Whitefly Densities Research Articles

Host plant selection and oviposition behaviour of whitefly Bemisia tabaci (Gennadius) in a mono and simulated polyculture crop habitat

The host plant selection, oviposition behaviour and survivorship of whitefly ( Bemisia tabac i (Gennadius) was evaluated in green house. Three host plants cultivars namely: Brinjal ( Solanum malagna ), chilli ( Capsicum annuum ) and tomato ( Solanum lycopersicu m) were placed in a multiple crop habitat at 34 - 36°C, 70 - 80% relative humidity (RH) in a normal day light. There was a variation of morphological characteristic among host plants (smooth and thick trichome leaves) where all developmental stages of pest were given free choice of foraging. Although the host plants genus C. annum and S. lycopersicum were also the potential host of B. tabaci but in the presence of S. malagna, the attack rate remains minimum on both the host plants. The feeding and egg laying was significantly higher on S. malagna L . leaves as compare to other two host plants in the open arena. S malagna L. was also preferred when pest was tested in confined cages for free choice probing on capsicum and S. malagna L. There was no significant but a slight difference in survivorship of all developmental stages of whitefly when Brinjal and Chilli and then Brinjal and tomato from two different experimental arenas were compared. There was no host plant varietal effect on the overall developmental time from egg eclosion to the adult. The leaves of S. malagna with thick trichomes were chosen significantly more higher for egg laying compared to other host plants. The morphological characters and plant architecture contribute to higher densities of adult whitefly (wf) compared to new leaves. No symptoms of viral infection has been observed on the chilli with Brinjal, whereas, the same variety of chilli was reasonably infected with virus in chilli monocrop arena. Key words: Multiple cropping, host plant selection, oviposition fecundity, intrinsic rate of increase.

Effect of host experience of the greenhouse whitefly, Trialeurodes vaporariorum, on trap cropping effectiveness

AbstractThis study evaluated whether experience of Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyrodidae) on a poinsettia cash crop, Euphorbia pulcherrima Willd. ex Koltz (Euphorbiaceae), influences the effectiveness of an eggplant trap crop, Solanum melongena L. (Solanaceae). Two whitefly strains were tested: one was reared on poinsettia (poinsettia‐strain) and a second was reared on bean [Phaseolus vulgaris L. (Fabaceae)] (bean‐strain). We first determined whether host experience altered the preference of adult whiteflies for eggplant and their survivorship on poinsettia. Then, we determined whether changes in the preference and/or survivorship influenced the effectiveness of the trap cropping. Adult whiteflies from both strains consistently redistributed and settled on an eggplant trap crop in significantly higher numbers compared to poinsettia. The adult survivorship of the poinsettia‐strain whiteflies was slightly higher on poinsettia than on the bean‐strain. In research greenhouse experiments, we found that the trap cropping consistently resulted in a decrease in the density of the poinsettia‐strain whiteflies on the cash crop compared to that in monoculture. However, higher adult whitefly survivorship on eggplant than on poinsettia could compromise its effectiveness as a trap crop in poinsettia. The effectiveness of trap cropping, as reflected by the whitefly density reduction on a poinsettia cash crop, was significantly smaller than the attractiveness of the trap crop, as indicated by the whitefly abundance on an eggplant trap crop.

Long-term efficacy of Encarsia dispersa Polaszek (Hymenoptera: Aphelinidae) for the biological control of Aleurodicus dispersus Russell (Hemiptera: Aleyrodidae) in tropical monsoon Australia

