Nancy Mabel Greco

Neoseiulus californicus (Acari: Phytoseiidae) as a potential control agent of Tetranychus urticae (Acari: Tetranychidae): effect of pest/predator ratio on pest abundance on strawberry.

Neoseiulus californicus (McGregor) is a promising agent for successful Tetranychus urticae Koch control through conservation techniques, in strawberry crops in La Plata (Buenos Aires, Argentina). In prey–predator interaction, initial relative densities have an important effect on system dynamics. The economic threshold level (ETL) used for this pest in the present study was 50 active mites per leaflet. In our laboratory experiments, initial T. urticae to N. californicus ratio had a significant effect on the population abundance of T. urticae at a 7-day period. When pest/predator ratio was 5/1 (at initial pest densities from 5 to 15 females/leaflet) the final number of active T. urticae/leaflet was significantly lower than the ETL, while at 20 females/leaflet this number did not differ from the ETL. At 7.5/1 ratio, the final number of active T. urticae/leaflet, at initial pest densities from 5 to 15 females/leaflet, reached the ETL without surpassing it. At 10/1 and 15/1 ratios, pest densities exceeded the ETL only at 15 initial T. urticae/leaflet. Most greenhouse and field observations were consistent with the predictions of a graphical model based on experimental results. This predator was very effective in limiting pest densities at a 7-day period and within the range of pest–predator ratios and absolute densities used in this study. Conservation of N. californicus promoting favorable pest/predator ratios may result in early control of T. urticae.

Spatial distribution and coincidence of Neoseiulus californicus and Tetranychus urticae (Acari: Phytoseiidae, Tetranychidae) on strawberry

In strawberry greenhouses in La Plata (Buenos Aires, Argentina), Tetranychus urticae is a major pest and Neoseiulus californicus is its most important established phytoseiid predator. The purpose of this study was to determine the spatial distribution and coincidence of N. californicus and T. urticae on strawberry. T. urticae populations exhibited density variations that were followed, with some temporal delay, by those of the predator. In general terms, N. californicus exhibited lower aggregation than T. urticae. The index of dispersion (I) of T. urticae had values significantly greater than 1 in 100% of the sampling dates. The percentage of infested leaflets increased with prey density in a curvilinear way, ranging from 80 to 100% when density was higher than 100 individuals/leaflet. N. californicus had values of I higher than unity in 86.7% (G1) and 53.8% (G2) of the cases. TaylorÕs b and IwaoÕs β values were greater than 1 for both populations, this indicating aggregation. IwaoÕs results showed larger aggregation units of T. urticae (α=53.24) than of N. californicus (α=3.61), probably due to its higher fecundity and oviposition behaviour of laying eggs in clumps. Both populations were overdispersed, although in general terms, N. californicus exhibited lower aggregation than T. urticae. This would create refuges for the prey, thus increasing the persistence of the system. Index of coincidence of predator (Ic) was high most of the time even at low densities of both populations, decreased abruptly at the end of an interaction cycle and was similar at greenhouse and plant spatial scales. The high spatial coincidence of N. californicus with T. urticae suggests an important dispersal capacity of the predator and a high ability to detect leaflets with prey. The refuge index for the prey decreased with predator density and was higher at greenhouse scale than at plant scale, both at similar predator densities. At greenhouse scale and despite high predator density, the prey had a 10%-refuge, which would lead to the system persistence. Results of the present study suggest that N. californicus is a promising established natural enemy for controlling T. urticae on strawberry.

Diversity of biting midges of the genus Culicoides Latreille (Diptera: Ceratopogonidae) in the area of the Yacyretá Dam Lake between Argentina and Paraguay

The Culicoides communities have been analyzed between 1993/1998 in the area influenced by the Yacyret Dam Lake (Paran River, Argentina-Paraguay). Adults of Culicoides were collected monthly by using CDC light traps exposed for 24 h in 9 sampling sites located at both margins of the river; 21 species were recorded. Highest values of species richness were recorded during 1993/1994, being Quiteria and Corpus the sites with the higest number of species (10 and 11, respectively). The species diversity was elevated in Quiteria, Zaim n, Candelaria, Santa Tecla, Capit n Meza and Corpus (Shannons diversity index 1.0-1.9) while Corate , Ituzaing and Aguapey showed less richness and diversity. The more abundant species were C. insignis, C. venezuelensis, C. leopoldoi, C. limai, C. flinti, C. debilipalpis, C. paraensis and C. guttatus. C. insignis, potential vector of bluetongue virus (BTV) to domestic and wild rumiants in the Neotropical region, is the predominant species in the area and was the only species widely distributed. C. paraensis, a proven vector of Oropouche virus to humans, is a common and abundant species. C. pusillus and C. lahillei, potential vectors of BTV and a filarial parasite, respectively, were occasionally collected. The taxonomic structure of communities was constant during the study period. The occasional species were not characteristic to one particular site and their presence could be related to non-intrinsic conditions.

