Efficacy of multiple biocontrol agents against the sweet potato whitefly Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) on tomato

Abstract: Biological control provides an environmentally harmonious and potentially stable management tactic to combat noxious pests such as Bemisia tabaci, notorious for its resistance to synthetic pesticides. Bioassays conducted under control chamber conditions integrating applications of the parasitoid Encarsia formosa, reared for 20 years on Trialeurodes vaporariorum, and the fungus Verticillium lecanii on the third‐fourth instar nymphs of B. tabaci on tomato, showed a comparable effect between the parasitoid‐fungus combined treatment and the fungus treatment alone (70.7% vs. 70.4%). Analysis of our results indicates antagonism between the two biocontrol agents related to the parasitoids’ ability to discriminate between infected and healthy B. tabaci nymphs. The parasitoid treatment alone produced 36.3% mortality, with no mortality in the distilled water controls. The behavioural performance of the parasitoid could have either genetic or environmental causes. Bioassays studying the feeding habit of the imported mirid predator Macrolophus caliginosus (adults) and the indigenous mirid Camptotylus reuteri (nymphs and adults) on eggs, or early second instar nymphs of B. tabaci, and choice preference tests indicated a significant difference in feeding between M. caliginosus and C. reuteri. There was no significant difference in percentage feeding of M. caliginosus on eggs (2.2%) or second instar nymphs (8.0%). There was a significant difference in feeding of M. caliginosus adults (18.6%) when offered eggs and second instars in the same arena compared with eggs or second instars offered separately. These results could be attributed to the biological behaviour of the predator having a type III functional response. Studies with the local C. reuteri species showed no significant difference in adult and nymphal consumption on second instars of B. tabaci compared with nymphs on eggs. However, C. reuteri adults fed less on eggs compared with nymphs. This local predatory species appears to be more efficient than M. caliginosus in feeding on particular stages of B. tabaci without depending on prey density. This is further supported by the low consumption of both adults and nymphs in the choice test (4% and 2.3%, respectively) compared with M. caliginosus adults (18.6%).     

Parasitism,host feeding and immature development of Encarsia formosa reared from Trialeurodes vaporariorum and Bemisia tabaci on Trialeurodes ricini

Encarsia formosa Gahan (Hymenoptera: Aphelinidae), a thelytokous parasitoid, is an important biological control agent of whiteflies because of its outstanding reproduction and host‐feeding ability. In this study, we evaluated the parasitism, host feeding and developmental time of E. formosa populations reared on Trialeurodes vaporariorum (Westwood) (EFT) or on Bemisia tabaci (Gennadius) (EFB) when different nymphal instars of the castor whitefly, Trialeurodes ricini (Misra), were offered as hosts, with an aim to understand the preference of the parasitoid on nymphal instars of T. ricini. Experiments were conducted on castor bean plants at 26 ± 2°C, 50–60% RH and 16 : 8 (L : D) photoperiod. The results showed that E. formosa successfully oviposited and fed on all nymphal instars of T. ricini. However, numbers of the first instars fed on by the E. formosa populations reared on T. vaporariorum (EFT) and B. tabaci (EFB) were significantly greater (45.9 and 31.3, respectively) than those of the second (EFT: 30.4 and EFB: 15.8), the third (EFT: 22.4 and EFB: 13.2) and the fourth nymphal instars (EFT: 6.0 and EFB: 3.8). The number of T. ricini nymphs parasitized by E. formosa varied significantly among different instars, and the parasitism rates on the first instar (EFT: 15.2; EFB: 7.7) and fourth instar (EFT: 19.3; EFB: 4.9) were greater than those on the second and third instars. Encarsia formosa reared on T. vaporariorum had a significantly higher host feeding and ovipositing potential on T. ricini than EFB. When parasitizing the fourth instar nymphs, E. formosa completed development in a significantly shorter time (12.9 day) than when ovipositing in other instars (17.8–19.1 day). These results showed that EFT had a better host adaption than EFB. The information from this study should be useful for us to better understand the performance and nymphal preference of E. formosa from T. vaporariorum and B. tabaci when they parasitized and fed on T. ricini, and the interactions of parasitoids with different host whitefly species.     

