Raul Narciso C. Guedes

Acute lethal and behavioral sublethal responses of two stored‐product psocids to surface insecticides

BACKGROUNDnThe psocids Liposcelis bostrychophila Badonnel and L. entomophila (Enderlein) (Liposcelididae) are emerging pests of stored products. Although their behavior, particularly their high mobility, may contribute to the reported relatively low efficacy of insecticides against them, studies to investigate this have not been conducted. The present study aimed to assess the label rate efficacy of three commercial insecticides (beta-cyfluthrin, chlorfenapyr and pyrethrins) applied on concrete surfaces against L. bostrychophila and L. entomophila, and also their sublethal effect on the mobility of these species.nnnRESULTSnThe synthetic insecticides beta-cyfluthrin and chlorfenapyr showed high short-term efficacy (LT(95)<or=15 h) against both psocid species, unlike the natural pyrethrins (LT(95) >or= 4 days). Liposcelis bostrychophila was slightly more tolerant (>or=1.2x) than L. entomophila to all three insecticides. Behavioral assays with fully sprayed and half-sprayed concrete arenas indicated that the insecticides reduced the mobility of both species. Pyrethrins seem to elicit weak repellence in L. bostrychophila.nnnCONCLUSIONnbeta-Cyfluthrin and chlorfenapyr were effective against both psocid species, but not pyrethrins. The mobility of both species does not seem to play a major role in the differential selectivity observed, but the lower mobility of L. bostrychophila may be a contributing factor to its higher insecticide tolerance.

Contest behaviour of maize weevil larvae when competing within seeds.

Food limitation induces severe competition for obligate seed-feeding insect larvae that are unable to leave the seed selected by their mother. The number of eggs laid per seed and the number of larvae hatched from the eggs are important determinants of whether larval behaviour within the seed will be of the scramble or the contest type. In maize weevils, Sitophilus zeamais, few adults emerge per seed regardless of the number of eggs laid, which may arise from scramble competition, if the optimum egg density (i.e. egg density leading to maximum total larval fitness) is low compared to the number of eggs laid per seed, or from contest competition due to direct interference among the larvae. The behavioural process and the ecological outcome of competition were assessed in two strains of the maize weevil. Neither strain showed a reduction in body mass with increased competition, and they had similar optimal egg densities (two eggs per seed). There was a hump in the larval fitness curve suggesting a scramble competition, but this conclusion is compromised because the optimum egg density was small (two eggs per seed). X-ray imaging of seeds was used to observe interactions of larvae within the seed and showed direct interference, with aggression among the larvae. This provides evidence of contest-like competition within seeds even when egg density was low. Hence, one should be cautious in inferring the underlying type of competitive behaviour from variables such as body mass and initial egg density per seed. Direct observation of behaviour is required to make such an inference.

Sublethal exposure, insecticide resistance, and community stress

Insecticides are an invaluable pest management tool and anthropogenic stressors of widespread environmental occurrence that are subject to biased perceptions based on the targeted application, market value of use, and regulatory requirements. As a result, short-term and simplistic efforts focusing on lethal effects toward individual species and populations prevail. Holistic and comprehensive studies exploring rather common sublethal insecticide exposures are rare, particularly considering their potential role in structuring populations and communities in diverse environmental settings and potentially interfering in a range of ecological interactions. Studies on insecticide resistance, for example, do not go beyond population-based studies, disregarding temporal and spatial effects in the associated community, and rarely considering the whole of sublethal exposure. Some of these knowledge gaps are here recognized and explored.

Differential Heat Shock Tolerance and Expression of Heat-Inducible Proteins in Two Stored-Product Psocids

Abstract The recent recognition of psocids as a major concern in stored products and also the reemergence of heat treatment as a control tactic of stored-product insects led to the present investigation. The objectives of this study were to determine whether there are differences in heat shock tolerance of two species of stored-product psocids—Lepinotus reticulatus Enderlein (Trogiidae) and Liposcelis entomophila (Enderlein) (Liposcelididae)—and to determine whether heat shock proteins (HSPs) underlay such tolerance. Time-response bioassays were therefore carried out at increasing temperatures for both psocids. The lethal time (LT)50 and LT95 estimates were correlated with the expression of heat shock proteins after exposure at the same range of temperatures for 30 min. The expression of HSP was determined through Western blot analyses using HSP 70 antibody. Liposcelis entomophila was more than two-fold more tolerant than L. reticulatus for nearly all of the range of temperatures (≥40.0°C). Expression of HSP 70 was not observed for either of the psocid species, but the expression of two low-molecular-mass heat-inducible proteins (HIPs; 23 and 27 kDa) was observed in L. entomophila. The expression of these small proteins was induced by exposure to higher temperatures, and the trend was particularly strong for HIP 27. In contrast, no expression of small heat-inducible proteins was detected in L. reticulatus, reflecting its higher susceptibility to heat treatments. The relatively high heat tolerance of L. entomophila might help explain its more common occurrence in grain stored in warmer regions of the world.

