Livy Williams

Nondestructive leaf area estimation of ‘Niagara’ and ‘DeChaunac’ grapevines

Abstract Leaves were sampled from Vitis labruscana Bailey cv. ‘Niagara’ and the interspecific hybrid ‘DeChaunac’ grapevines several times during a 2-year investigation of leaf area estimation. Linear measurements of leaf length and width were made and correlated with leaf area measurements made with a computerized image processing system. For each cultivar, nine regression equations were derived and compared. Most models resulted in high (R2=0.90) coefficients of determination, but the power model using leaf width provided a high R2 and relatively low standard error of the estimate (‘Niagara’: area=0.637W1.995, R2=0.9821, and S.E.=10.58; ‘DeChaunac’: area=0.672W1.963, R2=0.9632, S.E.=5.67). Use of single-variable equations facilitates simple, rapid, and accurate estimation of ‘Niagara’ and ‘DeChaunac’ leaf area.

Plant Volatiles Influence Electrophysiological and Behavioral Responses of Lygus hesperus

Previous laboratory studies have shown that the mirid Lygus hesperus is attracted to volatiles emitted from alfalfa; feeding damage increases the amounts of several of these volatiles, and visual cues can enhance attraction further. The present study tested single plant volatiles in electrophysiological and behavioral trials with L. hesperus. Electroantennogram (EAG) analyses indicated that antennae responded to most plant volatiles included in the test, and that when gender differences were observed, males usually were more responsive than females. Antennal responses to the alcohols ((E)-3-hexenol, (Z)-3-hexenol, 1-hexanol), the acetate (E)-2-hexenyl acetate, and the aldehyde (E)-2-hexenal were among the strongest. Moderate responses were observed for (E)-β-ocimene, (E,E)-α-farnesene, (±)-linalool, and methyl salicylate. A dose dependent response was not observed for several terpenes (β-myrcene, β-caryophyllene, (+)-limonene, or both (R)-(+)- and (S)-(−)-α-pinenes). EAG responses, however, were not always consistent with behavioral assays. In Y-tube bioassays, males did not exhibit a positive behavioral response to any of the compounds tested. Instead, males were repelled by (E)-2-hexenyl acetate, (±)-linalool, (E,E)-α-farnesene, and methyl salicylate. In contrast, female L. hesperus moved upwind towards (R)-(+)-α-pinene, (E)-β-ocimene, and (E,E)-α-farnesene, and showed a negative response towards (Z)-3-hexen-1-ol, (S)-(−)-α-pinene, and methyl salicylate. This study emphasizes the use of multiple approaches to better understand host plant finding in the generalist herbivore L. hesperus.

EAG-Active Herbivore-Induced Plant Volatiles Modify Behavioral Responses and Host Attack by An Egg Parasitoid

Volatiles emitted by plants in response to feeding by Lygus species were tested in neurophysiological, behavioral, and parasitism trials with Anaphes iole, an egg parasitoid of Lygus. Electroantennogram analyses indicated that A. iole antennae responded to most herbivore-induced plant volatiles (HIPVs) tested and that females were usually more responsive than males. Antennal responses to (Z)-3-hexenyl acetate and methyl salicylate were among the strongest. Behavioral assays in a four-arm olfactometer demonstrated that response of female wasps to (Z)-3-hexenyl acetate varied greatly depending on preconditioning regime. Preconditioning wasps to complex host-plant odors led to stronger preference than did a single preconditioning stimulus, i.e., (Z)-3-hexenyl acetate. In a horizontal wind tunnel, female wasps were attracted by methyl salicylate and α-farnesene. Parasitism of Lygus lineolaris eggs by A. iole in a cotton field was greater when the eggs were associated with (Z)-3-hexenyl acetate or α-farnesene than with controls. Overall, the results of this study show that A. iole can perceive a variety of plant volatiles released after its host damages plants, that the degree of associative learning in A. iole can be manipulated based on preconditioning regime, and that single synthetic HIPVs are attractive to A. iole and can be used to increase attack rates on host eggs. Therefore, it appears that HIPVs have potential for use in suppression of Lygus population densities.