Introductions of Encarsia dispersa Polaszek (Hymenoptera: Aphelinidae) for the biological control of spiralling whitefly, Aleurodicus dispersus Russell (Hemiptera: Aleyrodidae), were made directly onto Thursday Island, Torres Strait Queensland, in May 1994. Its efficacy was monitored intermittently for 7 years on 10 Acalypha wilkesiana Mull. Arg. (Euphorbiaceae) and two Terminalia catappa L. (Combretaceae) plants. Marked declines in whitefly densities were recorded on all hosts approximately 10 months after parasitoid releases. Declines in whitefly densities over the first 2 years were mostly matched with increases in per cent parasitism; these declines generally coinciding with wet seasons, with small resurgences occurring over the first dry season following the first decline. Whitefly densities and per cent leaves infested on T. catappa, and on A. wilkesiana with flat leaves were low by the latter half of the study. In contrast, A. wilkesiana with convoluted leaves seemed to inhibit the parasitoid by providing physical refuges for whitefly nymphs, resulting in higher whitefly numbers with low parasitism, particularly in dry conditions. Ten to 12 years after these initial declines, whitefly densities have remained low. Adults and larvae of Cryptolaemus affinis Crotch (Coleoptera: Coccinellidae) and larvae of Acletoxenus quadristriatus Duda (Diptera: Drosophilidae) were observed preying on whitefly nymphs during the study. Overall, sampling data suggested that A. dispersus was under biological control. It may be prudent to maintain A. wilkesiana with convoluted leaves (acting as host refuges) on the Torres Strait islands to retain parasitoid persistence, thereby maintaining controlled and stable populations of A. dispersus. In addition, because of the pest's wide host range and the ability for hosts with closed leaf structures, such as A. wilkesiana with convoluted leaves, to harbour higher pest numbers, movements of these plants within or out of tropical areas could pose a significant quarantine risk.

Influence of Cover Crop and Intercrop Systems onBemisia argentifolli(Hemiptera: Aleyrodidae) Infestation and Associated Squash Silverleaf Disorder in Zucchini

Field experiments were conducted to evaluate the effects of cover cropping and intercropping on population densities of silverleaf whitefly, Bemisia argentifolli Bellow and Perring, and the incidence of squash silverleaf disorder (SSL) in zucchini, Cucurbita pepo L., in Oahu, HI. Two cover crops, buckwheat (BW), Fagopyrum esculentum Moench, and white clover (WC), Trifolium repens L., or sunn hemp (SH), Crotolaria juncea L., and an intercropped vegetable, okra, Abelmonchus esculentus L., were evaluated during the 2003, 2005, and 2006 growing seasons, respectively. Population densities of whiteflies and SSL severity varied during the three field experiments. In 2003, the severity of SSL and percentage of leaves displaying symptoms were significantly lower on zucchini plants in WC than BW plots throughout the crops' growth cycle. Additionally, the percentage of leaves per plant displaying SSL symptoms was significantly greater in bare-ground (BG) compared with the pooled BW and WC treatments on each inspection date. In 2005, zucchini intercropped with okra had lower numbers of adult whiteflies and resulted in significantly lower severity of SSL than pooled BW and WC treatments. During 2006, zucchini grown with SH had significantly lower numbers of all whitefly stages (i.e., egg, immature, and adult) and less SSL severity symptoms than BW. Despite these differences in whitefly numbers and SSL severity, marketable yields were not significantly lower in BW compared with WC or SH treatment plots during the study. The mechanisms underlying these results and the feasibility of using cover crops and intercrops to manage B. argentifolli and SSL are discussed.

Effect of tomato yellow leaf curl disease on reproduction of Bemisia tabaci Q biotype (Hemiptera: Aleyrodidae) on tomato plants

Nonviruliferous adults of Bemisia tabaci Q biotype were released at different densities onto Tomato yellow leaf curl virus (TYLCV)-infected and healthy tomato plants (Lycopersicon esculentum ‘House Momotaro’) in the laboratory. The number of progeny was related to the release density of whiteflies but not to the presence of TYLCV infection on tomato plants. Q biotype adults reared on tomato plants that were either healthy or infected with TYLCV were allowed to oviposit on healthy tomato plants in the laboratory. There were no significant differences in survival rate and fecundity between viruliferous and nonviruliferous whiteflies. Q biotype adults were also released on TYLCV-infected and healthy tomato plants grown in glasshouses, and then the number of adults was investigated for two consecutive generations. In both generations there was no significant difference in the whitefly reproductive rate per generation between those released on infected and healthy tomato plants. These results suggest that symptomatic TYLCV infection of susceptible tomato plants has little effect on the reproduction of the virus's vector, the Q biotype of B. tabaci.