Fecundity,nymphal development and longevity of field‐collected tropical bedbugs,Cimex hemipterus

This study examined the fecundity, oviposition, nymphal development and longevity of field‐collected samples of the tropical bedbug, Cimex hemipterus (Fabricius) (Hemiptera: Cimicidae). Under environmental conditions of 26±2°C, 70 ± 5% relative humidity and a 12‐h photoperiod, with bloodmeals provided by a human host, six strains of tropical bedbug had a fecundity of up to 50 eggs per lifetime, over 11–14 oviposition cycles. Increased feeding frequency improved fecundity. After feeding and mating, adult females normally took 2–3 days to produce a first batch of eggs. The oviposition period lasted 2–7 days before cessation of the oviposition cycle. The egg incubation period usually lasted 5–7 days before the emergence of first instars. The nymphs underwent five stadia (the first four of which each took 3–4 days, whereas the last took 4–5 days) before becoming adults at a sex ratio of 1 : 1. More than five bloodmeals were required by the nymphs to ensure a successful moult. Unmated adults lived significantly longer than mated adults (P < 0.05). Unmated females lived up to almost 7 months, but the longevity of mated males and females did not differ significantly (P > 0.05).     

Opportunistic or selective? Stage-dependent feeding behavior in a wild vector of Chagas disease

The composition and contribution of different host species in the dynamics of vector-borne zoonotic parasites are particularly relevant for public health. Hence, the study of host selection by vectors is fundamental. Developmental stage and infection status are factors that may modulate vector feeding behavior. In the semi-arid Mediterranean ecosystem of South America, the transmission of Trypanosoma cruzi, the protozoan causing Chagas disease, includes the triatomine vector Mepraia spinolai and several vertebrate species. In this field study, we examined whether M. spinolai exhibits an opportunistic feeding behavior dependent upon developmental stage and/or infection status. We found that M. spinolai does not feed according to the relative availability of vertebrate species. In addition, early stage nymphs (first/second instars) fed on twice as many different species as middle (third/fourth instars) and late (fifth instars and adults) M. spinolai, with the former feeding on native rodents and lizards and the latter mostly on rabbits. Infected and uninfected M. spinolai showed similar feeding profiles. Wild triatomine species might be described as stage-dependent selective blood feeders, as a consequence of the temporal and spatial scale at which host-vector interactions occur, highlighting that all developmental stages might be infected and capable of transmitting T. cruzi.     

Geographic variation on biological parameters of Meccus picturatus (Usinger), 1939 (Hemiptera: Reduviidae)

The biological parameters related to the life cycles of three populations of Meccus picturatus (Usinger) (Hemiptera: Reduviidae), one of the main vectors of Trypanosoma cruzi Chagas (Trypanosomatida: Trypanosomatidae), were evaluated. A cohort of each of three populations from geographically isolated localities in western Mexico was maintained under similar laboratory conditions, after which all three populations were compared to each other. In each cohort, 50.9 to 72.1% of nymphs completed the cycle. The average time from N1 to adult was approximately 175 days and different for the three studied cohorts. The number of blood meals between molts varied from one to three. The hatching rates as well as the fecundity per female per day were different among the three studied populations. Our results show that the three isolated populations are statistically different in some parameters from each other, which points to the importance of studying the biological characteristics of local populations of triatomines for estimating their capacity of transmitting T. cruzi to reservoir hosts.     

Effect of Protein Intake on Bone Mineralization during Weight Loss: A 6‐Month Trial