Presence of trehalulose and other oligosaccharides in hemipteran honeydew, particularly Aleyrodidae

Abstract. Production by a bacterium of the disaccharide trehalulose was reported 30 years ago. The association between the fabrication of trehalulose and Insecta was found more recently. It was initially discovered in the honeydew, excreta, of the whitefly Bemisia tabaci feeding on Euphorbia pulcherrima where it was the predominant sugar. In the present study, B. tabaci‐produced trehalulose was again found in honeydew at significant levels, this time when the whitefly fed on 10 different plants. In seven of 10 Bemisia/host combinations, trehalulose accounted for more than 30% of the total carbohydrates found in their honeydew and was the principal oligosaccharide. Trehalulose constituents, glucose and fructose, were also present at lower levels, as were other oligosaccharides such as melezitose. Feeding by B. tabaci on the three other plant hosts also resulted in the production of trehalulose at relatively high levels (6.1–16.5%). Other whitefly species examined had little (e.g. B. afer and Aleurothrixus spp.), or no trehalulose (e.g. Trialeurodes spp. and Siphoninus phillyreae), in their honeydew. Trehalulose was also found in the honeydew of two aphid and one scale insect species. In insects whose honeydew had low levels or no trehalulose, sucrose, its constituents, or larger sugars predominated. The trisaccharide bemisiose was also found in the honeydew of half the whitefly species examined. Bemisiose was discovered here for the first time in the honeydew of three aphid species and three species of scale insects. Reasons for the production of trehalulose are often linked to high levels of dietary sucrose in whiteflies. This is probably true in our case because cotton and cucurbits, at least, are known to contain a great deal of sucrose. Although other functional possibilities were explored, the data suggest that trehalulose, being less susceptible to hydrolysis than sucrose, is involved in osmoregulation.

Different food sources affect the gustatory response of Anaphes iole, an egg parasitoid of Lygus Spp.

The gustatory response of female Anaphes iole wasps to naturally occurring carbohydrates, a commercial food source, and host (Lygus lineolaris) frass was determined. Wasps responded to all 14 of the sugars at the highest concentration tested (2 M). At this concentration, sucrose, glucose, maltose, melezitose, fructose, and erlose all elicited <90% acceptance. The lowest concentration that evoked a response (=acceptance threshold) for these sugars was >1/256 M, with the exception of glucose, which was 1/16 M. Raffinose, trehalose, mannose, galactose, melibiose, rhamnose, stachyose, and lactose led to >50% gustatory response by the wasps at 2 M, and were categorized as “moderately stimulatory sugars.” The acceptance threshold for these sugars was <1/4 M, with the exception of raffinose, which was 1/256 M. In trials with moderately stimulatory sugars combined with either sucrose or maltose, only the rhamnose+maltose mixture significantly inhibited the gustatory response of A. iole. Food and water deprived parasitoids readily accepted the moderately stimulatory sugars. EliminadeTM, a commercial food supplement, was readily accepted (92%) by A. iole. Conversely, the wasps did not feed on host frass. Chemical analysis of L. lineolaris frass demonstrated the presence of glucose, sucrose, fructose, trehalulose, and melezitose, apparently at concentrations below those perceived by A. iole. The latter two compounds were previously known only from homopteran honeydew (trehalulose and melezitose) and bacteria (trehalulose). With respect to gustatory response to nectar and honeydew sugars, A. iole differs markedly from other hymenopterans that have been studied in that this parasitoid accepted all the naturally occurring sugars with which it was tested. Moreover, this parasitoid had lower acceptance thresholds than other hymenopterans for many of the sugars. This broad and sensitive range of gustatory perception might be helpful in the development of a food source for the wasp that is not exploited by the host.

