When bigger is not better: intraspecific competition for pollination increases with population size in invasive milkweeds

One of the essential requirements for an introduced plant species to become invasive is an ability to reproduce outside the native range, particularly when initial populations are small. If a reproductive Allee effect is operating, plants in small populations will have reduced reproductive success relative to plants in larger populations. Alternatively, if plants in small populations experience less competition for pollination than those in large populations, they may actually have higher levels of reproductive success than plants in large populations. To resolve this uncertainty, we investigated how the per capita fecundity of plants was affected by population size in three invasive milkweed species. Field surveys of seed production in natural populations of different sizes but similar densities were conducted for three pollinator-dependent invasive species, namely Asclepias curassavica, Gomphocarpus fruticosus and G. physocarpus. Additionally, supplemental hand-pollinations were performed in small and large populations in order to determine whether reproductive output was limited by pollinator activity in these populations. Reproductive Allee effects were not detected in any of the study species. Instead, plants in small populations exhibited remarkably high levels of reproductive output compared to those in large populations. Increased fruit production following supplemental hand-pollinations suggested that the lower reproductive output of naturally pollinated plants in large populations is a consequence of pollen limitation rather than limitation due to abiotic resources. This is consistent with increased intraspecific competition for pollination amongst plants in large populations. It is likely that the invasion of these milkweed species in Australia has been enhanced because plants in small founding populations experience less intraspecific competition for pollinators than those in large populations, and thus have the ability to produce copious amounts of seeds.     

Behavioral and electrophysiological analysis of general anesthesia in 3 background strains of Drosophila melanogaster

General anesthetics achieve behavioral unresponsiveness via a mechanism that is incompletely understood. The study of genetic model systems such as the fruit fly Drosophila melanogaster is crucial to advancing our understanding of how anesthetic drugs render animals unresponsive. Previous studies have shown that wild-type control strains differ significantly in their sensitivity to general anesthetics, which potentially introduces confounding factors for comparing genetic mutations placed on these wild-type backgrounds. Here, we examined a variety of behavioral and electrophysiological endpoints in Drosophila, in both adult and larval animals. We characterized these endpoints in 3 commonly used fly strains: wild-type Canton Special (CS), and 2 commonly used white-eyed strains, isoCJ1 and w¹¹¹⁸. We found that CS and isoCJ1 show remarkably similar sensitivity to isoflurane across a variety of behavioral and electrophysiological endpoints. In contrast, w¹¹¹⁸ is resistant to isoflurane compared to the other 2 strains at both the adult and larval stages. This resistance is however not reflected at the level of neurotransmitter release at the larval neuromuscular junction (NMJ). This suggests that the w¹¹¹⁸ strain harbors another mutation that produces isoflurane resistance, by acting on an arousal pathway that is most likely preserved between larval and adult brains. This mutation probably also affects sleep, as marked differences between isoCJ1 and w¹¹¹⁸ have also recently been found for behavioral responsiveness and sleep intensity measures.     

Is the effect of priming plants,and a functional JAR1, negligible on the foraging behaviour and development of a generalist lepidopteran,Helicoverpa armigera?

Theory and recent literature suggest strong effects of induced plant defences in some plant herbivore systems. Few have studied behavioural effects on intact plants. Differences in foraging behaviour as well as weight gain were determined for first instar Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) on Arabidopsis thaliana (L.) Heynh. (Brassicaceae) mutant and wild type plants, non‐primed, or primed by herbivore feeding or methyl jasmonate. The differences in feeding were primarily in the length of feeding time as opposed to the area fed on, feeding location, or frequency. More larvae dispersed from plants after priming by mite feeding than dispersed after caterpillar feeding. Other behavioural activities such as resting were not significantly affected. Early instars gained less weight feeding on ein2 (ethylene insensitive) mutant, but there was no difference in weight gain between larvae feeding on induced and non‐induced plants of the same type. We concluded that there are fitness consequences for neonates of the generalist H. armigera after feeding on induced plant tissues in some cases, and that distinct changes in behaviour are recognisable both at the fine scale and at grosser levels (dispersal). However, these changes are more subtle than might be expected.     

Fresh,frozen or fake: A comparison of predation rates measured by various types of sentinel prey

Arthropod predators and parasitoids support the health and functioning of the world's ecosystems, most notably by supplying biological control services to agricultural landscapes. Quantifying the impact that these organisms have on their prey can be challenging, as direct observation and measurement of arthropod predation is difficult. The use of sentinel prey is one method to measure predator impact; however, despite widespread use, few studies have compared predation on different prey types within a single experiment. This study evaluated the predation rates on four sentinel prey items in grass and wheat fields in south-east Queensland, Australia. Attack rates on live and dead Helicoverpa armigera eggs, and dead H. armigera larvae and artificial plasticine larvae, were compared and the predators that were attracted to each prey type were documented with the use of field cameras. There was no significant difference in predation rates between sentinel eggs, while dead larvae were significantly more attacked than artificial larvae. Prey were attacked by a diverse range of predators, including ants, beetles, various nymph and juvenile insects and small mammals. Different predators were active in grass and crop fields, with predator activity peaking around dawn and dusk. The same trends were observed within and between the two habitats studied, providing a measure of confidence in the sentinel prey method. A range of different sentinel prey types could be suitable for use in most comparative studies; however, each prey type has its own benefits and limitations, and these should be carefully evaluated to determine which is most suitable to address the research questions.     