Population suppression of Bemisia tabaci (Hemiptera: Aleyrodidae) using yellow sticky traps and Eretmocerus nr. rajasthanicus (Hymenoptera: Aphelinidae) on tomato plants in greenhouses

AbstractWe conducted three experiments for management of Bemisia tabaci (Gennadius) biotype ‘B’ on tomatoes under greenhouse conditions: (i) vertically placing yellow sticky cards either parallel or perpendicular to tomato rows at a rate of 1 per 3‐m row; (ii) releasing Eretmocerus sp. nr. rajasthanicus once at 30 adults/m2 in the high whitefly density greenhouses (> 10 adults/plant), or twice at 15 adults/m2 at a 5‐day interval in the low whitefly density greenhouses (< 10 adults/plant); and (iii) using combinations of yellow sticky cards that were placed vertically parallel to tomato rows and parasitoids released once at 30/m2 in high whitefly density greenhouses or twice at 15/m2 at a 5‐day interval in low whitefly density greenhouses. Our data show that yellow sticky cards trapped B. tabaci adults and significantly reduced whitefly populations on tomato. The yellow sticky cards that were placed parallel to tomato rows caught significantly more whitefly adults than those placed perpendicular to tomato rows on every sampling date. In the treatment where parasitoids were released once at 30/m2 in high whitefly density greenhouses, the number of live whitefly nymphs were reduced from 4.6/leaf to 2.9/leaf in 40 days as compared with those on untreated plants on which live whitefly nymphs increased from 4.4/leaf to 8.9/leaf. In the treatment where parasitoids were released twice at 15/m2 in low whitefly density greenhouses, the numbers of live nymphs of B. tabaci on tomato leaves were reduced from 2.1/leaf to 1.7/leaf in 20 days as compared with those on untreated plants on which numbers of live nymphs of B. tabaci increased from 2.2/leaf to 4.5/leaf. In the treatment of yellow sticky cards and parasitoid release once at 30/m2 in high whitefly density greenhouses, the numbers of live nymphs of B. tabaci on tomato leaves were reduced from 7.2/leaf to 1.9/leaf, and in the treatment of yellow sticky cards and parasitoid release twice at 15/m2 at a 5‐day interval at low whitefly density, the numbers of live nymphs of B. tabaci on tomato leaves were reduced from 2.5/leaf to 0.8/leaf; whereas the numbers of live nymphs of B. tabaci on untreated plants increased from 4.4/leaf to 8.9/leaf. An integrated program for management of B. tabaci on greenhouse vegetables by using yellow sticky cards, parasitoids and biorational insecticides is discussed.

Density estimation of Bemisia tabaci (Hemiptera: Aleyrodidae) in a greenhouse using sticky traps in conjunction with an image processing system

Accurate forecasting of pest density is essential for effective pest management. In this study, a simple image processing system that automatically estimated the density of whiteflies on sticky traps was developed. The estimated densities of samples in a laboratory and a greenhouse were in accordance with the actual values. The detection system was especially efficient when the whitefly densities were at medium to high levels, however, the development of identification algorithms and more appropriate collection methods may improve the image processing system. Taken together, the results of this study indicate that the image processing system described here may be useful for evaluating the population dynamics.

Efecto de la densidad de población de Trialeurodes vaporariorum (Hemiptera: Aleyrodidae) sobre la eficiencia del depredador Delphastus pusillus (Coleoptera: Coccinellidae)

Los estudios de dinámica de población de los enemigos naturales de las plagas han demostrado que cambios en la densidad de la presa o del huésped inciden en la efectividad biocontroladora de los insectos depredadores o parasitoides. En el presente estudio se evaluó el efecto de la densidad de población de la mosca blanca de los invernaderos, Trialeurodes vaporariorum, sobre la eficiencia de búsqueda del depredador Delphastus pusillus, mediante la determinación de la respuesta funcional. A partir de plantas de fríjol infestadas con T. vaporariorum se tomaron foliolos con presencia de huevos o ninfas de la plaga y se colocaron en cajas de Petri de 5 cm de diámetro con papel de filtro humedecido. En cada caja se introdujo un individuo del depredador, en estado de larva o adulto, y se dejó en contacto durante 24 h con cada una de las densidades de la presa de 8, 16, 32, 64 y 128 individuos. Se encontró que, tanto el adulto como la larva de D. pusillus, presentan una respuesta funcional de tipo II al depredar ninfas de la mosca blanca. Sin embargo, cuando se les suministraron huevos, la curva no mostró una tendencia clara con las densidades de presa ofrecidas. El adulto del depredador presentó un alto consumo del estado de huevo en todas las densidades, con tasas de ataque (a’) de 0,1480 presas consumidas/densidad y un tiempo de manipulación (Th) de 108 segundos, mientras que la larva mostró poca preferencia por el estado de huevo.

Whitefly control in cut gerbera: is it possible to control Trialeurodes vaporariorum with Encarsia formosa?

