The impact of Lygus bugs on cotton yield is poorly understood. Growers have long observed that there is a large amount of unexplained variability between the number of Lygus observed in a cotton field and the amount of damage the crop sustains. We have begun to investigate this phenomenon by asking the question: can differences in age structure and sex ratio among Lygus populations affect accuracy and/or damage to cotton squares? In 2002 we documented a wide range of age structures (adults versus nymphs) and sex ratios (males versus females) across ten different Lygus populations in California. Experiments using whole cage sampling allowed us to examine the efficiency of sweep for assessing the absolute age structures and sex ratios of these populations. Results suggest that sweep samples do predict Lygus presence, but that they are more efficient in capturing adults versus nymphs. The underestimation of nymphs due to sweep bias presents a problem for Lygus management because nymphs feed on squares as much as (or more than) adults. Sweep nets were also more efficient in capturing adult males versus adult females. This underestimation of female adults may pose a similar problem for Lygus management because adult female Lygus spend more time feeding on squares relative to adult males. Adult female density was positively correlated with egg density, suggesting that the propensity of sweep nets to favor adult males could be an additional problem for predicting future generations of Lygus in a field.
Abstract 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.
Abstract We reviewed the literature on aphid parasitoids to determine the occurrence, nature and outcome of intraguild interactions. Intraguild interactions were described for larval, pupal and adult aphid parasitoids and by the type of natural enemy (fungus, predator, or parasitoid). They appear to be prevalent in most aphid parasitoid systems and, except for parasitoid‐parasitoid interactions, they are mostly asymmetric, with aphidophagous predators and pathogens killing parasitoids. The limited experimental evidence from field studies is insufficient to provide a comprehensive pattern of the consequences of intraguild interactions for aphid parasitoid populations in general and, more specifically, for the efficacy of biological control. However, because intraguild interactions are widespread in aphid‐natural enemy communities and mostly detrimental to aphid parasitoids, we conclude that intraguild interactions have a primary effect in driving fluctuations in aphid parasitoid populations. Drawing on case studies, we further argue that intraguild interactions can substantially alter the effectiveness of aphid parasitoids as biological‐control agents.
Abstract Cannibalism, once viewed as a rare or aberrant behavior, is now recognized to be widespread and to contribute broadly to the self‐regulation of many populations. Cannibalism can produce endogenous negative feedback on population growth because it is expressed as a conditional behavior, responding to the deteriorating ecological conditions that flow, directly or indirectly, from increasing densities of conspecifics. Thus, cannibalism emerges as a strongly density‐dependent source of mortality. In this synthesis, we review recent research that has revealed a rich diversity of pathways through which rising density elicits increased cannibalism, including both factors that (a) elevate the rate of dangerous encounters between conspecifics and (b) enhance the likelihood that such encounters will lead to successful cannibalistic attacks. These pathways include both features of the autecology of cannibal populations and features of interactions with other species, including food resources and pathogens. Using mathematical models, we explore the consequences of including density‐dependent cannibal attack rates on population dynamics. The conditional expression of cannibalism generally enhances stability and population regulation in single‐species models but also may increase opportunities for alternative states and prey population escape from control by cannibalistic predators.
Abstract In a previous study we demonstrated greater abundance of the parasitoid Anagrus epos (Girault) in grape vineyards located downwind of prune trees that function as overwintering habitats. This study examines whether these higher A. epos numbers translated into higher egg parasitism rates of the grape leafhopper, Erythroneura elegantula (Osborn). Paired commercial wine‐grape vineyard plots, one with and one without adjacent prune trees, were studied within a complete block design in northern and central California. A. epos was the key mortality factor affecting E. elegantula eggs. Point estimates of A. epos parasitism rates were significantly greater in vineyards associated with prune trees during the first E. elegantula generation in both 1991 and 1992. No consistent differences in parasitism rates were observed during the second or third generations. The results indicated that prune trees enhance early season parasitism rates. Cumulative estimates of egg parasitism across E. elegantula generations demonstrated that enhanced early‐season parasitism resulted in a net season‐long increase in the degree of mortality imposed by A. epos on E. elegantula eggs. Two factors were found to influence parasitism rates: the abundance of early‐season A. epos adults moving into vineyards and the density of E. elegantula eggs in vineyards. Our results indicate that diversification of vineyards using prune trees supports overwintering populations of a specialist parasitoid and thereby alters host‐parasitoid interactions to favor enhanced parasitism in vineyards.
1. In many species of parasitoid Hymenoptera, adult females can use hosts either for oviposition or «host feeding». Oviposition is current reproduction and host feeding represents investment towards future reproduction. 2. We investigated the influences of egg load, diet, age, experience and host size on host-feeding behaviour in the aphelinid parasitoid Aphytis melinus attacking oleander scale Aspidiotus nerii (Homoptera: Diaspididae). Hosts were either rejected, used exclusively for host feeding, used for both oviposition and host feeding concurrently, or used exclusively for oviposition. When hosts were used for both oviposition and host feeding, parasitoid progeny did not develop to adulthood. 3. Behavioural records were analysed with the logistic regression model, a statistical tool for assessing the relative contributions of multiple, potentially correlated, variables. 4. Variation in egg load was obtained by (i) using diet and age treatments to manipulate the rate of egg resorption by parasitoids and (ii) taking advantage of size-related differences in egg load. Parasitoids with lower egg loads were more likely to host feed than to ovipost. This result is consistent with recently developed theory. 5. During their first-ever encounter with a host, parasitoids that had been fed a pure sucrose diet during their adult life were more likely to host feed than were parasitoids fed a sucrose diet supplemented with yeast. 6. Contrary to theoretical predictions, younger parasitoids were not more likely to host feed than older parasitoids. 7. Smaller hosts were (i) more frequently used for host feeding than were larger hosts, and (ii) less suitable for progeny development than were larger hosts
Increasing diversity on farms can enhance many key ecosystem services to and from agriculture, and natural control of arthropod pests is often presumed to be among them. The expectation that increasing the size of monocultural crop plantings exacerbates the impact of pests is common throughout the agroecological literature. However, the theoretical basis for this expectation is uncertain; mechanistic mathematical models suggest instead that increasing field size can have positive, negative, neutral, or even nonlinear effects on arthropod pest densities. Here, we report a broad survey of crop field-size effects: across 14 pest species, 5 crops, and 20,000 field years of observations, we quantify the impact of field size on pest densities, pesticide applications, and crop yield. We find no evidence that larger fields cause consistently worse pest impacts. The most common outcome (9 of 14 species) was for pest severity to be independent of field size; larger fields resulted in less severe pest problems for four species, and only one species exhibited the expected trend of larger fields worsening pest severity. Importantly, pest responses to field size strongly correlated with their responses to the fraction of the surrounding landscape planted to the focal crop, suggesting that shared ecological processes produce parallel responses to crop simplification across spatial scales. We conclude that the idea that larger field sizes consistently disrupt natural pest control services is without foundation in either the theoretical or empirical record.
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