Identification of endo- and exo-polygalacturonase activity in Lygus hesperus (Knight) salivary glands

Polygalacturonase (PG) activity found in the salivary gland apparatus of the western tarnished plant bug (WTPB, Lygus hesperus Knight) has been thought to be the main chemical cause of the damage inflicted by this mirid when feeding on its plant hosts. Early viscosity and thermal stability studies of the PG activity in L. hesperus protein extracts were difficult to interpret. Thus, it has been suggested that one or more PG protein(s) with different hydrolytic modes of action are produced by this mirid. In order to understand the quantitative complexity of the WTPB salivary PG activity, PG purification from a protein extract from salivary glands excised from L. hesperus insects was performed using affinity and ion exchange chromatography. To elucidate the qualitative complexity of the purified PGs, the digestion products generated by the PGs were separated using high performance anion exchange chromatography with pulsed amperometric detection. At least five PG proteins were detected; these differing in terms of their glycosylation, mass-to-charge ratios, and/or molecular mass. The characterization of the products generated by these PGs showed that endo- and exo-acting PGs are produced by WTPB. Although none of the PGs was purified to homogeneity, the present work provides biochemical evidence of a multiplicity of PGs that degrade the pectin component of the plant tissue in different fashions. The implications of these findings affect the understanding of WTPB feeding damage and, potentially, help identify ways to control this important crop pest. Arch. Insect Biochem. Physiol. 2008. © 2008 Wiley-Liss, Inc.     

Relative preference of Lygus hesperus (Hemiptera: Miridae) to selected host plants in the field

Abstract Information on host plant preference of agriculturally important insect pests, such as Lygus hesperus (Knight), can be helpful in predicting its occurrence and future movement among crop and non-crop host plants. A field study was conducted during 2005 and 2006 to evaluate the host preference of Lygus to cotton and four other host plants in the Texas High Plains, including alfalfa (Medicago sativa L.), wild sunflower (Helianthus annuus L.), Russian thistle (Salsola iberica L.) and pigweed (Amaranthus palmeri L.). Sampling for both nymphs and adults during 2005 (July to November) and 2006 (June to November) showed that alfalfa and Russian thistle were the two most preferred hosts out of the five hosts evaluated. Abundance of nymphs (numbers per 50 sweeps per host plant) during the sampling period also indicated the superior reproductive suitability of alfalfa and Russian thistle. Cotton appeared to be the least attractive host plant for Lygus when Russian thistle and alfalfa were available in the host mosaic. Seasonal abundance of Lygus was found to be lower during 2006 compared to 2005, which may be explained by the difference in rainfall patterns during these two years. In terms of species dominance, L. hesperus was the most dominant species in the sampled population followed by an inconsiderable fraction of L. elisus and L. lineolaris.     

Characterization of microsatellite loci in the western tarnished plant bug,Lygus hesperus Knight (Hemiptera: Miridae)

A microsatellite‐enriched partial genomic DNA library of Lygus hesperus was generated and screened by sequencing. Ten polymorphic microsatellite marker loci were characterized by genotyping 92 insect samples. The observed number of alleles ranged from three to seven with an average of 4.5 (SE ± 0.45), while the effective number of alleles ranged from 1.21 to 3.02 with an average of 2.14 (SE ± 0.20). Significant deviations from Hardy–Weinberg expectations were detected at three loci. Significant linkage disequilibrium was also detected between the loci LhMS2‐54 and LhMS3‐32. Seven of the L. hesperus markers could be transferred to Lygus lineolaris.     

