Functional response of Telenomus remus Nixon(Hymenoptera, Scelionidae) to Spodoptera frugiperda (J. E. Smith) (Lepidoptera, Noctuidae) eggs: effect of female age

Functional response of Telenomus remus Nixon (Hymenoptera, Scelionidae) to Spodoptera frugiperda (J. E. Smith) (Lepidoptera, Noctuidae) eggs: effect of female age. Functional response of 24-h and 48-h-old Telenomus remus adults was studied on Spodoptera frugiperda eggs. The study was carried out in climatic chamber regulated at 25 ± 1°C, 70 ± 10% RH and 12:12h (L: D). Females of T. remus were honey fed and individualized in glass vials along with 25, 50, 75, 100, 150, 200, 250 or 300 eggs of S. frugiperda for 24 h. Complete randomized design with ten replications was adopted. The parameters evaluated to construct the functional response curve were daily average parasitism, searching rate and oviposition time. It was observed that the higher the egg density, the higher the parasitism for 24-h and 48-h-old females although there was a tendency of parasitism stabilization at 150-egg density. The results showed a type II functional response curve for both 24-h and 48-h-old female.

Searching efficiency and the functional response of a pause-travel forager

1. The feeding rates of many predators and parasitoids exhibit type II functional responses, with a decelerating rate of increase to reach an asymptotic value as the density of their prey or hosts increases. Holling's disc equation describes such relationships and predicts that the asymptotic feeding rate at high prey densities is set by handling time, while the rate at which feeding rate increases with increased prey density is determined by searching efficiency. Searching efficiency and handling time are also parameters in other models which describe the functional response. Models which incorporate functional responses in order to make predictions of the effects of food shortage thus rely upon a clear understanding and accurate quantification of searching efficiency and handling time. 2. Blackbird Turdus merula exhibit a type II functional response and use pause-travel foraging, a foraging technique in which animals search for prey while stationary and then move to capture prey. Pause-travel foraging allows accurate direct measurement of feeding rate and both searching efficiency and handling time. We use Blackbirds as a model species to: (i) compare observed measures of both searching efficiency and handling time with those estimated by statistically fitting the disc equation to the observed functional response; and (ii) investigate alternative measures of searching efficiency derived by the established method where search area is assumed to be circular and a new method that we propose where it is not. 3. We find that the disc equation can adequately explain the functional response of blackbirds feeding on artificial prey. However, this depends critically upon how searching efficiency is measured. Two variations on the previous method of measuring search area (a component of searching efficiency) overestimated searching efficiency, and hence predicted feeding rates higher than those observed. Two variations of our alternative approach produced lower estimates of searching efficiency, closer to that estimated by fitting the disc equation, and hence more accurately predicted feeding rate. Our study shows the limitations of the previous method of measuring searching efficiency, and describes a new method for measuring searching efficiency more accurately.

Predicting the functional response of a farmland bird

1. Mechanistic models may be able to predict how changes in agricultural practice influence farmland bird populations. A key component of these models is the link between food and competitor densities and the rate at which birds consume food, i.e. the functional response. 2. This paper tests whether the functional response of a farmland bird, the rook Corvus frugilegus, can be predicted from three parameters: searching speed, food detection distance and handling time. It is often difficult to measure the functional response of farmland birds directly, but it may be possible to measure behavioural parameters more quickly. 3. We performed experiments in which rooks fed on a range of artificial food densities in two grass sward heights. Food detection distance was greater in the shorter sward, but sward height did not influence searching speed or handling time. The functional response could be accurately predicted in both sward heights. 4. We show that the functional response of a farmland bird can be predicted from parameters that can be measured more quickly than the alternative of measuring the functional response directly. This implies that the functional responses of other farmland birds may be predicted using a minimum of information.

Functional response of Telenomus remus Nixon(Hymenoptera, Scelionidae) to Spodoptera frugiperda (J. E. Smith) (Lepidoptera, Noctuidae) eggs: effect of female age

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Cannibalistic behavior and functional response in Chrysomya albiceps (Diptera : Calliphoridae)

Chrysomya albiceps is a facultative predator and cannibal species during the larval stage. Very little is known about cannibalism and prey size preference, especially in blowflies. The purpose of this investigation was to determine the influence of prey size and larval density on cannibalism by third-instar larvae of C. albiceps under laboratory conditions. Our results indicate that no cannibalism occurs by third-instar larvae on first- and second-instar larvae, but third-instar larvae do eat second-instar larvae. The functional response on second-instar larvae is consistent with Holling type II. The consequences of consuming second-, compared to first- or third-, instar larvae as well as the implications of cannibalism for the population dynamics of C. albiceps are discussed.