We investigated the impact of inundative releases of the parasitoid, Encarsia formosa Gahan (Hymenoptera: Aphelinidae), for control of greenhouse whitefly, Trialeurodes vaporariorum (Westwood), on cut gerbera (Gerbera jamesonii L.) under controlled greenhouse conditions. Experimental units consisted of ten plants covered and separated from other units by gauze tents. We assessed three release rates of the aphelinid parasitoid: a 7-week experiment with a standard release rate (10 m−2/14 days), and a subsequent 3-month trial with high (100 m−2/week) and very high (1,000 m−2/week) release rates. Experimental units without release of parasitoids served as control treatment. Gerbera plants were infested initially with 50–100 juvenile and 50–70 adult whiteflies in the first experiment, and in the second experiment with less than 50 juveniles per plant and 50–70 adults. Whitefly and parasitoid population density were assessed in weekly intervals using infestation and activity categories. Results show that parasitized whiteflies were present in all treatments within 2 weeks after initial release. Unfortunately, it was not possible to control whiteflies with standard release rates of E. formosa. Although parasitism rates slightly increased, the effect on whitefly populations was negligible. Large amounts of honeydew and growth of sooty mold fungi caused the termination of the first experiment. In a second experiment, E. formosa was tested at 10–100 times higher release densities. In contrast to the first experiment, whitefly densities increased steadily during the first 8 weeks, but remained constant until the end of the experiment in both treatments. Parasitism by E. formosa reached its maximum after 8 weeks. We discuss possible reasons for the low efficiency of E. formosa as a whitefly antagonist in greenhouse production of gerbera.

Management and Yield Impact of the Greenhouse Whitefly (Trialeurodes vaporariorum) on California Strawberries

Reductions in strawberry yields were observed in association with infestations of the greenhouse whitefly, Trialeurodes vaporariorum (Westwood). The magnitude of yield impact is estimated using data from a field experiment evaluating nine management approaches for the greenhouse whitefly on strawberries, var. Camarosa. A range of effectiveness and yields was found. Imidacloprid (Admire) applied at transplanting was found to provide the greatest effect on adult whitefly densities. Pyriproxyfen (Esteem) had no immediate effect on adult density but reduced nymphal densities over time. Pairing the use of these products at transplant (Admire) and early spring (Esteem) provided the greatest reduction in whitefly density observed relative to an untreated control. Reductions in strawberry yields were found to increase as whitefly densities grow or as populations persist over time. For example, the estimates from our analysis suggest that at 20 weeks after planting, an average whitefly population may reduce average weekly yields in an untreated field by ≈80 g/plant.

Evaluation of Cotton Germ Plasm for Resistance to the Whitefly and Cotton Leaf Crumple (CLCr) Disease and Etiology of CLCr in California's Imperial Valley.

Cotton (Gossypium hirsutum) entries were evaluated for resistance to the whitefly (Bemisia tabaci biotype B) and cotton leaf crumple (CLCr) disease during the 1999 to 2001 growing seasons in the Imperial Valley of California. Entries were evaluated for densities of whitefly adults and nymphs, and for CLCr, by visual rating and squash/dot blot hybridization analyses. Differences in whitefly densities were detected among entries, but none were highly resistant, nor was there any correlation with CLCr disease severity. Entries AP 4103 and AP 6101 had relatively low whitefly densities and were highly susceptible (high CLCr disease severity ratings and viral titers), whereas NK 2387C and DPX 1883 also had low whitefly densities but were highly resistant (no symptoms or detectable viral titers). Other entries showed moderate CLCr resistance, which was independent of whitefly density. Geminivirus DNA-A and DNA-B components were consistently detected in cotton leaves with CLCr symptoms by polymerase chain reaction (PCR) with degenerate begomovirus primers, and full-length DNA-A and DNA-B clones were obtained. Cotton seedlings inoculated with these cloned DNAs by particle bombardment developed CLCr symptoms, and progeny virus was whitefly-transmissible. Sequence analysis revealed that these clones comprised the genome of a California isolate of the bipartite begomovirus Cotton leaf crumple virus (CLCrV-CA). Thus, CLCr disease in the Imperial Valley is caused by CLCrV-CA, and cotton entries with high levels of resistance were identified.