Reproduction of the western tarnished plant bug, Lygus hesperus, in relation to age, gonadal activity and mating status

Understanding the basic life history and underlying regulatory mechanisms for a pest insect is essential for developing targeted control strategies, but for many insects relatively little is known. Although the western tarnished plant bug, Lygus hesperus Knight (Heteroptera: Miridae) has a substantial negative impact in the western U.S., its basic biology is poorly characterized. To elucidate the regulation of L. hesperus reproductive dynamics, the onset times of gonadal activation and mating behavior were examined in young adults. Newly emerged adults reared under laboratory conditions at 25 °C were monitored daily for changes in gamete production and willingness to mate. Males matured more quickly than females. Sperm was present at emergence and a small proportion of males were willing to mate as early as 2 days post-emergence. Females were unwilling to mate until at least 5 days post-emergence, although many produced choriogenic oocytes by 4 days. Males appeared to discriminate female age and were more likely to attempt mating with females >5 days post-emergence than with younger females. Males were also able to detect previous mating and attempted to mount virgins more often than recently inseminated females. Collectively these results indicate that the changes in the mating behaviors of L. hesperus are linked to reproductive status, although there is a lag between gamete production and willingness to mate. The results also suggest that interactions of the sexes are chemically mediated.     

Evaluating female remating rates in light of spermatophore degradation in Heliconius butterflies: pupal‐mating monandry versus adult‐mating polyandry

1. Butterflies are frequently used in comparative studies of sexual selection because of their diverse mating systems. In Heliconius, the two major clades in the genus are characterised by contrasting pupal‐mating and adult‐mating strategies. Adult‐mating females are considered to be promiscuous whereas pupal‐mating females are thought to be monandrous. 2. Counting spermatophores in female Lepidoptera is a common method for assessing patterns of female remating. However, in pupal‐mating Heliconius butterflies spermatophores can become completely degraded, potentially leading to underestimation of female remating rates. 3. We qualitatively characterised the different states of spermatophore degradation, and showed that complete degradation takes approximately 3 weeks in captive‐bred H. erato females. 4. We counted spermatophores and/or assayed spermatophore degradation in > 500 Heliconius females across 28 species sampled from natural populations. Among pupal‐maters these observations yielded a few rare observations of double mating by recently eclosed females, but generally indicated a lack of rematings. In contrast, approximately 25% of sampled adult‐mating females remated at least once. 5. Using a novel statistical analysis, we estimated remating rates from patterns of spermatophore degradation or from counts stratified by age, as indicated by wing‐wear. This analysis showed no statistically significant evidence for remating for the pupal‐mating H. erato whereas significant remating rates were detected for adult‐mating species. 6. The present results support the established view of Heliconius mating systems in which pupal‐maters are largely monandrous, whereas adult‐maters are polyandrous.     

Diel activity pattern and predation rate of the generalist predator Dicyphus hesperus

Abstract This study examined the diel activity pattern and the effect of diel activity pattern on predation rate and prey finding of Dicyphus hesperus Knight (Heteroptera: Miridae). To determine the diel activity pattern of D. hesperus, starved females were placed on tomato leaflets Lycopersicon esculentum Mill. (Solanaceae) under zero, low, or high light intensities at 02:00, 08:00, and 14:00 h, respectively, and the amount of time spent walking or resting during a 30‐min interval was recorded. Predation rates of D. hesperus females on Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) egg patches were determined under either a L16:D8 (long day) or L8:D16 (short day) diel period. Egg patches were removed from D. hesperus females after either 8 or 16 h of dark or 16 or 8 h of light, and the number of eggs consumed was counted. Dicyphus hesperus females spent more time searching for prey at night than during the day. Females ate eggs at a higher rate during the night than during the day. Overall, D. hesperus females had higher predation rates when reared under a long day diel cycle compared with females reared under a short day diel cycle. More females reared at the L16:D8 diel cycle found the egg patch during the night than during the day. There was no difference in egg patch finding between night and day for females reared at L8:D16. Overall, L16:D8 reared females found more egg patches than females reared at L8:D16. Therefore, D. hesperus females are more active and find and consume prey at a higher rate at night than day.     

