituted the main mortality factor for the first two larval stages, whereas predation by birds and arthropods and parasitism were more important in older larvae and pupae. Mortality caused by leaf senescence was often the dominant mortality factor in the last annual generation. The cohort method detected higher mortality rates than the grab sampling method. In particular, mortality by plant defence reaction and leaf senescence were better assessed using the cohort method, which is, therefore, recommended for life table studies on leaf miners.     

The influence of Salix leaf abscission on leaf-miner survival and life history

Abstract. 1. Early abscission of mined leaves was an important mortality factor of a Phyllonorycter species (Lepidoptera: Gracillariidae) on Salix lasiolepis Benth. (Salicaceae). A larger percentage of mined leaves abscised early (34.4% in 1990; 24.5% in 1991), and Phyllonorycter survival was greatly reduced in these abscised leaves.
2. Leaf-mining by Phyllonorycter was associated with increased early leaf abscission. An egg removal experiment demonstrated that leaf mining induced this increase in leaf abscission.
3. The induction of early leaf abscission was dependent upon the timing of herbivory and simulated herbivory (mechanical damage). Early mechanical damage induced leaf abscission, late mechanical damage did not. Mines which expanded early were more likely to induce leaf abscission than mines which expanded more slowly.     

Intraspecific competition as a selective pressure on the choice of oviposition site in a phytophagous insect

Our aim was to clarify the factors providing selective pressures on the choice of oviposition sites in phytophagous insects. To this end, we examined the spatial distribution of eggs of the sawfly leaf miner, Profenusa japonica . The study was conducted over five consecutive years on three subpopulations found in riverbeds in central Japan. Profenusa japonica lays eggs on leaflets of Rosa multiflora , and its larvae mine a single leaflet for the entire larval stage. The egg distribution among leaflets was uniform (regular) at all sites examined in each year. The distribution within leaflets was also uniform, and it appeared that females avoided ovipositing on areas of the leaf that already contained eggs. Few parasitoids were found in eggs or larval stages, despite intensive examination. Larval mortality increased, mainly as a result of exploitative competition, when there were two or three larvae per leaflet. Starvation caused by resource competition for leaflet tissues was an important mortality factor. Therefore, we suggest that P. japonica females avoided ovipositing on areas of the leaflet that already contained eggs to prevent exploitative competition among larvae sharing the same leaflet.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 641–650.     

Parasitism and ant protection alter the survival of the lycaenid Hemiargus isola

Abstract. 1. Although the majority of lycaenid–ant associations is facultative, few studies have documented the protection benefits provided by ants to lycaenids that are tended facultatively (Pierce & Easteal, 1986; Peterson, 1993).
2. Larvae of Hemiargus isola (Lepidoptera: Lycaenidae) feeding on Dalea albiflora are tended facultatively by several species of ant. In 1999 and 2000, the levels of parasitism and the identities of attendant ants were determined for larvae of H. isola . In addition, the presence of ants was manipulated experimentally to determine the efficacy of protection provided by attendant ants to H. isola .
3. Lycaenids were parasitised by a braconid wasp, Cotesia cyaniridis (Riley), and a tachinid fly, Aplomya theclarum (Scudder). In 1999 and 2000, 62 and 65% of larvae were parasitised; the percentage of the population parasitised did not differ significantly between years. In both 1999 and 2000, parasitism by the braconid wasp C. cyaniridis accounted for > 99% of all parasitism events.
4. Four species of ant, Crematogaster sp., Dorymyrmex sp., Forelius sp., and Formica sp., were associated with 88–99% of the tended lycaenids collected in both 1999 and 2000. For both years, there was a single, numerically dominant species associated with >80% of the tended larvae collected, but the identity of this numerically dominant ant differed between years.
5. Experimental exclusion of ants from D. albiflora plants resulted in 78% larval mortality as a result of parasitism, nearly twice that of larvae that were tended by ants on unmanipulated plants.     