The effect of hunger level on predation dynamics in the spider Nesticodes rufipes: a functional response study

It is well known that a predator has the potential to regulate a prey population only if the predator responds to increases in prey density and inflicts greater mortality rates. Predators may cause such density-dependent mortality depending on the nature of the functional and numerical responses. As spiders are usually faced with a shortage of prey, the killing behavior of the spider Nesticodes rufipes at varying densities of Musca domestica was examined here through laboratory functional response experiments where spiders were deprived of food for 5 (well-fed) or 20 days (hungry). An additional laboratory experiment was also carried out to assess handling time of spiders. The number of prey killed by spiders over 24- and 168-h periods of predator-prey interaction was recorded. Logistic regression analyses revealed the type II functional response for both well-fed and hungry spiders. We found that the lower predation of hungry spiders during the first hours of experimentation was offset later by an increase in predation ( explained by estimated handling times), resulting in similarity of functional response curves for well-fed and hungry spiders. It was also observed that the higher number of prey killed by well-fed spiders over a 24- h period of spider-prey interaction probably occurred due to their greater weights than hungry spiders. We concluded that hungry spiders may be more voracious than well-fed spiders only over longer time periods, since hungry spiders may spend more time handling their first prey items than well-fed spiders.

The role of habitat heterogeneity for the functional response of the spider Nesticodes rufipes (Araneae : Theridiidae) to houseflies

We investigated whether or not different degrees of refuge for prey influence the characteristic of functional response exhibited by the spider Nesticodes rufipes on Musca domestica, comparing the inherent ability of N. rufipes to kill individual houseflies in such environments at two distinct time intervals. To investigate these questions, two artificial habitats were elaborated in the laboratory. For 168 h of predator-prey interaction, logistic regression analyses revealed a type 11 functional response, and a significant decrease in prey capture in the highest prey density was observed when habitat complexity was increased. Data from habitat 1 (less complex) presented a greater coefficient of determination than those from habitat 2 (more complex), indicating a higher variation of predation of the latter. For a 24 h period of predator-prey interaction, spiders killed significantly fewer prey in habitat 2 than in habitat 1. Although prey capture did not enable data to fit properly in the random predator equation in this case, predation data from habitat 2 presented a higher variation than data from habitat 1, corroborating results from 168 h of interaction. The high variability observed on data from habitat 2 (more complex habitat) is an interesting result because it reinforces the importance of refuge in promoting spatial heterogeneity, which can affect the extent of predator-prey interactions.

Consequences of refuge for the functional response of Dermestes ater (Coleoptera : Dermestidae) to Musca domestica (Diptera : Muscidae)

It is well known that a predator has the potential to regulate a prey population only if the predator responds to increases in prey density and inflicts greater mortality rates. Predators may cause such density-dependent mortality depending on the nature of the functional and numerical responses. Yet, few studies have examined the relationship between the addition of refuges and the characteristic of functional response fits. We investigated whether addition of a refuge changed the type of functional response exhibited by Dermestes ater on Musca domestica, comparing the inherent ability of D. ater to kill houseflies in the absence and in the presence of refuge. An additional laboratory experiment was also carried out to assess handling and searching times exhibited by D. ater. Logistic regression analyses revealed a type III functional response for predator-prey interaction without refuge, and results were described by the random predator equation. The mean number of prey killed did not differ between experimental habitats, indicating that the addition of refuge did not inhibit predation. However, predators that interacted with prey without refuge spent less time searching for prey at higher densities, increasing predatory interaction. We concluded that this interaction may be weak, because data from experiments with refuge fitted poorly to models. However, the high variability and the nonsignificance of the data from the experiment with refuge show the importance of refuge for promoting spatial heterogeneity, which may prevent prey extinction.

Dynamics of handling time and functional response by larvae of Chrysomya albiceps (Dipt., Calliphoridae) on different prey species

One way to understand the behavioural patterns exhibited by a predator in response to prey density is to evaluate its functional response. Such evaluation yields information about basic mechanisms of prey-predator dynamics, and is an essential component of prey-predator models. In this paper we analysed experimentally the functional response and the handling time spent by Chrysomya albiceps on different prey species and larval instars of blowflies. The type II functional response was observed when second instar larvae of Chrysomya megacephala and Chrysomya macellaria were consumed. The handling time spent by the predator was significantly different between instars and species. The implications of the functional response and handling time for the interaction dynamics of Brazilian Chrysomyinae species are discussed.