Effect of planting date and nitrogen fertilization on photosynthesis and soluble carbohydrate contents of cotton in relation to silverleaf whitefly (Bemisia tabaci biotype "B") populations

Abstract The impacts of planting date and nitrogen fertilization on cotton (Gossypium hirsutum L.) photosynthesis and soluble carbohydrate contents in relation to silverleaf whitefly, Bemisia tabaci (Gennadius) biotype “B”, populations were examined in field experiments. Cotton planted in late April and early June was treated with 0,112, 168 and 224 kg/N hectare in soil using urea fertilizer. The mean photosynthetic rate of April-planted cotton was 4%J20% higher than that of June-planted cotton early in the season, but 10%J 18% lower than that of June-planted cotton late in the season. The photosynthetic rates for both planting dates were positively correlated with levels of added nitrogen. While levels of glucose for both planting dates were positively correlated with nitrogen levels, fructose and sucrose levels were not. The mean levels of fructose were up to 40% lower, while that of sucrose were up to 59% higher, in April-planted cotton than in June-planted cotton. Levels of photosynthetic rate or stomatal conductance were not correlated with adult whitefly densities for either planting date. Levels of glucose and fructose were positively correlated with whitefly densities only for June-planted cotton late in the season.

Trials of Beauveria bassiana, Paecilomyces fumosoroseus and imidacloprid for management of Trialeurodes vaporariorum (Homoptera: Aleyrodidae) on greenhouse grown lettuce

Mineral oil-based emulsifiable preparations of Beauveria bassiana (Bb) and Paecilomyces fumosoroseus (Pfr) conidia were separately applied alone or together with low rates of imidacloprid 10% WP at 4.7% (Im 1), 14.0% (Im 2), and 23.3% (Im 3) of its recommended application rate, respectively, against the greenhouse whitefly, Trialeurodes vaporariorum, on lettuce grown in the greenhouse. Besides eight fungal treatments, the three low application rates of imidacloprid in the oil-based carrier and a blank control (CK) were also included as treatments of the trials conducted in 2002 and 2003. For the 12 treatments of each trial with three replicates, 1,000-fold aqueous dilutions were sprayed twice on transplanted lettuce at a 15-day interval at a rate of ∼1.43×1013 conidia ha−1 for each fungal treatment or at one of the low rates of imidacloprid using a backpack hand-operated hydraulic sprayer. Based on whitefly densities, mortalities, relative efficacies and percent density declines estimated from whitefly counts made at 5-day intervals, all B. bassiana and P. fumosoroseus sprays were highly effective against T. vaporariorum compared to CK and Im 1–3 in both trials. In trial 1, the estimates of whitefly density decline and relative efficacy ranged from 44 and 72% (Bb) to 79 and 90% (Pfr+Im 2–3) on day 10 after the first spray and exceeded 94% for all fungal treatments 10 days after the second spray. Similar trends in whitefly control were also achieved in the corresponding treatments of trial 2. A more desirable and faster control resulted from fungal sprays containing more imidacloprid, but none of the three low imidacoprid rates alone suppressed the whitefly population more effectively than any fungal treatment despite a varying degree of efficacy. Moreover, P. fumosoroseus tended to be more effective against T. vaporariorum than B. bassiana when applied alone or together with the same low rate of imidacloprid but the difference in whitefly control eventually achieved was not significant between the two fungal agents. Thus, the emulsifiable preparations of both fungal agents can be considered as alternatives for whitefly management and for a slightly faster control can be applied together with imidacloprid at around 15% of its common application rate. An economic balance between the efficacy of whitefly control and the ease and cost of conidial production is needed when both fungi are incorporated into whitefly management.

DOES AN AUTOPARASITOID DISRUPT HOST SUPPRESSION PROVIDED BY A PRIMARY PARASITOID?