Spatial, bottom-up, and top-down effects on the abundance of a leaf miner

Many plant and animal species have higher densities at the centre of their distribution, with a gradual decline in abundance towards the edge of the range, though reasons for this pattern is not well known. We examined the abundance of the leaf miner Cameraria sp. nova over the range of its host plant Quercus myrtifolia in Florida and addressed how bottom-up and top-down factors varied over its whole distribution. Leaf miner densities, plant quality and natural enemy effects on mine survivorship were evaluated in 40 sites and spatially structured models were used to determine the effects of spatial location on the abundance of Cameraria and effects of both bottom-up (tannin concentration, foliar nitrogen, soil nitrogen, and leaf area) and top-down factors (larval parasitism and predation) on abundance and survivorship. Cameraria mines were, on average, three times more abundant on edge/coastal sites compared to centre/inland sites and did not support the hypothesis of higher abundance on the centre of the distribution. Differences in plant quality, larval parasitism and successful emergence of mines on edge versus central sites might be partially responsible for this finding. A trend surface equation with latitude and longitude combined explained almost 52% of the variation in Cameraria density and a trend surface map also revealed peaks of Cameraria abundance on the edges of the plant distribution. Correlograms also indicated a significant spatial structure of Cameraria as mines were positively spatially autocorrelated at small distances (≈122 km). Partial regression analyses indicated that 69% of the variation in Cameraria abundance was explained by the effects of latitude, longitude, elevation and percentage of foliar nitrogen. Our results indicated that variation in Cameraria abundance was mostly explained by spatial position and significant effects of bottom-up and top-down factors were not detected in our large-scale study.     

Offspring performance in dynamic habitats: key factors for a riparian carabid beetle

Abstract.  1. Knowledge of the ecology of carabid species is largely restricted to adults, although larval mortality is assumed to be the key factor in overall mortality. As low-mobility larvae are unable to avoid unfavourable conditions, habitat selection of reproducing adults should be clearly affected by the habitat factors which determine offspring performance.
2. The present study examines the key habitat factors governing the distribution patterns of Bembidion velox larvae and adults on the river banks of the River Elbe by means of habitat suitability models. The validity of the determined habitat factors for offspring performance and survival was tested in laboratory experiments.
3. In the field, B. velox adults as well as larvae show a strong association with semi-terrestrial, sandy, open soil habitats.
4. In the laboratory, overall mortality of larvae reared in different substrates was lowest in sand of medium grain size mixed with fine and coarse sand. The first larval instars in particular reacted sensitively to variations in grain size. Furthermore, flood resistance of eggs was demonstrated, as 90% of larvae hatched under permanently flooded conditions.
5. Short development times were recorded, with 4–7 days for hatching of young larvae from eggs after oviposition and approximately 28 days for the development of adults from newly hatched larvae. This probably increases the survival probability for the pre-imaginal stages in dynamic habitats.
6. In conclusion, it can be said that reproductive success is strongly dependent on oviposition site selection by adults as this reflects the ecological demands of the immature stages.     

Growth of an invertebrate shredder on native (Populus) and non-native (Tamarix, Elaeagnus) leaf litter

1. Large-scale invasions of riparian trees can alter the quantity and quality of allochthonous inputs of leaf litter to streams and thus have the potential to alter stream organic matter dynamics. Non-native saltcedar ( Tamarix sp.) and Russian olive ( Elaeagnus angustifolia ) are now among the most common trees in riparian zones in western North America, yet their impacts on energy flow in streams are virtually unknown.
2. We conducted a laboratory feeding experiment to compare the growth of the aquatic crane fly Tipula (Diptera: Tipulidae) on leaf litter from native cottonwood ( Populus ) and non-native Tamarix and Elaeagnus . Tipula showed positive growth on leaf litter of all three species; however, after 7 weeks, larvae fed Tamarix leaves averaged 1.7 and 2.5 times the mass of those fed Elaeagnus and Populus , respectively. Tipula survival was highest on Populus , intermediate on Tamarix and lowest on Elaeagnus .
3. High Tipula growth on Tamarix probably reflects a combination of leaf chemistry and morphology. Conditioned Tamarix leaf litter had intermediate carbon : nitrogen values (33 : 1) compared to Populus (40 : 1) and Elaeagnus (26 : 1), and it had intermediate proportions of structural carbon (42%) compared to Elaeagnus (57%) and Populus (35%). Tamarix leaves are also relatively small and possibly more easily ingested by Tipula than either Elaeagnus or Populus .
4. Field surveys of streams in the western U.S.A. revealed that Tamarix and Elaeagnus leaf packs were rare compared to native Populus , probably due to the elongate shape and small size of the non-native leaves. Thus we conclude that, in general, the impact of non-native riparian invasion on aquatic shredders will depend not only on leaf decomposition rate and palatability but also on rates of leaf litter input to the stream coupled with streambed retention and subsequent availability to consumers.     