Pest management plan for the two-spotted spider mite, Tetranychus urticae, based on the natural occurrence of the predatory mite Neoseiulus californicus in strawberries

We developed and validated a management plan for Tetranychus urticae in strawberries, based on the natural control exerted by Neoseiulus californicus and on acaricide applications made only when necessary. The plan has two components: a sampling protocol and a decision chart. Systematic presence–absence sampling of active T. urticae and N. californicus was used to predict prey and predator densities relying only on the proportion of T. urticae-infested leaflets, once the occurrence of the predator was detected in at least one of them. The decision chart, which was constructed taking into account the pest and predator densities and the pests rate of increase, determines the range in the proportion of T. urticae-infested leaflets that will require different actions: to use selective acaricides and re-check at 7 days, to take no action but re-check at 7 days, or to take no action but re-check at 14 days. The management plan was potentially effective and feasible, showing that natural populations of N. californicus can consistently exert strong top-down suppression of T. urticae. Thus, N. californicus is a promising candidate for conservation biological control.

Presence-absence sampling for Tetranychus urticae and its predator Neoseiulus californicus (Acari: Tetranychidae; Phytoseiidae) on strawberries

We propose a systematic presence–absence sampling plan for estimating densities of both Tetranychus urticae Koch (Acari: Tetranychidae) and its predator Neoseiulus californicus (McGregor) (Acari: Phytoseiidae) on strawberries. It is based on the synchrony and spatial coincidence between prey and predator. Active stages of both species on each leaflet and the proportion of T. urticae-free leaflets were recorded from 87 samples collected during two growing seasons on five commercial farms. Seventy to 160 leaflets in a sample were taken at intervals of 5 – 10 m in each row. The distribution-free equation Ln(m) = a + b Ln(-Ln(p)) was used to describe the relationships between the mean number (m) of T. urticae and N. californicus per leaflet and the proportion (p) of T. urticae-free leaflets. We devised the following density estimators: T. urticae: Tu = exp(2.78 + 1.31Ln(−Ln p)) and N. californicus: Nc = exp(− 0.04 + 0.76 Ln(−Ln p)). These estimates, complemented with the knowledge of the economic level of damage and the number of predators necessary to control different densities of T. urticae, are likely to help reduce the use of acaricides in the context of integrated pest management of T. urticae on strawberry.

Negative interaction between twospotted spider mites and aphids mediated by feeding damage and honeydew

Among the herbivorous arthropods that feed on strawberry, the most important are the two-spotted spider mite (TSSM), Tetranychus urticae Koch, and several species of aphids. Mites and aphids belong to different guilds that coexist in the field and feed on the undersides of strawberry leaflets. However, the occurrence of large numbers of individuals of both species on the same leaflet is rarely recorded. We hypothesize that negative interactions between TSSM and aphids explain the intraplant distribution of these herbivores. We first examined the spatial coincidence of both herbivores in the field. Under experimental conditions, we then analyzed: (i) the rate of increase of TSSM and the aphid Chaetosiphon fragaefolii (Cockerell), growing individually and together; (ii) the effect of honeydew on TSSM preference; and (iii) the effect of previous strawberry leaflet damage by TSSM on C. fragaefolii preference. The proportion of TSSM that coincided with at least one aphid decreased as the percentage of leaflets with TSSM increased. The spatial coincidence index between aphids and TSSM increased together with the percentage of TSSM-infested leaflets. TSSM showed both a lower rate of increase when they shared the same leaflet with C. fragaefolii and lower fecundity on strawberry discs with honeydew. The rate of increase of C. fragaefolii did not change on co-occupied leaves, but the aphid species moved to the other side of leaflets shared with TSSM. Negative interactions resulting in a tendency for species to avoid each other, such as demonstrated herein, can affect distribution and performance of herbivorous arthropods.

Diversity of Herbivorous Communities in Blueberry Crops of Different Regions of Argentina

ABSTRACT When a crop is introduced into a new region the herbivorous insect communities are structured from the regional pool of species, and the spatio—temporal heterogeneity of the landscape and the spatial arrangement of habitat elements are essential for species diversity. The blueberry, Vaccinium corymbosum L., is a crop native to the northern hemisphere and was introduced in Argentina in the 1990s. We have analyzed the richness and diversity of insect herbivore communities in several blueberry crops located in different agricultural landscapes of Buenos Aires and Entre Ríos, in Argentina. In total, the number of families recorded throughout the study period was 31. The richness and the diversity showed significant differences among phenological stages and years, although the sites had an effect on the richness, in Buenos Aires. Aphidae was the most abundant family in all sites, followed by different families depending on the surrounding crops and their more common herbivores. The highest values of cumulative alpha diversity were in crops located in more changing landscapes. There were high values of temporal beta diversity everywhere. Spatial beta diversity was higher in Buenos Aires than in Entre Ríos, probably because the crops of the latter are located in similar landscapes. Gamma diversity was 29 families for communities of Buenos Aires and it was 15 families in Entre Ríos. The herbivorous communities in blueberry crops of Argentina are shaped by ectophagous generalist (South American and cosmopolitan) from the regional pool of species, characteristic of the landscape where the crops were introduced.