Objective: The long‐term effect of dietary protein on bone mineralization is not well understood. Research Methods and Procedures: Sixty‐five overweight (body mass index, 25 to 29.9 kg/m²) or obese (≥30 kg/m²) subjects were enrolled in a randomized, placebo‐controlled, 6‐month dietary‐intervention study comparing two controlled ad libitum diets with matched fat contents: high protein (HP) or low protein (LP). Body composition was assessed by DXA. Results: In the HP group, dietary‐protein intake increased from 91.4 g/d to a 6‐month intervention mean of 107.8 g/d (p < 0.05) and decreased in the LP group from 91.1 g/d to 70.4 g/d (p < 0.05). Total weight loss after 6 months was 8.9 kg in the HP group, 5.1 kg in the LP group, and none in the control group. After 6 months, bone mineral content (BMC) had declined by 111 ± 13 g (4%) in the HP group and by 85 ± 13 g (3%) in the LP group (not significant). Loss of BMC was more positively correlated with loss of body fat mass (r = 0.83; p < 0.0001) than with loss of body weight. Six‐month BMC loss, adjusted for differences in fat loss, was greater in the LP group than in the HP group [difference in LP vs. HP, 44.8 g (95% confidence interval, 16 to 73.8 g); p < 0.05]. Independent of change in body weight and composition during the intervention, highprotein intake was associated with a diminished loss of BMC (p < 0.01). Discussion: Body‐fat loss was the major determinant of loss of BMC, and we found no adverse effects of 6 months of high‐protein intake on BMC.     

Excretion/defecation patterns in Triatoma infestans populations that are,respectively, susceptible and resistant to deltamethrin

Pyrethroid resistance has been detected in Triatoma infestans (Klug) (Hemiptera: Reduviidae) specimens from different areas of Argentina and Bolivia. Genes conferring resistance can have a pleiotropic effect with epidemiological and evolutionary consequences. This research studied excretion/defecation patterns in deltamethrin‐resistant T. infestans in order to elucidate its biological performance, adaptive consequences and role in the transmission of Chagas' disease. One deltamethrin‐susceptible strain and two deltamethrin‐resistant strains were used. Fifth‐instar nymphs were fed ad libitum and their defecations recorded during and after the first or second feeding in the stadium. Resistant insects began to defecate later, defecated less, showed a lower proportion of defecating individuals and lower defecation indices compared with susceptible insects during the first hour after feeding. The number of bloodmeals in the stadium did not affect the main variables determining the pattern of defecation. The present study suggests that alterations in the excretion/defecation pattern in resistant insects entail an adaptive cost and, considering only this pattern, determine a lower capacity for transmission of Trypanosoma cruzi (Kinetoplastida: Trypanosomatidae) compared with susceptible insects.     

Superfluous killing in spiders: a consequence of adaptation to food-limited environments?

The hypothesis that superfluous killing, partial consumption,and abandonment of prey is a consequence of adaptation to food-limited environments was tested in two feeding trials on a desert spider, Agelenopsis aperta. First, we made comparisons among populations inhabiting sites of high prey (HP) or low prey (LP) availabilitythat differed in their degree of genetic isolation. Typically,A. aperta entirely consumed one or two of the prey items itcaptured in a feeding bout. Additional prey were partiallyconsumed or abandoned without eating. Spiders from the geneticallyisolated HP population, however, captured fewer prey and showeda higher incidence of full feeding on prey than did individualsfrom the other populations. Only one spider from this populationcaptured a prey item that it failed to feed on, whereas spidersfrom LP populations failed to feed on high numbers of capturedprey. The greatest variability in feeding behavior was exhibitedin the HP population that experienced gene flow. The secondtest was based on the finding that aggressiveness is largelya sex-linked trait in A. aperta: the aggressiveness of thefemale parent only is inherited by male offspring, whereasboth parents contribute to this trait in female offspring.All female F₁ hybrids between LP and HP parental types exhibitedhigh levels of superfluous killing, as did male F₁ hybridsderived from LP females. F₁ hybrid males derived from HP femalesexhibited extremely low levels of superfluous killing. Superfluouskilling thus has its basis in the genetic control of levelsof aggression.     

Effect of mating combinations on the host parasitisation and sex allocation in solitary endoparasitoid,Aenasius arizonensis (Hymenoptera: Encyrtidae)