Diet affects female mating behaviour in a seed‐feeding beetle

In species where males provide nuptial gifts, females can improve their nutritional status and thus increase their fecundity by mating when in need of resources. However, mating can be costly, so females should only mate to acquire resources when the need for resources is large, such as when females are nutritionally‐deprived. Two populations of the seed‐feeding beetle Callosobruchus maculatus, a species that produces relatively large nuptial gifts, are used to test whether female nutritional status affects mating behaviour. Female access to water, sugar and yeast are manipulated and the fitness consequences of these manipulations are examined together with the effects of diet on the propensity of nonvirgin females to mate. Access to water has a small but significant effect on mass loss over time, lifespan and fecundity of females, relative to unfed controls. Access to sugar (dissolved in water) improves female fecundity and lifespan above that of hydrated females but access to yeast has no positive effects on female survival or reproduction. Diet has a large effect on both receptivity of nonvirgin females to a male and how quickly they accept that male. Unfed females are both more likely to mate, and accept a mate more quickly, than females provided access to water, which are more likely to mate and accept a mate more quickly than females provided with sugar. This rank order of behaviours matches the order predicted if females increase their mating rate when nutritionally deprived (i.e. it matches the effect of diet on female fitness). The results obtained also suggest that mate choice may be condition‐dependent: females from one population (Burkina Faso) show a preference for large males when well‐fed but not when unfed, although this result is not found in a second population (South India). It is concluded that nutritionally‐deprived females are more receptive to mates than are well‐fed females, consistent with the hypothesis that females ‘forage’ for nuptial gifts, or at least more willingly accept sperm in exchange for nuptial gifts, when they are nutritionally deprived.     

Eight polymorphic microsatellite markers developed in the green leaf bug,Lygus lucorum Meyer‐Dür (Hemiptera: Miridae)

We developed eight polymorphic microsatellite markers for the green leaf bug, Lygus lucorum Meyer‐Dür. The number of observed alleles per locus in 48 individuals ranged from three to 13, whereas the observed and expected heterozygosities ranged from 0.133 to 0.810 and from 0.236 to 0.886, respectively. Cross‐species amplifications were conducted both within and between generas, and the amplification results indicated that these primers enriched for L. lucorum Meyer‐Dür are species‐specific.     

Gustatory discrimination of sugars, amino acids, and selected allelochemicals by the tarnished plant bug, Lygus lineolaris

ABSTRACT. Selected sugars, amino acids and allelochemicals were tested over a range of concentrations to determine their effect on the ingestion rates of adult tarnished plant bugs. Experiments were performed with a two-choice bioassay technique utilizing artificial membranes. Sucrose was the most effective feeding stimulant of all the compounds tested. Glucose was an effective stimulant but was 8 times less potent than sucrose. Fructose was 50 times less potent than sucrose. Methionine and phenylalanine were the only amino acids of seven tested which stimulated feeding. Leucine produced a deterrent effect. Cotton tannin was the most effective deterrent of all allelochemicals tested and was 10 times more potent than gossypol. The quinone, 1, 4-napthoquinone, and the alkaloid, tomatine, were also effective feeding deterrents. Sinigrin and its aglycone, ethyl isothiocyanate, were among the weakest deterrents of all the allelochemicals tested. Hydroquinone was the least potent deterrent of all compounds tested. The results indicate a strong chemosensory component regulating the feeding response of the tarnished plant bug. It is suggested that this response is mediated by epipharyngeal chemosensitive sensilla.     

Prey feeding increases water stress in the omnivorous predator Dicyphus hesperus

The effects of water stress (produced by water deprivation and prey feeding) on plant feeding were investigated in the omnivorous predator Dicyphus hesperus Knight (Hemiptera: Miridae). The objective was to determine if prey feeding aggravated water deficits and thus increased plant feeding. We measured plant feeding in a factorial experiment where female D. hesperus were prepared for experiments by providing or withholding water and/or prey for 24 h. We then evaluated the amount of plant feeding on Nicotiana tabacum seedlings by the direct observation of insects at three different densities of the prey, Ephestia kuehniella eggs. The amount of plant feeding, as measured by frequency of plant feeding bouts and time spent plant feeding during observation, was significantly greater for water‐deprived individuals than for those that had been provided with water. Individuals that had been provided with prey fed on plants at a significantly higher frequency than prey‐deprived individuals at two of the prey densities used in the experiment. These results support the hypothesis that plant feeding in zoophytophagous Hemiptera facilitates prey feeding by providing water that is essential for predation.     