Multiparasitism of Piezodorus guildinii eggs by Telenomus podisi and Trissolcus urichi

Multiparasitism involves competition between larvae inside the host. Telenomus podisi (Ashmead) and Trissolcus urichi (Crawford) (Hymenoptera: Platygastridae) are solitary egg parasitoids of Piezodorus guildinii Westwood (Hemiptera: Pentatomidae), an important soybean pest. Egg masses partially parasitized by one species were offered to females of the other species. Both species attacked randomly unparasitized and parasitized hosts. Emergence from multiparasitized eggs was greater for T. urichi than for T. podisi, although it was lower than emergence from eggs parasitized by T. urichi alone. Emergence of each species was independent of the order in which they parasitized and of time elapsed between ovipositions. Progeny sex ratio obtained from multiparasitized and from parasitized eggs were similar for both parasitoids. Our results suggest that T. urichi is a better intrinsic competitor than T. podisi for P. guildinii eggs. In the field, however, T. podisi was the dominant species, and T. urichi could be using other pentatomid eggs as resource.

Abundance of Icerya purchasi (Hemiptera: Margarodidae) and Its Parasitoid Cryptochaetum iceryae (Diptera: Cryptochaetidae) in Argentina Blueberry Crops

ABSTRACT Icerya purchasi Maskell is a polyphagous scale pest of ornamentals and several other crops. It has two natural specialist enemies that maintain its populations at low levels: Rodolia cardinalis (Mulsant) and Cryptochaetum iceryae (Williston). I. purchasi was recently observed in blueberry crops (Vaccinium corymbosum L.) in Argentina. Seven blueberry fields located in different agricultural landscapes were sampled from July 2006 to September 2008. I. purchasi was present only in the citrus areas of Gobernador Castro, San Pedro, and Concordia. In the last locality, it was present in January 2007, and the abundance was very low. In Buenos Aires, I. purchasi was more abundant in Gobernador Castro where it was present throughout all study period. In San Pedro, it was only registered in August 2007 and March, April, and June 2008. In Gobernador Castro, where the abundance was highest, the dominant crop near blueberries is citrus. The low abundance in San Pedro and Concordia fields may be attributable to the replacement of citrus crops by nonhosts. In Concordia and San Pedro fields, no parasitized I. purchasi were found. In Gobernador Castro, the proportion of adults parasitized was 0.805 ± 0.286. When the risk of parasitism was analyzed for three density categories, there were no significant differences. R. cardinalis was present only in Gobernador Castro, and very few individuals were observed. The presence and abundance of I. purchasi in blueberry crops in Argentina seems to be linked to the structure of the landscape at the regional level, and the variable incidence of C. iceryae suggests major constraints to the scales population growth.

Effect of Telenomus podisi, Trissolcus urichi, and Trissolcus basalis (Hymenoptera: Platygastridae) age on attack of Piezodorus guildinii (Hemiptera: Pentatomidae) eggs.

ABSTRACT Parasitoids performance is subject to parasitism decisions influenced by host quality and parasitoids age. We evaluated parasitism, emergence, and the progeny sex ratio proportions of Telenomus podisi (Ashmead), Trissolcus urichi (Crawford), and Trissolcus basalis (Wollaston) (Hymenoptera: Platygastridae) females of different age attacking Piezodorus guildinii (Westwood) (Hemiptera: Pentatomidae), an important soybean pest. Eggs were offered to females of T. podisi 1–10 d old, T. urichi 1, 3, 4, or 6 d old, or T. basalis 1–6 or 8 d old. Average egg parasitism proportions were 0.71 and 0.79 to T. podisi and T. urichi, respectively; and preimaginal survivorship was>97% for both species. Trissolcus basalis females 1, 2, and 3 d old did not parasitize P. guildinii eggs, and high parasitism proportions were recorded from older females, but only 0.5 of females attacked host eggs. A greater proportion of males were produced in the progeny of T. podisi and T. urichi as maternal age increased, but no differences were found in the progeny of T. basalis wasps of different ages. The results from this study indicate that performance of T. podisi and T. urichi would not be affected by the age of the females. However, aging would induce T. basalis females to attack P. guildinii, although it is known that their main host is Nezara viridula (L.) (Hemiptera: Pentatomidae). Under certain circumstances, such as shortage of preferred host availability, high egg load, and short life expectancy, this wasp may include P. guildinii as a host.