Aenasius arizonensis (Girault) is an important solitary endoparasitoid of Phenacoccus solenopsis Tinsley. To optimise the mass production of high-quality females, it is important to assess the influence of mating regimes on the progeny fitness and sex allocation. We, therefore, hypothesise that mating combinations in A. arizonensis adults emerged from different host instars may influence parasitism and sex allocation in the subsequent generation. Therefore, we compared three nymphal instars (1st, 2nd and 3rd) and adults host stages of P. solenopsis for parasitism and sex allocation by A. arizonensis. Further, F₁ female progeny of the parasitoid emerged from different host instars was henceforth evaluated for its fitness in six mating combinations. A. arizonensis females parasitised all the host stages except the 1st instar nymphs. The parasitised 2nd instar nymphs yielded only males, while the sex ratio in the later host instars was strongly female-biased. The parasitoid females preferred 3rd instar nymphs with respect to higher parasitism (74.0–84.0%) and produced more females in the F₁ progeny as compared to other host stages. F₁ females that emerged from 3rd instar nymphs produced significantly higher parasitism (74.0–79.0%). These mating combinations also yielded more female progeny in the F₂ generation. However, parasitism by F₁ females was significantly lower (9.0–12.0%) when mated with males that emerged from 2nd instar P. solenopsis nymphs. Moreover, latter combinations yielded only male progeny in F₂ generation. These findings can be used in laboratory mass rearing of this parasitoid vis-à-vis biological control of P. solenopsis.     

Stage-dependent feeding behavior by western tarnished plant bugs influences flower bud abscission in cotton

Unexplained variability in the relationship between the number of herbivores in a field and the amount of crop damage can arise if there is a large amount of variation among herbivore individuals in the amount of feeding damage each generates. In California, populations of the western tarnished plant bug, Lygus hesperus Knight (Heteroptera: Miridae), produce highly variable levels of damage to cotton plants (Gossypium hirsutum L.) (Malvaceae), even when found at low densities. Because L. hesperus populations are also highly variable in their overall stage structure, we hypothesize that differences in crop damage might result from varying impact by each L. hesperus stage on cotton flower buds (termed squares). Laboratory measurements of L. hesperus mouth‐parts and distance to anther sacs, a preferred feeding site, revealed that 1st?3rd instar L. hesperus nymphs will not be able to feed on anther sacs of larger squares (over 8 mm in length) but will be able to feed on squares that are most sensitive to L. hesperus damage (<7 mm). Because even the 1st instars can feed on the most sensitive ‘pinhead’ squares, size constraints do not rule out damaging effects from the youngest L. hesperus. Laboratory observations revealed that later developmental stages, and adults, spend more time feeding on cotton squares relative to 2nd and 3rd instars. In addition, a field experiment revealed no effect of 2nd instars on square retention (relative to control cages) but did reveal a significant decrease in square retention generated by adult L. hesperus (4th instar L. hesperus resulted in an intermediate level of square retention). In a final study we sampled L. hesperus stage structure and density across 38 cotton fields. Multiple regression revealed that the densities of 1st?3rd instars of L. hesperus are not correlated with anther sac damage or square retention. However, in 2 years 4th and 5th instars were positively correlated with anther sac damage and negatively correlated with square retention. In the a third year, adult L. hesperus showed correlations in the same direction, across fields and across sites within fields. Overall, these results suggest that the adults and the largest nymphs of L. hesperus (4th and 5th instars) are particularly damaging to cotton squares, with the 1st?3rd instars of L. hesperus causing little damage to plants.     

An adaptive benefit of facultative coprophagy in the German cockroach Blattella germanica

1. A possible adaptive benefit of coprophagy was investigated in nymphs of the German cockroach Blattella germanica (L.). 2. Newly ecdysed first instars, given no source of food other than conspecific faeces, survived significantly longer than first instars deprived of faeces. The faeces of adult males and females may be of different quality, however, because nymphs given female faeces were more likely to moult into the second stadium than nymphs given male faeces. 3. In contrast to first instars, second instars provided adult faeces survived only slightly longer than starved counterparts. Faecal feeding is therefore stage‐specific, as is the benefit derived from it. 4. The relationship between the nutrient composition of faeces and the survival of nymphs was also examined. First instars fed the faeces of adults that had been maintained on a high (50%) protein diet, died more slowly than first instars fed the faeces of adults that had been maintained on medium (22.5%) and low (5%) protein diets. Chemical analysis of faeces showed that the concentration of protein in adult faeces increased with the level of protein in the diet. Moreover, food choice assays showed that first instars, unlike adults, ingested more of the high‐protein diets. 5. These data support the idea that coprophagy is a stage‐specific adaptive behaviour that permits first instars to moult into the second stadium with minimal foraging.     