State-dependent sampling bias in insects: implications for monitoring western tarnished plant bugs

Insect populations vary in the proportion of individuals exhibiting a particular ‘state’ (e.g., developmental stage, sex, egg load, or nutritional status). Because an insect's developmental state often determines both its size and its behavior, it is likely that this will also affect the probability of being sampled. We propose that a comprehensive approach to pest management must consider the interaction between the structure of a pest population (i.e., the relative number of individuals in each state) and any state‐dependent sampling bias. To illustrate the usefulness of this method we sampled populations of the western tarnished pest bug, Lygus hesperus Knight (Heteroptera: Miridae), in cotton fields. Our sampling technique utilized large cages to measure the absolute densities of each L. hesperus stage and adult sex within a population. This technique allowed us to document a wide range of absolute stage structures and sex ratios across 10 L. hesperus populations in California. Using a combination of cage samples and sweep net samples, we quantified the state‐dependent sampling bias by calculating the efficiency of sweep sampling as a function of L. hesperus developmental stage and sex. We found that the efficiency of sweep nets increased steadily with each successive developmental stage (i.e., nymphal instar) of L. hesperus. We also found that sweep nets are slightly more efficient in capturing male vs. female L. hesperus adults. Since other studies have documented that the stage and sex of L. hesperus can affect feeding impact on cotton flower buds, our results suggest that accurate predictions of Lygus damage will need to incorporate stage and sex‐dependent sampling biases.     

绿盲蝽雌虫的浸提物分析

采用浸泡法和气质联(GC-MS)用对绿盲蝽Lygus lucorum Meyer-dür雌虫不同部位的浸提物进行鉴定。结果表明:确定10种成分为绿盲蝽雌虫的浸提物中的主要组分,包括醇类,酸类和酯类。相对含量较高的依次有反-2-丁酸己烯酯、反-2-己烯醇、丁酸、顺-3-己烯醇、丁酸己酯。绿盲蝽雌虫在完成性召唤活动后,丁酸己酯、反-2-丁酸己烯酯和反-2-己烯醇的含量都明显减少,其中反-2-丁酸己烯酯从召唤前的7200ng减少到召唤后的250ng,表明在绿盲蝽雌虫性召唤活动中,有大量的反-2-丁酸己烯酯释放到体外。比较绿盲蝽雌虫的不同部位的浸提物的组分发现反-2-丁酸己烯酯主要存在于虫体胸部,占总量的89.75%,且反-2-己烯醇也主要存在于胸部。因此推测反-2-丁酸己烯酯可能是绿盲蝽雌虫的性信息素成分或组分之一,并主要由胸部内的腺体分泌。     

Mating and social contact change egg production and longevity in adult females of the mirid Lygus hesperus

There is frequently a tradeoff between fecundity and longevity, but the relationship is inconsistent across species and influenced by various exogenous and endogenous factors. Previous studies of Lygus hesperus Knight (Hemiptera: Miridae) established that egg production is promoted by insemination, at least temporarily, but little is known about the long‐term effects of mating and nonsexual interactions with conspecifics on egg production and female lifespan. To elucidate these relationships, survivorship and oviposition rate were tracked daily in females that were isolated or paired with a fertile male or another female throughout their adult lives. Mating rates were determined by postmortem examination. Results indicate that male‐specific stimuli accelerate female reproductive maturation, and that mating elevates oviposition rate. However, females paired with either a female or male companion had shortened lifespans, suggesting that social contacts exact a significant cost in this solitary species. Despite the negative impact of conspecific interactions and the finding that a singly mated female has sufficient sperm to fertilize a lifetime supply of eggs, many females were found to have mated more than once. Multiply mated females had higher sustained oviposition rates, lived longer, and had greater lifetime fecundities. Collectively, no strong evidence was found of a direct physiological link between fecundity and longevity, but environmental factors and mating were found to significantly influence both traits.     