Theory predicts that intraguild consumers such as predators or parasitoids may displace more specialized heterospecific competitors and thereby actually increase the population densities of a shared host or prey. We tested this idea with a native primary parasitoid, Eretmocerus eremicus, and an exotic autoparasitoid Encarsia sophia, both attacking the sweetpotato whitefly Bemisia tabaci. Autoparasitoids are intraguild consumers that attack and kill both the immature stages of hemipteran hosts, such as whiteflies, and heterospecific and conspecific parasitoids. In population cages on cotton plants in the field in 1997 and 1998, we introduced whiteflies and then parasitoids in a replacement design with constant total numbers of parasitoids, as follows: (1) control (whiteflies only), (2) E. eremicus only, (3) E. sophia only, and (4) both E. eremicus and E. sophia. Destructive samples of plants were taken 2, 4, and 6 wk after wasp releases, and immature whitefly and wasp stages were censused. In 1997, there were no significant differences in whitefly densities among treatments. In 1998, the control treatment densities were significantly greater than parasitoid treatments, but there was no difference among the parasitoid treatments, indicating equivalent suppression of whitefly populations by the two parasitoid species. In both years, similar patterns in parasitoid dynamics were observed. Densities of E. eremicus were significantly higher in the absence of E. sophia. In contrast, E. sophia densities were unaffected by the presence of E. eremicus. The results suggest that interference by the autoparasitoid reduced primary parasitoid density, but with no concommitant disruption of host suppression. The results support theoretical predictions that no disruption should occur when both parasitoids are equally efficient and suggest that an autoparasitoid may be as efficient as a primary parasitoid in suppressing host densities.

A field experiment on the effectiveness of spiders and carabid beetles as biocontrol agents in soybean

Abstract1 Spiders and carabid beetles are abundant generalist predators that prey upon insect pests of soybean. A field experiment was conducted to determine the impact of spiders and carabids on soybean yield. Prior to planting, three 7 × 7 m plots were fenced in order to reduce spider and carabid immigration. Carabids that emerged within the plots were not removed, but spiders that ballooned into these predator‐reduction plots or that entered by climbing the fence were removed by pitfall trapping and searching the vegetation. Three unmanipulated, unfenced plots served as the control treatment.2 Densities of spiders on soybean vegetation, and activity‐densities of spiders and carabids determined by pitfall trapping, were c. 75% lower in the spider‐carabid reduction treatment than in control plots. Despite clear differences between treatments in numbers and activity of these major generalist predators, the weight of soybeans harvested did not differ between control and spider‐carabid reduction plots.3 Paralleling the absence of an effect of predator reduction on soybean yield was the absence of any significant difference between treatments in densities of whiteflies (Aleyrodidae), leafhoppers (Cicadellidae), thrips (Thysanoptera), Lepidoptera larvae and herbivorous Coleoptera.4 Our experiment provides no evidence that spiders and carabid beetles at ambient densities affect soybean yield. Low populations of pest species or low predation pressure on soybean pests by spiders and carabids at the ambient densities of this experiment could be responsible for this result.

Distribution and sampling of the whiteflies Aleurothrixus floccosus, Dialeurodes citri, and Parabemisia myricae (Homoptera: Aleyrodidae) in citrus in Spain.

From 1993 to 1995 data sets were collected from four citrus groves in Valencia, Spain, to determine the distribution patterns of eggs and nymphs of Aleurothrixus floccosus (Maskell), Dialeurodes citri (Ashmead), and Parabemisia myricae (Kuwana) on leaves, and to develop reliable sampling plans for estimating densities of immature whiteflies. A. floccosus showed higher aggregation than the other two species. The dispersion index b from the Taylor power law did not vary between different developing stages for A. floccosus and D. citri, reaching overall values of 1.70 and 1.53, respectively. In P. myricae, b was 1.60 for eggs and N1, and 1.46 for the remaining nymphs. The minimum number of leaves to estimate the population density with a coefficient of variation of 0.25 for densities above 10 immature whiteflies per leaf was 40 for D. citri and P. myricae, and 250 for A. floccosus. Binomial sampling programs for the three species were rejected for pest management purposes due to the high sample sizes required. The enumerative procedure of counting the number of insects per leaf appears to be the most suitable method for D. citri and P. myricae. For A. floccosus an index of occupation (from 0 to 10) linearly related to the proportion of the leaf undersurface occupied by this insect was found to be reliable and time-saving. Examining 150 leaves with this index achieves the desired relative variation level of 0.25 for most population densities usually found in commercial groves.

Colonisation of tomato fields by predatory mirid bugs (Hemiptera: Heteroptera) in northern Spain