Identifying the limitations for growth in low performing piglets from birth until 10 weeks of age

The evolution of hyper-prolific pig breeds has led to a higher within-litter variation in birth weight and in BW gain during the nursery phase. Based on an algorithm developed in previous research, two populations from a pool of 368 clinically healthy piglets at 6 weeks of age were selected: a low (LP) and a high (HP) performing population and their development was monitored until the end of the nursery phase (10 weeks of age). To understand the cause of the variation in growth between these populations we characterized the LP and HP piglets in terms of body morphology, behaviour, voluntary feed intake, BW gain, and apparent total tract and ileal nutrient digestibility. Piglets were housed individually and were fed a highly digestible diet. At selection, 6 weeks of age, the BW of LP and HP piglets were 6.8±0.1 and 12.2±0.1 kg, respectively. Compared with the LP piglets the HP piglets grew faster (203 g/day), ate more (275 g/day) from 6 to 10 weeks of age and were heavier at 10 weeks (30.0 v. 18.8 kg, all P<0.01). Yet, the differences in average daily gain and average daily feed intake disappeared when compared per kg BW^0.75. Assuming similar maintenance requirements per kg BW^0.75 the efficiency of feed utilization above maintenance was 0.1 g/g lower for the LP piglets (P=0.09).The gain : feed ratio was similar for both groups. LP piglets tended to take more time to touch a novel object (P=0.10), and spent more time eating (P<0.05). At 10 weeks, LP piglets had a higher body length and head circumference relative to BW (P<0.01). Relative to BW, LP had a 21% higher small intestine weight; 36% longer length, and relative to average FI, the small intestinal weight was 4 g/kg higher (both P=<0.01). Apparent total tract and ileal dry matter, N and gross energy digestibility were similar between groups (P>0.10). We concluded that the low performance of the LP piglets was due to their inability to engage compensatory gain or compensatory feed intake as efficiency of nutrient utilization and feed intake per kg BW^0.75 was unaffected. LP piglets tend to be more fearful towards novel objects. The morphological comparisons, increased body length and head circumference relative to BW imply that LP piglets have an increased priority for skeletal growth.     

An analysis of the feeding behavior of three stages of Toxoptera citricida by DC electrical penetration graph waveforms

With the purpose of studying the feeding behavior of the brown citrus aphid pest, Toxoptera citricida (Kirkaldy) (Hemiptera: Aphididae), we compared stylet probing behaviors of third and fourth instars and adults on Citrus unshiu Marc (Rutaceae) seedlings using the electrical penetration graph (EPG) technique. EPG waveforms exhibited the full suite of stylet behaviors – stylet pathway, intracellular stylet puncture, phloem salivation (E1), sieve ingestion (E2), and xylem sap ingestion activities, plus the non‐penetration (Np) waveform. Before the phloem phase, the number of probes was significantly higher for third‐instar nymphs than for adults. Overall duration of Np events by adults was significantly lower than the duration of third and fourth instars. The number of short probes of the fourth instars was significantly higher than that of the adults. In the phloem phase, adults made more frequent and longer E1 events than the third and fourth instars. Third instars made more frequent but shorter E2 events, whereas adults made fewer but longer events. These results showed adults gained nutrients by increasing feeding time during phloem ingestion. Thus, the probability of phloem‐associated virus acquisition and transmission of T. citricida was higher in adults than in nymphs.     

Phosphorus availability mediates plasticity in life-history traits and predator–prey interactions in Daphnia

We analysed growth plasticity of two Daphnia pulex clones under low‐phosphorus (LP) and high phosphorus (HP) conditions, in the presence of Chaoborus kairomones to examine how food quality (P‐availability) might impact life‐history responses and vulnerability to predation. Overall, clone 1 grew faster, and was larger at maturity. Under HP, both clones responded to kairomones by increasing growth, age and size at maturity, and decreasing fecundity. Under LP, both clones suffered reduced growth, and fecundity. However, the magnitude of response to kairomones depended on a clone by P‐availability interaction. Chaoborus presented a 1 : 1 clonal mixture under HP or LP, consumed more individuals under LP. Moreover, fewer clone 1 individuals were consumed. Studying the effects of P‐availability on life histories, and predator–prey interactions may help us understand the mechanisms generating and maintaining plasticity, as well as influencing genotypic diversity and microevolutionary processes in natural populations.     