Predicting population dynamics of weed biological control agents: science or gazing into crystal balls?

Abstract  Various factors can influence the population dynamics of phytophages post introduction, of which climate is fundamental. Here we present an approach, using a mechanistic modelling package (CLIMEX), that at least enables one to make predictions of likely dynamics based on climate alone. As biological control programs will have minimal funding for basic work (particularly on population dynamics), we show how predictions can be made using a species geographical distribution, relative abundance across its range, seasonal phenology and laboratory rearing data. Many of these data sets are more likely to be available than long-term population data, and some can be incorporated into the exploratory phase of a biocontrol program. Although models are likely to be more robust the more information is available, useful models can be developed using information on species distribution alone. The fitted model estimates a species average response to climate, and can be used to predict likely geographical distribution if introduced, where the agent is likely to be more abundant (i.e. good locations) and more importantly for interpretation of release success, the likely variation in abundance over time due to intra- and inter-year climate variability. The latter will be useful in predicting both the seasonal and long-term impacts of the potential biocontrol agent on the target weed. We believe this tool may not only aid in the agent selection process, but also in the design of release strategies, and for interpretation of post-introduction dynamics and impacts. More importantly we are making testable predictions. If biological control is to become more of a science making and testing such hypothesis will be a key component.     

Detrimental effects of latex and cardiac glycosides on survival and growth of first-instar monarch butterfly larvae Danaus plexippus feeding on the sandhill milkweed Asclepias humistrata

1. A novel experimental method was developed to study negative physical and chemical effects of latex and cardiac glycosides on first‐instar monarch butterfly larvae in their natural environment in north central Florida. Forceps were used to nibble through the petioles of leaves of the sandhill milkweed Asclepias humistrata, mimicking the behaviour of mature monarch larvae. This notching cut off the supply of latex to the leaves without significantly reducing either their cardiac glycoside concentration or water content. 2. The mean cardiac glycoside concentration in larvae that fed on intact leaves was nearly two and a half times greater than in larvae that fed on notched leaves. This was probably because more latex is present in the gut of the larvae that fed on the intact leaves. Supporting this is the fact that the mean concentration of cardiac glycosides in the latex was 34–47 times that in the leaves. 3. Wet weights, dry weights, and growth rates of first‐instar larvae that fed on intact leaves over a 72‐h period were less than half those of larvae that fed on notched leaves. 4. Mortality due to miring in the latex was 27% on the intact leaves compared with 2% on the notched leaves. 5. Latex, cardiac glycosides, and other as yet undetermined plant factors all have a negative effect on first‐instar larval survival. 6. Video‐analyses indicated that ingestion of latex caused the larvae to become cataleptic and increased their chances of being mired on the leaf by the setting latex glue. Dysfunction resulting from latex ingestion may lead to the larvae falling off the plant and being killed by invertebrate predators. 7. The difficulty of neonate monarch larvae surviving on A. humistrata – one of the principal milkweed species fed on each spring as monarchs remigrate from Mexico into the southern U.S.A. – is evidence that a co‐evolutionary arms race is operating in this plant–herbivore system.     

It’s the first bites that count: Survival of first‐instar monarchs on milkweeds

Mortality of first instars is generally very high, but variable, and is caused by many factors, including physical and chemical plant characters, weather and natural enemies. Here, a summary of detailed field‐based studies of the early‐stage survival of a specialist lepidopteran herbivore is presented. First‐instar larvae of the monarch butterfly, Danaus plexippus, a milkweed specialist, generally grew faster and survived better on leaves when latex flow was reduced by partial severance of the leaf petiole. The outcome depended on milkweed species, and was related to the amount of latex produced, as well as other plant characters, such as leaf hairs, microclimate and concentration of secondary metabolites. Even for a so‐called ‘milkweed specialist’, larval performance and survival appears to be related to the concentration of cardenolides produced by the plants (a potential chemical defence against herbivory). This case study of monarchs and milkweeds highlights the need for field‐based experiments to assess the effect of plant characters on the usually poor survival of early instar phytophagous insects. Few similar studies concerning the performance and survival of first‐instar, eucalypt‐specific herbivores have been conducted, but this type of study is considered essential based on the findings obtained using D. plexippus.