Mathematical analysis and simulations involving chemotherapy and surgery on large human tumours under a suitable cell-kill functional response

Dosage and frequency of treatment schedules are important for successful chemotherapy. However, in this work we argue that cell-kill response and tumoral growth should not be seen as separate and therefore are essential in a mathematical cancer model. This paper presents a mathematical model for sequencing of cancer chemotherapy and surgery. Our purpose is to investigate treatments for large human tumours considering a suitable cell-kill dynamics. We use some biological and pharmacological data in a numerical approach, where drug administration occurs in cycles (periodic infusion) and surgery is performed instantaneously. Moreover, we also present an analysis of stability for a chemotherapeutic model with continuous drug administration. According to Norton & Simon [22], our results indicate that chemotherapy is less eficient in treating tumours that have reached a plateau level of growing and that a combination with surgical treatment can provide better outcomes.

Functional response of Ulmus minor Mill. to drought, flooding and dutch elm disease

Ulmus minor es una especie arbórea originaria de Europa cuyas poblaciones han sido diezmadas por el hongo patógeno causante de la enfermedad de la grafiosis. La conservación de los olmos exige plantearse su propagación a través de plantaciones y conocer mejor su ecología y biología. Ulmus minor es un árbol de ribera, pero frecuentemente se encuentra alejado del cauce de arroyos y ríos, donde la capa freática sufre fuertes oscilaciones. Por ello, nuestra hipótesis general es que esta especie es moderadamente resistente tanto a la inundación como a la sequía. El principal objetivo de esta tesis doctoral es entender desde un punto de vista funcional la respuesta de U. minor a la inundación, la sequía y la infección por O. novo-ulmi; los factores que posiblemente más influyen en la distribución actual de U. minor. Con este objetivo se persigue dar continuidad a los esfuerzos de conservación de esta especie que desde hace años se dedican en varios centros de investigación a nivel mundial, ya que, entender mejor los mecanismos que contribuyen a la resistencia de U. minor ante la inoculación con O. novo-ulmi y factores de estrés abiótico ayudará en la selección y propagación de genotipos resistentes a la grafiosis. Se han planteado tres experimentos en este sentido. Primero, se ha comparado la tolerancia de brinzales de U. minor y U. laevis – otro olmo ibérico – a una inmersión controlada con el fin de evaluar su tolerancia a la inundación y comprender los mecanismos de aclimatación. Segundo, se ha comparado la tolerancia de brinzales de U. minor y Quercus ilex – una especie típica de ambientes Mediterránea secos – a la falta de agua en el suelo con el fin de evaluar el grado de tolerancia y los mecanismos de aclimatación a la sequía. El hecho de comparar dos especies contrastadas responde al interés en entender mejor cuales son los procesos que conducen a la muerte de una planta en condiciones de sequía – asunto sobre el que hay una interesante discusión desde hace algunos años. En tercer lugar, con el fin de entender mejor la resistencia de algunos genotipos de U. minor a la grafiosis, se han estudiado las diferencias fisiológicas y químicas constitutivas e inducidas por O. novo-ulmi entre clones de U. minor seleccionados a priori por su variable grado de resistencia a esta enfermedad. En el primer experimento se observó que los brinzales de U. minor sobrevivieron 60 días inmersos en una piscina con agua no estancada hasta una altura de 2-3 cm por encima del cuello de la raíz. A los 60 días, los brinzales de U. laevis se sacaron de la piscina y, a lo largo de las siguientes semanas, fueron capaces de recuperar las funciones fisiológicas que habían sido alteradas anteriormente. La conductividad hidráulica de las raíces y la tasa de asimilación de CO2 neta disminuyeron en ambas especies. Por el contrario, la tasa de respiración de hojas, tallos y raíces aumentó en las primeras semanas de la inundación, posiblemente en relación al aumento de energía necesario para desarrollar mecanismos de aclimatación a la inundación, como la hipertrofia de las lenticelas que se observó en ambas especies. Por ello, el desequilibrio del balance de carbono de la planta podría ser un factor relevante en la mortalidad de las plantas ante inundaciones prolongadas. Las plantas de U. minor (cultivadas en envases de 16 litros a media sombra) sobrevivieron por un prolongado periodo de tiempo en verano sin riego; la mitad de las plantas murieron tras 90 días sin riego. El cierre de los estomas y la pérdida de hojas contribuyeron a ralentizar las pérdidas de agua y tolerar la sequía en U. minor. Las obvias diferencias en tolerancia a la sequía con respecto a Q. ilex se reflejaron en la distinta capacidad para ralentizar la aparición del estrés hídrico tras dejar de regar y para transportar agua en condiciones de elevada tensión en el xilema. Más relevante es que las plantas con evidentes síntomas de decaimiento previo a su muerte exhibieron pérdidas de conductividad hidráulica en las raíces del 80% en ambas especies, mientras que las reservas de carbohidratos apenas variaron y lo hicieron de forma desigual en ambas especies. Árboles de U. minor de 5 y 6 años de edad (plantados en eras con riego mantenido) exhibieron una respuesta a la inoculación con O. novo-ulmi consistente con ensayos previos de resistencia. La conductividad hidráulica del tallo, el potencial hídrico foliar y la tasa de asimilación de CO2 neta disminuyeron significativamente en relación a árboles inoculados con agua, pero solo en los clones susceptibles. Este hecho enlaza con el perfil químico “más defensivo” de los clones resistentes, es decir, con los mayores niveles de suberina, ácidos grasos y compuestos fenólicos en estos clones que en los susceptibles. Ello podría