Olfactory Responses of Anaphes iole (Hymenoptera: Mymaridae) to Volatile Signals Derived from Host Habitats

Anaphes iole Girault is a frequent parasitoid of Lygus spp. eggs in the United States, and has potential as a biological control agent against Lygus hesperus Knight in different crops. Feeding and oviposition by L. hesperus induce emission of plant volatiles, but studies to date do not address the role of plant volatiles in the host-searching behavior of A. iole. In this study, a four-arm olfactometer was used to test the responses of female parasitoids to odors emanating from cotton (Gossypium hirsutum L., Malvaceae) plants damaged by L. hesperus females, L. hesperus males, larvae of the nonhost Spodoptera exigua Hubner, or mechanically, or to odors from L. hesperus females alone. In addition, various plants damaged by L. hesperus females were evaluated in the olfactometer: cotton, alfalfa (Medicago sativa L., Fabaceae), common groundsel (Senecio vulgaris L., Asteraceae), annual ragweed (Ambrosia artemisifolia L., Asteraceae), and redroot pigweed (Amaranthus retroflexus L., Amaranthaceae). In all olfactometry bioassays, treatment odors were compared against three controls (humidified air). Results showed that A. iole females were consistently attracted to odors derived from different plant–L. hesperus complexes, while odors from plants subjected to nonhost (S. exigua) or mechanical damage and L. hesperus females alone were not attractive or only variably attractive. These findings suggest that while searching for hosts A. iole females use specific volatiles induced by L. hesperus feeding and oviposition to locate hosts inhabiting a wide variety of plants, including annual and perennial species from four plant families. It was suggested that future research should seek to identify the chemical elicitors involved in the release of plant volatiles attractive to A. iole females.

Tethered and Untethered Flight by Lygus hesperus and Lygus lineolaris (Heteroptera: Miridae)

Abstract We compared the flight behavior of Lygus hesperus Knight and Lygus lineolaris (Palisot de Beauvois) relative to age, sex, and time of day by using tethered (flight mills) and untethered flight (vertical flight chamber) assays. Both species and sexes initiated flights throughout the day, and flight mill assays recorded flights throughout the night. For both species and flight systems, most flights were <5 min in duration, with longest flights occurring from 1 to 3 wk of age. Number of flights and flight duration were influenced by age and sex for tethered individuals and by sex for untethered individuals. Species differences were less apparent, but tethered L. lineolaris had more sustained flights (individual flights >5 min) that were of longer cumulative duration compared with L. hesperus. The longest flights were obtained with flight mills and were 17–18 times longer than the longest flight (22 min) in the flight chamber. Determination of flight periodicity, throughout the day and night, was only possible for tethered insects, and females exhibited more distinct periodicities for sustained flights than males. For L. hesperus females, sustained flights followed a diurnal to crepuscular periodicity, whereas sustained flights by L. lineolaris females were nocturnal. No significant correlations were found between egg load and any of the flight parameters when grouped by species, but there was a positive correlation between the number of spermatophores and several of the flight parameters for female L. hesperus. In the vertical flight chamber, takeoffs began at low light levels and were always higher for L. lineolaris than L. hesperus. Rates of climb toward the skylight cue were ≈50 cm/s, indicating a capacity for strong, self-directed flight by both species.