Dicyphus tamaninii and Macrolophus caliginosus are predatory bugs that significantly contribute to control of pests in tomato crops following early colonisation in spring from winter refuges. This paper aimed to determine the influence of the richness of nearby potential predator sources at the time of arrival, amount and establishment of the mirid bugs in tomato fields. For this, six tomato fields were selected that showed a gradient of vegetational settings in the landscape. Crops and non-crop vegetation were sampled with a vacuum suction sampler and abundance of potential colonising predators was estimated. Soon after transplantation of tomatoes, weekly visual counts on the crop were initiated and continued for 8 weeks to estimate densities of Trialeurodes vaporariorum adults, the most abundant pest, and predator adults and nymphs. Time of predator arrival in each field, which varied between 6 and 8 weeks after seedling transplantation, was not related to the respective predator abundance in the surrounding vegetation. Eleven weeks after transplantation, fields differed in their cumulative numbers of insect-days (greenhouse whitefly, predator adults and nymphs). Differences in predator-days were not related to densities of greenhouse whitefly-days but to predator abundance in field surroundings. Fields were also colonised by predators coming from distant sources. Mirid bugs were more abundant in outer rows, particularly in fields with close predator sources and were concentrated on plants with higher whitefly densities. Predator nymphs were detected soon after adult arrival and distributed in the field mostly according to adult predator or prey spatial pattern. Presence of winter refuges for predators in nearby habitats increases the amount of colonisation. However, in order to rely on predator colonisation into fields, the stimuli involved in the predator movement from winter refuges to crops and the predator dispersal capacity should be further investigated.

Relationship of Silverleaf Whitefly Population Density to Severity of Irregular Ripening of Tomato

The silverleaf whitefly (Bemisia argentifolii Bellows & Perring) is an important pest of tomatoes in Florida and elsewhere. Associated with populations of the whitefly is an irregular ripening disorder of fruit characterized by inhibited or incomplete ripening of longitudinal sections of fruit and by an increase in the amount of interior white tissue. Experiments were conducted during the spring and fall tomato production seasons of 1995 and 1996 to elucidate the relationship of nymphal and pupal density with severity of the disorder. Insecticides or insecticide combinations were applied at predetermined densities of whitefly nymphs and pupae and the subsequent severity of the disorder was rated separately for external and internal symptoms on red ripe fruit harvested weekly. Expression of irregular ripening symptoms, especially external symptoms, were correlated positively to the density of whitefly nymphs and pupae (number·10-1 terminal leaflets on the seventh to eighth leaf from the top of a main or lateral stem) increased. Expression of external symptoms tended to be better correlated with whitefly density when symptom severity was rated 1 and 3 weeks after estimating whitefly density for the spring and fall seasons, respectively. Expression of internal symptoms tended to be more consistently correlated with whitefly density when symptom severity was rated 2 and 3 weeks after estimating whitefly density for the spring and fall seasons, respectively.

Susceptibility of Upland Cotton Cultivars to <I>Bemisia tabaci</I> Biotype B (Homoptera: Aleyrodidae) in Relation to Leaf Age and Trichome Density

The relationships between leaf trichome densities, leaf age, and sweetpotato whitefly, Bemisia tabaci (Gennadius) biotype B, infestations of 13 upland cotton, Gossypium hirsutum L., cultivars were investigated in 1990 and 2000. Stoneville 474 supported higher numbers of B. tabaci biotype B eggs, nymphs and adults, and also had higher numbers of stellate trichomes on abaxial leaf surfaces compared with other cotton cultivars. Siokra L-23, in general, had fewer stellate trichomes and also fewer whiteflies. However, the positive trichome–whitefly density relationships were affected by the ages of leaves from different main stem cotton nodes. The youngest leaves on main stem node 1 below the terminal for all cultivars had higher numbers of stellate trichomes but fewer whiteflies compared with older leaves.

Density-yield relationships and economic injury levels for Bemisia argentifolii (Homoptera: Aleyrodidae) in cantaloupe in Texas.

A range of infestation levels of the whitefly Bemisia argentifolii Bellows & Perring were established across experimental field plots of cantaloupe by varying insecticide treatments to evaluate the relationships between whitefly density and crop yield and quality. High levels of whitefly adults and immatures were associated with significant reductions in fruit yield, a decrease in fruit size, and an increase in the percentage of fruits with sooty mold. Yield loss rates (kg/ha/adult or nymph) decreased in a nonlinear fashion with increasing whitefly numbers. Estimated economic injury levels varied as a function of whitefly density, whitefly stage, control cost, crop cultivar, and crop season. Estimates of economic injury levels for one ($31.2/ha), five ($156/ha), and 10 ($312/ha) insecticide treatments ranged from 0.02 to 0.39, from 0.12 to 1.96, and from 0.24 to 3.92 adults/leaf, and from 0.20 to 5.43, from 0.98 to 27.17, and from 1.97 to 54.35 nymphs/6.45 cm2, respectively.