restringir la propagación del hongo en el árbol y preservar el comportamiento fisiológico de los clones resistentes al inocularlos con el patógeno. Los datos indican una respuesta fisiológica común de U. minor a la inundación, la sequía y la infección por O. novo-ulmi: pérdida de conductividad hidráulica, estrés hídrico y pérdida de ganancia neta de carbono. Pese a ello, U. minor desarrolla varios mecanismos que le confieren una capacidad moderada para vivir en suelos temporalmente anegados o secos. Por otro lado, el perfil químico es un factor relevante en la resistencia de ciertos genotipos a la grafiosis. Futuros estudios deberían examinar como este perfil químico y la resistencia a la grafiosis se ven alteradas por el estrés abiótico. ABSTRACT Ulmus minor is a native European elm species whose populations have been decimated by the Dutch elm disease (DED). An active conservation of this species requires large-scale plantations and a better understanding of its biology and ecology. U. minor generally grows close to water channels. However, of the Iberian riparian tree species, U. minor is the one that spread farther away from rivers and streams. For these reasons, we hypothesize that this species is moderately tolerant to both flooding and drought stresses. The main aim of the present PhD thesis is to better understand the functional response of U. minor to the abiotic stresses – flooding and drought – and the biotic stress – DED – that can be most influential on its distribution. The overarching goal is to aid in the conservation of this emblematic species through a better understanding of the mechanisms that contribute to resistance to abiotic and biotic stresses; an information that can help in the selection of resistant genotypes and their expansion in large-scale plantations. To this end, three experiments were set up. First, we compared the tolerance to experimental immersion between seedlings of U. minor and U. laevis – another European riparian elm species – in order to assess their degree of tolerance and understand the mechanisms of acclimation to this stress. Second, we investigated the tolerance to drought of U. minor seedlings in comparison with Quercus ilex (an oak species typical of dry Mediterranean habitats). Besides assessing and understanding U. minor tolerance to drought at the seedling stage, the aim was to shed light into the functional alterations that trigger drought-induced plant mortality – a matter of controversy in the last years. Third, we studied constitutive and induced physiological and biochemical differences among clones of variable DED resistance, before and following inoculation with Ophiostoma novo-ulmi. The goal is to shed light into the factors of DED resistance that is evident in some genotypes of U. minor, but not others. Potted seedlings of U. minor survived for 60 days immersed in a pool with running water to approximately 2-3 cm above the stem collar. By this time, U. minor seedlings died, whereas U. laevis seedlings moved out of the pool were able to recover most physiological functions that had been altered by flooding. For example, root hydraulic conductivity and leaf photosynthetic CO2 uptake decreased in both species; while respiration initially increased with flooding in leaves, stems and roots possibly to respond to energy demands associated to mechanisms of acclimation to soil oxygen deficiency; as example, a remarkable hypertrophy of lenticels was soon observed in flooded seedlings of both species. Therefore, the inability to maintain a positive carbon balance somehow compromises seedling survival under flooding, earlier in U. minor than U. laevis, partly explaining their differential habitats. Potted seedlings of U. minor survived for a remarkable long time without irrigation – half of plants dying only after 90 days of no irrigation in conditions of high vapour pressure deficit typical of summer. Some mechanisms that contributed to tolerate drought were leaf shedding and stomata closure, which reduced water loss and the risk of xylem cavitation. Obviously, U. minor was less tolerant to drought than Q. ilex, differences in drought tolerance resulting mostly from the distinct capacity to postpone water stress and conduct water under high xylem tension among species. More relevant was that plants of both species exhibited similar symptoms of root hydraulic failure (i.e. approximately 80% loss of hydraulic conductivity), but a slight and variable depletion of non-structural carbohydrate reserves preceding dieback. Five- and six-year-old trees of U. minor (planted in the field with supplementary watering) belonging to clones of contrasted susceptibility to DED exhibited a different physiological response to inoculation with O. novo-ulmi. Stem hydraulic conductivity, leaf water potential and photosynthetic CO2 uptake decreased significantly relative to control trees inoculated with water only in DED susceptible clones. This is consistent with the “more defensive” chemical profile observed in resistant clones, i.e. with higher levels of saturated hydrocarbons (suberin and fatty acids) and phenolic compounds than in susceptible clones. These compounds could restrict the spread of O. novo-ulmi and contribute to preserving the near-normal physiological function of resistant trees when exposed to the pathogen. These results evidence common physiological responses of U. minor to flooding, drought and pathogen infection leading to xylem water disruption, leaf water stress and reduced net carbon gain. Still, seedlings of U. minor develop various mechanisms of acclimation to abiotic stresses that can play a role in surviving moderate periods of flood and drought. The chemical profile appears to be an important factor for the resistance of some genotypes of U. minor to DED. How abiotic stresses such as flooding and drought affect the capacity of resistant U. minor clones to face O. novo-ulmi is a key question that must be contemplated in future research.