Developmental response of the brown-winged green bug, Plautia stali (Heteroptera: Pentatomidae), to food shortage

To study developmental response of the brown-winged green bug, Plautia stali Scott, to food shortage we reared nymphs under restricted feeding conditions produced by shortening the feeding period after molt or withholding food from second to fifth instars. For second instars, molting rates were significantly reduced as the feeding period was shortened. Shortening the feeding periods for third to fifth instars also reduced molting rates, but less so; some nymphs were able to complete their developments even if no food was given to any of the instars. Compared with controls for which feeding was not restricted, nymphs that successfully reached the next instar had reduced postmolt body size as a result of restricted feeding for all instars except the fifth (2-day feeding for second instar, and 1-day feeding for third and fourth instars), whereas instar duration was unchanged or only slightly prolonged for all instars. These results suggest that only nymphs with nutritional accumulation over a specific threshold in each instar can progress to the next instar, and that, particularly for second to fourth instars, nymphs develop on schedule without prolonging instar durations to compensate for reduced growth under conditions of food shortage.     

Estimating potential stylet penetration of southern green stink bug – a mathematical modeling approach

Southern green stink bugs, Nezara viridula (L.) (Hemiptera: Pentatomidae), and related species are significant pests of cotton, Gossypium hirsutum L. (Malvaceae), in the USA Cotton Belt. Using their stylets, adults introduce pathogens of cotton into cotton bolls, and preliminary data indicate nymphs can also ingest these pathogens. However, data are lacking regarding stylet penetration potential of N. viridula nymphs, and records of stylet penetration by adults are typically determined after damage has occurred. In this study, rostral segments of all developmental stages of N. viridula were measured to estimate potential stylet penetration depth using a novel mathematical model. Overall mean stylet penetration estimates for all stages ranged from 135.3 μm for first instars to 2 389.3 μm for adult females. Potential stylet penetration significantly increased as the insect progressed through nymphal stages. Penetration was also significantly affected by insect posture while feeding. Overall minimum and maximum observed lengths of rostrum ranged from 835.3 μm (first instars) to 7 088.2 μm (adult females), and mean rostral lengths were significantly different between all stages. This report establishes conservative baselines of potential stylet penetration depths by all nymphal stages and both adult sexes of N. viridula. Additionally, the model presented here can be used to estimate potential stylet penetration for other Hemiptera and closely related insects with similar modes of feeding. In conjunction with crop phenology data, accurate estimates of potential stylet penetration will allow more proactive approaches to pest management in a wide range of high‐value cash crops affected worldwide by N. viridula.     

Laboratory assessment of the potential of Paranosema locustae to control immature stages of Schistocerca gregaria and Oedaleus senegalensis and vertical transmission of the pathogen in host populations

We tested the effects of Paranosema locustae spores in wheat bran formulation on the immature stages of Schistocerca gregaria and Oedaleus senegalensis under laboratory conditions. Younger instars were the most sensitive to the pathogen. While 100% infection was recorded in younger instar nymphs, older instars were less sensitive, with 16–27% of the inoculated nymphs remaining uninfected at the end of the experiment. Mortality of each instar increased with increased spore concentration. Immature survival time was significantly reduced by the pathogen and none of the nymphs inoculated as first, second, and third instar nymphs developed to adulthood (6–30% and 55–74% of nymphs inoculated as fourth and fifth instar, respectively). Sublethal effects such as delayed host growth, reduced host size, and abnormal wing and leg development (37% of emerging adults) were noted. Almost half the infected adults showed morphological abnormalities at emergence. Moreover, infection in S. gregaria and O. senegalensis by P. locustae did not affect female oviposition. However, 60% of S. gregaria and 52% of O. senegalensis progeny clearly showed infection by P. locustae with infection intensity of 1.08±0.27×10¹ and 1.19±0.32×10² spores/nymph, respectively. In view of the mortality rates, immature survival, host growth, and abnormal development in the P. locustae treatments, and the high prevalence of the pathogen in offspring from infected parents, it can be expected that the reduction in the impact of the two acridid species in the field will be considerable.     