Mating behaviour and reproductive output in insecticide-resistant and -susceptible strains of the maize weevil (Sitophilus zeamais )

Insecticide resistance is a broadly recognised and well-studied management problem resulting from intensive insecticide use, which also provides useful evolutionary models of newly adapted phenotypes to changing environments. Two common assumptions in such models are the existence of fitness costs associated with insecticide resistance, which will place the resistant individuals at a disadvantage in insecticide-free environments, and the prevalence of random mating among insecticide-resistant and -susceptible individuals. However, cases of insecticide resistance lacking apparent fitness disadvantages do exist impacting the evolution and management of insecticide resistance. Assortative mating, although rarely considered, may also favour the evolution and spread of insecticide resistance. Thus, the possible existence of both conditions in the maize weevil (Sitophilus zeamais), a key pest of stored cereals, led to the assessment of the mating behaviour and reproductive fitness of insecticide-resistant and -susceptible weevil strains and their reciprocal crosses. The patterns of female and male mating choice also were assessed. Although mating behaviour within and between weevil strains was similar without mate choice, mating within the resistant strain led to higher reproductive output than within the susceptible strain; inter-strain matings led to even higher fertility. Thus, no apparent fitness cost associated with resistance seems to exist in these weevils, favouring the evolution of this phenotype that is further aided by the higher fertility of inter-strain matings. Mate choice reduced latency to mate and no inter-strain preference was detected, but female weevils were consistent in their mate selection between 1st and 2nd matings indicating existence of female mating preference among maize weevils. Therefore, if female mate selection comes to favour trait(s) associated with insecticide resistance, higher reproductive fitness will be the outcome of such matings favouring the evolution and spread of insecticide resistance among maize weevil populations reverting into a management concern.

Pesticides and passive dispersal: acaricide- and starvation-induced take-off of the predatory mite Neoseiulus baraki

BACKGROUNDnAn understanding of the causes and consequences of dispersal is vital for managing populations. Environmental contaminants, such as pesticides, provide potential environmental context-dependent stimuli for dispersal of targeted and non-targeted species, which may occur not only for active but also for passive dispersal, although such a possibility is frequently neglected. Here, we assessed the potential of food deprivation and acaricides to interfere with the take-off for passive (wind) dispersal of the predatory mite Neoseiulus baraki.nnnRESULTSnWind tunnel bioassays indicated that starvation favoured the take-off for wind dispersal by the mite predator, which also varied with wind velocity, and dispersal increased at higher velocities within the 1-7 (m s-1 ) range tested. For the acaricides tested, particularly the biopesticide azadirachtin but also abamectin and fenpyroximate, the rate of predator take-off for dispersal increased, and further increased with wind velocity up to 7 m/s. Such responses were associated with changes in the predator behavioural preparation for wind-mediated passive dispersal, with a greater incidence of the standing posture that permitted take-off.nnnCONCLUSIONnThe rate of take-off for passive dispersal by N. baraki increased with food deprivation and exposure to the residues of agricultural acaricides. Azadirachtin exposure resulted in a particularly strong response, although abamectin and fenpyroximate also stimulated dispersal.

Larval cannibalism and fitness in the stored grain weevils Sitophilus granarius and Sitophilus zeamais

Resource limitation is an important determinant of life history and behavior while mediating competition and reproduction among organisms. Discreet and closed systems such as grain kernels and seeds impose drastic restrictions to grain beetles that spend their immature stages within a single kernel selected by their mother. This is the case of internally feeding stored grain beetles, such as the grain weevils. Female egg-laying decisions and larval competition largely determine resource limitation for such insects where clustered egg distribution and contest competition with larval interference and cannibalism take place. As the clustered eggs within a grain lead to larval competition and conspecific weevil larvae face each other off during development allowing the emergence of one or two larvae per kernel, we hypothesized that such competition and consequent cannibalism will have fitness consequences for the competing individuals and their offspring. Thus, larvae of the granary weevil (Sitophilus granarius L.) and the maize weevil (Sitophilus zeamais Motsch.) were subjected to larval competition with cannibalism, and lack of it, to assess the potential fitness consequence of cannibalism on these non-carnivorous pest species of stored grains. Larval cannibalism reduced developmental time of maize weevil, but not of granary weevil. However, such condition led to heavier adult weevils of both species exhibiting higher reproductive output generating more and better quality progeny than non-cannibal weevils. These findings indicate direct nutritional benefits of cannibalism to grain weevils favoring their status of key pest species of stored cereal grains.