Intake rates and the functional response in shorebirds (Charadriiformes) eating macro-invertebrates

As field determinations take much effort, it would be useful to be able to predict easily the coefficients describing the functional response of free-living predators, the function relating food intake rate to the abundance of food organisms in the environment. As a means easily to parameterise an individual-based model of shorebird Charadriiformes populations, we attempted this for shorebirds eating macro-invertebrates. Intake rate is measured as the ash-free dry mass (AFDM) per second of active foraging; i.e. excluding time spent on digestive pauses and other activities, such as preening. The present and previous studies show that the general shape of the functional response in shorebirds eating approximately the same size of prey across the full range of prey density is a decelerating rise to a plateau, thus approximating the Holling type 11 ('disc equation') formulation. But field studies confirmed that the asymptote was not set by handling time, as assumed by the disc equation, because only about half the foraging time was spent in successfully or unsuccessfully attacking and handling prey, the rest being devoted to searching. A review of 30 functional responses showed that intake rate in free-living shorebirds varied independently of prey density over a wide range, with the asymptote being reached at very low prey densities (< 150/m(-2)). Accordingly, most of the many studies of shorebird intake rate have probably been conducted at or near the asymptote of the functional response, suggesting that equations that predict intake rate should also predict the asymptote. A multivariate analysis of 468 'spot' estimates of intake rates from 26 shorebirds identified ten variables, representing prey and shorebird characteristics, that accounted for 81 % of the variance in logarithm-transformed intake rate. But four-variables accounted for almost as much (77.3 %), these being bird size, prey size, whether the bird was an oystercatcher Haematopus ostralegus eating mussels Mytilus edulis, or breeding. The four variable equation under-predicted, on average, the observed 30 estimates of the asymptote by 11.6%, but this discrepancy was reduced to 0.2% when two suspect estimates from one early study in the 1960s were removed. The equation therefore predicted the observed asymptote very successfully in 93 % of cases. We conclude that the asymptote can be reliably predicted from just four easily measured variables. Indeed, if the birds are not breeding and are not oystercatchers eating mussels, reliable predictions can be obtained using just two variables, bird and prey sizes. A multivariate analysis of 23 estimates of the half-asymptote constant suggested they were smaller when prey were small but greater when the birds were large, especially in oystercatchers. The resulting equation could be used to predict the half-asymptote constant, but its predictive power has yet to be tested. As well as predicting the asymptote of the functional response, the equations will enable research workers engaged in many areas of shorebird ecology and behaviour to estimate intake rate without the need for conventional time-consuming field studies, including species for which it has not yet proved possible to measure intake rate in the field.