Detecting the Egg Parasitoid Anaphes iole (Hymenoptera: Mymaridae) in Tarnished Plant Bug (Heteroptera: Miridae) Eggs by Using a Molecular Approach

Abstract Polymerase chain reaction (PCR) technique was used to develop a specific molecular marker for detection of immature stages of the egg parasitoid Anaphes iole Girault developing within its host, Lygus lineolaris (Palisot de Beauvois). Ribosomal DNA sequences for the internal transcribed spacer 2 (ITS2) were cloned and sequenced from adult A. iole and L. lineolaris. PCR amplification of genomic DNA using primers designed on the basis of these rDNA sequences, followed by agarose gel electrophoresis, successfully detected eggs and larvae of the parasitoid within L. lineolaris eggs. This molecular method is highly specific and sensitive. A 661-bp wasp DNA fragment was consistently amplified from pure DNA extracted from a single wasp, from a DNA mixture prepared from a wasp and an L. lineolaris egg, and from a DNA mixture prepared from a wasp, an L. lineolaris egg, and alfalfa tissue. The presence of plant tissue did not hinder detection of A. iole within host eggs. With this PCR technique, as low as 5 × 10−6 wasp DNA equivalent (1.2 pg DNA) could be easily detected. The PCR technique provided more rapid detection of parasitism rates than observed by rearing parasitized host eggs in artificial oviposition packets. PCR technology shows promise for early and sensitive detection and identification of single and multiple species of egg parasitoids in agricultural and natural systems.

Potential of Detection and Identification of Nymphal Parasitoids (Hymenoptera: Braconidae) of Lygus Bugs (Heteroptera: Miridae) by Using Polymerase Chain Reaction and ITS2 Sequence Analysis Techniques

Abstract Ribosomal ITS2 DNA fragments were sequenced from four Peristenus species, two Leiophron species, and two Lygus species. Specific primers for polymerase chain reaction (PCR) amplification were designed from ITS2 DNA sequences to separate each species from the others. Using this molecular approach, we were able to determine whether Lygus hesperus Knight and Lygus lineolaris (Palisot de Beauvois) were parasitized by Peristenus and Leiophron parasitoids. The PCR technique was very sensitive and could detect Peristenus stygicus Loan DNA at a concentration of 0.01 pg/μl or 7.5 × 10−7 wasp DNA equivalents. Detection of P. stygicus eggs confirmed that early detection of parasitoids was possible. Parasitoid DNA was readily recovered from all L. hesperus nymphs that were parasitized by a single P. stygicus after 1 h of contact between the parasitoid and putative hosts. This study demonstrates the effectiveness of a molecular technique for detecting parasitoids developing inside their hosts.

Electrophysiological and behavioral characterization of bioactive compounds of the Thymus vulgaris, Cymbopogon winterianus, Cuminum cyminum and Cinnamomum zeylanicum essential oils against Anopheles gambiae and prospects for their use as bednet treatments

BackgroundLaboratory and field studies showed that repellent, irritant and toxic actions of common public health insecticides reduce human-vector contact and thereby interrupt disease transmission. One of the more effective strategies to reduce disease risk involves the use of long-lasting treated bednets. However, development of insecticide resistance in mosquito populations makes it imperative to find alternatives to these insecticides. Our previous study identified four essential oils as alternatives to pyrethroids: Thymus vulgaris, Cymbopogon winterianus, Cuminum cyminum, Cinnamomum zeylanicum. The objectives of this study were to identify active compounds of these essential oils, to characterize their biological activity, and to examine their potential as a treatment for bednets.MethodsWe evaluated the electrophysiological, behavioural (repellency, irritancy) and toxic effects of the major compounds of these oils against Anopheles gambiae strain ‘Kisumu’.ResultsAldehydes elicited the strongest responses and monoterpenes the weakest responses in electroantennogram (EAG) trials. However, EAG responses did not correlate consistently with results of behavioral assays. In behavioral and toxicity studies, several of the single compounds did exhibit repellency, irritancy or toxicity in An. gambiae; however, the activity of essential oils did not always correlate with activity expected from the major components. On the contrary, the biological activity of essential oils appeared complex, suggesting interactions between individual compounds and the insect under study. Data also indicated that the three effects appeared independent, suggesting that repellency mechanism(s) may differ from mechanisms of irritancy and toxicity.ConclusionsBased on the bioassays reported here, some of the compounds merit consideration as alternative bednet treatments.