Feeding of Perla grandis nymphs (Plecoptera: Perlidae) in an Apennine first order stream (Rio Berga,NW Italy)

Feeding habits of Perla grandis (Rambur) nymphs have been investigated in the Rio Berga, an Apenninic stream of Northwestern Italy. In this study, we analysed gut contents of 50 nymphs of this species, with the aim to investigate feeding preferences. Nymphs were collected from a single riffle, of which benthic coenosis was also determined. We detected a change in the diet during ontogenesis, with small instars feeding mainly on detritus and large instars strictly carnivorous. We also detected the existence of an evident trophic selection: diet was almost entirely dominated by Chironomidae, independently from their availability on the substratum. This finding is discussed on the basis of ecological and ethological considerations.     

Bemisia tabaci (Homoptera: Aleyrodidae) instar effects on rate of parasitism by Eretmocerus mundus and Encarsia pergandiella (Hymenoptera: Aphelinidae)

Studies were conducted to compare preference among Bemisia tabaci Gennadius, biotype B instars for parasitization by Eretmocerus mundus Mercet and Encarsia pergandiella Howard when provided one instar only, two different instars, and four different instars simultaneously. In the single‐instar no choice treatment, Er. mundus was more successful in parasitizing the younger host instars, while En. pergandiella parasitized a greater proportion of the older instars. Similar results were observed when parasitoids were provided a choice of two instars in six different pair combinations. When all four instars were provided simultaneously, the numbers of first, second, and third instars parasitized by Er. mundus were not significantly different from each other (range 10.3–16.4%), but all were significantly higher than parasitism of fourth instar nymphs (2.1%). The highest percentage parasitization by En. pergandiella was in third instar (17.2%), and the lowest in first instar (2.8%).     

Life-cycle and host specificity of Amblyomma tigrinum (Acari: Ixodidae) under laboratory conditions

Biological data of three generations of Amblyomma tigrinum in the laboratory are reported and the suitability of different host species for immature ticks are compared. Grouping the three generations, infestations by both the larval and nymphal stages were performed on chickens (Gallus gallus), wistar rats (Rattus norvegicus), rabbits (Oryctolagus cuniculus),wild mice (Calomys callosus), dogs (Canis familiaris) and opossums (Didelphis albiventris). Only dogs were used for infestations by adult ticks. Tick developmental periods were observed in an incubator at 27°C and RH 90%. The proportion of engorged larvae recovered from chickens (21.7% of the exposed larvae) was significantly larger (p<0.001) than those from the five mammal species used in the infestations (maximum of 3.1%). A significant larger (p<0.01) proportion of engorged larvae successfully molted after being fed on chickens than on mammal hosts. The proportion of engorged nymphs recovered from chickens (28.8% of the exposed nymphs) was significantly larger (p<0.001) than those from mammal hosts (range: 0–2.1%). Larvae showed similar feeding periods on exposure to different host species, except for those larvae fed on C. callosus, which showed significantly longer (p<0.001) feeding periods. Engorged larvae detachment peaked on the 5^th feeding day, followed by the 6^th day, on all hosts except for C. callosus. Larval premolt periods were similar for engorged ticks exposed to different host species, except for larvae fed on dogs, which showed significantly longer (p<0.001) premolt periods. Host detachment of engorged nymphs peaked on the 6^th feeding day on chickens. Although nymphal detachment on rats peaked on the 8^th day, only 15 nymphs were recovered from this host species. In a sample of 144 F₃ nymphs fed on chickens no significant difference (p>0.10) was found between the feeding or premolt periods of 82 males and 62 females, but female nymphs were significantly heavier (p<0.005) than male nymphs. Sixteen engorged females (61.5% of the exposed ticks) were recovered after being fed on dogs, and all these females laid viable eggs. Chickens, the only avian host, were the most suitable host when compared with the five mammal species. Dogs were demonstrated to be a suitable host for adults of A. tigrinum, which is consistent with, several reports of adult A. tigrinum ticks parasitizing dogs in different areas of South America. Our results reinforce that in these same areas avian species are the major hosts for immature stages of this tick species. This revised version was published online in July 2006 with corrections to the Cover Date.