Resource selection by female moths in a heterogeneous environment: what is a poor girl to do?

1. According to the preference-performance hypothesis, female insects select resources that maximize offspring performance. To achieve high fitness, leaf miner females should then adjust their oviposition behaviour in response to leaf attributes signalling high host quality. 2. Here we investigate resource selection in Tischeria ekebladella, a leaf-mining moth of the pedunculate oak (Quercus robur), in relation to two alternative hypotheses: (1) females select their resources with respect to their future quality for developing larvae; or (2) temporal changes in resource quality prevent females from selecting the best larval resources. 3. Specifically, we test whether females show the strongest selection at the levels at which quality varies the most (shoots and leaves); whether they respond to specific leaf attributes (leaf size, phenolic content and conspecific eggs); and whether female preference is reflected in offspring performance. 4. Female choice of leaves was found to be non-random. Within trees, the females preferred certain shoots, but when the shoots were on different trees the degree of discrimination was about four times larger than when they were on the same trees. 5. While females typically lay more eggs on large leaves, this is not a result of active selection of large leaves, but rather a result of females moving at random and ovipositing at regular intervals. 6. The females in our study did not adjust their oviposition behaviour in response to leaf phenolic contents (as measured by the time of larval feeding). Neither did they avoid leaves with conspecific eggs. 7. Female choice of oviposition sites did not match patterns of offspring performance: there was no positive association between offspring survival and counts of eggs. 8. We propose that temporal variation in resource quality may prevent female moths from evaluating resource quality reliably. To compensate for this, females may adopt a risk-spreading strategy when selecting their resources.     

Testing of selected insecticides to assess the viability of the integrated pest management of the Eucalyptus snout-beetle Gonipterus scutellatus in north-west Spain

Abstract:  We investigated, by means of topical, residual and field assays, the toxicity of four insecticides ( Bacillus thuringiensis , azadirachtin, flufenoxuron and ethofenprox) to adult, larvae and eggs of the Eucalyptus snout-beetle Gonipterus scutellatus and their side-effects on the adults of the solitary egg parasitoid Anaphes nitens , its natural enemy. In the topical toxicity experiment, both flufenoxuron and ethofenprox severely affected G. scutellatus , causing 100% adult mortality after 7 days, and 75 and 100% larval mortality, respectively. Ethofenprox also caused 93% larval pre-eclosion mortality and 100% parasitoid mortality. In the residual experiment, azadirachtin acted as an effective larvicide when ingested (73% mortality after 7 days). Ethofenprox showed the highest toxicity, with 100% mortality of adults and larvae of G. scutellatus after 24 h. On the day 7 of observation, flufenoxuron resulted in 93% adult mortality and 100% larval mortality. After contact with residuals, only ethofenprox and flufenoxuron had adversely affected A. nitens (98% mortality after 48 h). Bacillus thuringiensis exhibited only a slight activity against the pest or the parasitoid. The field experiment was carried out on three localities, with a preliminary sampling, and three samplings after the applications of the products, on days 7, 14 and 30. We evaluated the parasitism rate, parasitoid sex ratio, parasitoid pre-eclosion mortality, the abundance of G. scutellatus adults, larvae and egg capsules, the pre-eclosion mortality rate of weevil eggs and larvae and the foliar damage. Only fluenoxuron and ethofenprox reduced the pest population and the foliar damages, but ethofenprox also significantly decreased the rate of parasitism (17%) after 7 days. Given our experimental results, we suggest that flufenoxuron and azadirachtin could be used in association with the natural enemy.