Bidirectional selection of walking velocity, associated behavioral syndrome and reproductive output in the maize weevil Sitophilus zeamais

Individual behavioral types (i.e., personality traits) and their interrelations (i.e., behavioral syndrome) are recognized in arthropods and are of ecological and evolutionary importance. This aspect is also important in applied entomology, as distinct personality types may exhibit different fitness consequences. Here, we attempt to understand how within-population variation in a personality trait responds to selection and affects other personality traits. Our main focus was a quantifiable behavioral trait (i.e., walking velocity), its potential association with other personality traits, and how these traits are related to the reproductive output. The maize weevil (Sitophilus zeamais), a key pest of stored cereal grains, was used to test these hypotheses, and we detected that walking velocity is amenable to selection in this species. Furthermore, bidirectional selection with regard to walking velocity (i.e., either high or low) had an impact on the behavioral type, thereby allowing the recognition of different behavioral patterns among the strains. The generated multi-dimensional behavioral constructs of personality indicate consistent divergence among the strains with a stronger contribution of the activity dimension. Walking velocity was associated with other behavioral traits, particularly within the activity dimension, indicating the existence of a behavioral syndrome in the maize weevil. Furthermore, this behavioral dimension (i.e., activity), and particularly walking velocity, was associated with the reproductive output of maize weevil.

Spinosad- and Deltamethrin-Induced Impact on Mating and Reproductive Output of the Maize Weevil Sitophilus zeamais

Assessments of acute insecticide toxicity frequently focus on the lethal effects on individual arthropod pest species and populations neglecting the impacts and consequences of sublethal exposure. However, the sublethal effects of insecticides may lead to harmful, neutral, or even beneficial responses that may affect (or not) the behavior and sexual fitness of the exposed insects. Intriguingly, little is known about such effects on stored product insect pests in general and the maize weevil in particular. Thus, we assessed the sublethal effects of spinosad and deltamethrin on female mate-searching, mating behavior, progeny emergence, and grain consumption by maize weevils. Insecticide exposure did not affect the resting time, number of stops, and duration of mate-searching by female weevils, but their walking velocity was compromised. Maize weevil couples sublethally exposed to deltamethrin and spinosad exhibited altered reproductive behavior (walking, interacting, mounting, and copulating), but deltamethrin caused greater impairment. Curiously, higher grain consumption and increased progeny emergence were observed in deltamethrin-exposed insects, suggesting that this pyrethroid insecticide elicits hormesis in maize weevils that may compromise control efficacy by this compound. Although spinosad has less of an impact on weevil reproductive behavior than deltamethrin, this bioinsecticide also benefited weevil progeny emergence, but did not affect grain consumption. Therefore, our findings suggest caution using either compound, and particularly deltamethrin, for controlling the maize weevil, as they may actually favor this species population growth when in sublethal exposure requiring further assessments. The same concern may be valid for other insecticides as well, what deserves future attention.

Acaricide-Mediated Colonization of Mite-Infested Coconuts by the Predatory Phytoseiid Neoseiulus baraki (Acari: Phytoseiidae)

Abstract Walking is important to dispersal on plants and colonization of new plants by predatory mites, and this activity is potentially affected by the presence of acaricides. This possibility was investigated in coconut fruits infested with the coconut mite, Aceria guerreronis Keifer (Acari: Eriophyidae), where colonization by the predator Neoseiulus baraki (Athias-Henriot) (Acari: Phytoseiidae) was monitored. The following acaricides were evaluated for influence on the process of colonization by the predatory mite: abamectin, azadirachtin, and fenpyroximate. Water-treated fruits were used for comparison. Experiments were conducted with and without freedom of choice on coconut fruits with the release and recapture of females of N. baraki marked with fluorescent ink. A confinement experiment was also carried out on coconut bunch rachis sprayed or not sprayed with the acaricides. The predatory mite N. baraki avoided contact with acaricide-contaminated areas. After the predators were released on the fruits or bunch rachis, larger numbers were recaptured under the bracts than on the surface of the fruits. The number of predators recaptured in all experiments was lower in the treatments with acaricides than in the control. Among the acaricides tested, azadirachtin least affected N. baraki colonization. Therefore, the presence of the tested acaricides indeed interferes with N. baraki dispersal within plants and the coconut fruit colonization.