Random size‐assortative mating despite size‐dependent fecundity in a Neotropical amphibian with explosive reproduction

Sexual selection theory predicts that, when body size is correlated with fecundity, there should be fitness advantages for mate choice of the largest females. Moreover, because larger males are expected to monopolise the largest females, this should result in an assortative mating based on body size. Although such patterns could be expected in both explosive and prolonged breeders, non‐assortative mating should be more widespread in species under time constraints. However, patterns of sexual selection are largely unexplored in explosive breeding species, and contrasting patterns have been found previously. We expect that the active choice of partners may be particularly risky when the time period during which sexual partners are available is severely limited. Therefore, to avoid missing an entire reproductive act, males and females should pair irrespective of traits, such as body size. We tested this hypothesis by investigating the mating patterns of the Pacific horned toad, Ceratophrys stolzmanni, a short‐lived fossorial species inhabiting Neotropical dry forests. This species is particularly adequate to test our prediction because it reproduces explosively over the course of a single night per year. Although the number of eggs laid was proportional to the size of females, and individuals of both sexes showed variation in body size, there was no assortative mating based either on size, body condition or age of mates. Egg size was not influenced by either female size or clutch size. The larger body size of females compared to males is likely due to fecundity selection, that is, the selective pressure that enhances reproductive output. Although we cannot dismiss the possibility that individuals could select their partners based on other criteria than those related to size or age, the results fit well our prediction, showing that the explosive breeding makes improbable an active choice of partners in both sexes and therefore favours a random mating pattern.     

Assessment of trap plants to regulate numbers of the European tarnished plant bug, Lygus rugulipennis, on late-season strawberries

Various species of plants in the family Asteraceae were shown to be hosts for the European tarnished plant bug Lygus rugulipennis and one, Matricaria recutita, was chosen as a potential trap plant for adults of this capsid species on their migration into fields of late-season strawberries from other host plants in July. There was a delay in the build-up of populations of nymphs of L. rugulipennis on strawberries surrounded by a barrier strip of M. recutita compared to those without a barrier, but overall there were no consistent reductions in populations of the pest. Numbers of L. rugulipennis on the trap plant were small until late August. In a similar experiment using an alternative trap plant, Medicago sativa, no significant reductions in the numbers of L. rugulipennis were found on strawberries with the trap barrier, despite large numbers of the pest insect being found on this trap plant.Although predatory arthropods such as spiders, Orius spp., and nabids, and hymenopterous parasitoids were found on the trap plants, there was no indication that they became more numerous on the strawberry plots inside these barriers than on those without surrounding trap plants.     

The functions of plant feeding in the omnivorous predator Dicyphus hesperus: water places limits on predation

1. Dicyphus hesperus Knight (Heteroptera: Miridae) nymphs were fed from egg hatch to the adult stage on Ephestia kuehniella eggs provided either alone or in combination with tomato leaves or with a supplementary water source. 2. Only 6% of individuals completed nymphal development on a diet of eggs alone. In contrast, a high proportion of nymphs completed development on a diet of eggs when either tomato leaves (97%) or a supplementary water source (88%) were provided. 3. The development times of nymphs given access to leaves were significantly shorter than those of nymphs given access only to supplementary water. 4. Adult female D. hesperus that were given access to tomato leaves prior to feeding trials consumed significantly more eggs in a 4‐h period than females that were dehydrated before trials. Dehydrated females that were allowed access to water for 3 h before trials consumed an intermediate number of eggs. 5. Plant feeding or access to some other water source is required for prey feeding, growth, and development in D. hesperus, and acquisition of water is proposed as a primary function of plant feeding. In addition, D. hesperus derives nutrients from plant feeding that increase the rate of nymphal development, although nymphs cannot complete development when provided only with tomato leaves. 6. Three simple models are presented of feeding behaviour in predatory Heteroptera where the amount of plant feeding either decreases, increases, or is constant as a function of the amount of prey feeding. The models are discussed with reference to the results and the probable multifunctional nature of plant feeding in predatory Heteroptera.