Opposing roles of plant laticifer cells in the resistance to insect herbivores and fungal pathogens.

More than 12,000 plant species (ca. 10% of flowering plants) exude latex when their tissues are injured. Latex is produced and stored in specialized cells named "laticifers". Laticifers form a tubing system composed of rows of elongated cells that branch and create an internal network encompassing the entire plant. Laticifers constitute a recent evolutionary achievement in ecophysiological adaptation to specific natural environments; however, their fitness benefit to the plant still remains to be proven. The identification of Euphorbia lathyris mutants (pil mutants) deficient in laticifer cells or latex metabolism, and therefore compromised in latex production, allowed us to test the importance of laticifers in pest resistance. We provided genetic evidence indicating that laticifers represent a cellular adaptation for an essential defense strategy to fend off arthropod herbivores with different feeding habits, such as Spodoptera exigua and Tetranychus urticae. In marked contrast, we also discovered that a lack of laticifer cells causes complete resistance to the fungal pathogen Botrytis cinerea. Thereafter, a latex-derived factor required for conidia germination on the leaf surface was identified. This factor promoted disease susceptibility enhancement even in the non-latex-bearing plant Arabidopsis. We speculate on the role of laticifers in the co-evolutionary arms race between plants and their enemies.

Tracking mite trophic interactions by multiplex PCR.

BACKGROUND: A thorough knowledge of trophic webs in agroecosystems is essential to achieve successful biological pest control. Phytoseiid mites are the most efficient natural enemies of tetranychid mites, which include several important pests worldwide. Nevertheless, phytoseiids may feed on other food sources including other microarthropods, plants and even other phytoseiids (intraguild predation), which can interfere with biological control services. Molecular gut content analysis is a valuable tool for characterizing trophic interactions, mainly when working on microarthropods such as mites. We have designed new primers for Phytoseiidae, Tetranychidae and Thysanoptera identification and they have been multiplexed in a polymerase chain reaction (PCR) together with universal plant primers. Additionally, we have estimated prey DNA detectability success over time (DS50 ) considering the most probable events in Spanish citrus orchards: the phytoseiid Euseius stipulatus as a predator, the phytoseiid Phytoseiulus persimilis as intraguild prey, and the thrips Frankliniella occidentalis and Anaphothrips obscurus as alternative prey to Tetranychus urticae. RESULTS: The designed multiplex PCR allows the identification of phytoseiids (both predator and intraguild prey) and detects alternative food sources mentioned above in the gut of the phytoseiid predator. DS50 for E. stipulatus as the predator were 1.3, 2.3 and 18.7 h post feeding for F. occidentalis, A. obscurus and P. persimilis as prey, respectively. CONCLUSION: Tracking of the trophic relationships within the citrus acarofauna, and the unveiling of the role of alternative food sources will pave the way for enhancing T. urticae biological control. This multiplex PCR approach could be applicable for these purposes in similar agroecosystems. © 2019 Society of Chemical Industry.

When do predatory mites (Phytoseiidae) attack? Understanding their diel and seasonal predation patterns.

Predatory mites of the Phytoseiidae family are considered one of the most important groups of natural enemies used in biological control. The behavioral patterns of arthropods can differ greatly daily and seasonally; however, there is a lack of literature related to Phytoseiidae diel and seasonal predation patterns. The predatory activity of three phytoseiid species (two Tetranychidae-specialists, Phytoseiulus persimilis and Neoseiulus californicus, and one omnivore, Euseius stipulatus) that occur naturally in Spanish citrus orchards was observed under laboratory conditions in winter and summer. The temperature and photoperiod of the climatic chamber where the mites were reared did not change during the experiment. Our study demonstrates that phytoseiids can exhibit diel and seasonal predatory patterns when feeding on Tetranychus urticae (Acari: Tetranychidae). Neoseiulus californicus was revealed to be a nocturnal predator in summer but diurnal in winter. In contrast, P. persimilis activity was maximal during the daytime, and E. stipulatus showed no clear daily predation patterns. The predatory patterns described in this study should be taken into account when designing laboratory studies and also in field samplings, especially when applying molecular techniques to unveil trophic relationships.

Can we forecast the effects of climate change on entomophagous biological control agents?

The worldwide climate has been changing rapidly over the past decades. Air temperatures have been increasing in most regions and will probably continue to rise for most of the present century, regardless of any mitigation policy put in place. Although increased herbivory from enhanced biomass production and changes in plant quality are generally accepted as a consequence of global warming, the eventual status of any pest species will mostly depend on the relative effects of climate change on its own versus its natural enemies' complex. Because a bottom-up amplification effect often occurs in trophic webs subjected to any kind of disturbance, natural enemies are expected to suffer the effects of climate change to a greater extent than their phytophagous hosts/preys. A deeper understanding of the genotypic diversity of the populations of natural enemies and their target pests will allow an informed reaction to climate change. New strategies for the selection of exotic natural enemies and their release and establishment will have to be adopted. Conservation biological control will probably become the keystone for the successful management of these biological control agents.

Economic threshold for Tetranychus urticae (Acari: Tetranychidae) in clementine mandarins Citrus clementina.

Tetranychus urticae is a key pest of citrus in Spain, especially of clementine mandarin trees. The effects of this mite on fruit production were assessed in 24 clementine trees for three consecutive years. Trees were visited weekly and spider mite and phytoseiid mite populations and leaf flush patterns were estimated. At the end of the season, mandarins were harvested, weighed, and mite damage (scarring on the fruit) characterized. Negative relationships between spider mite density and yield (kg/tree) and fruit damage (% scarred fruit rind) were found. The multivariate regressions highlighted the key role of phytoseiid mites and leaf flush patterns, which were negatively related to fruit damage. The shortest sampling period that satisfactorily predicted fruit damage at harvest, extended from August to mid-October. For IPM purposes, an action threshold of 31.1 mites m⁻² of symptomatic leaf was estimated. Taking into account spider mite dynamics, the economic threshold ranged from 10 to 15 mites m⁻² of symptomatic leaf. When this threshold is exceeded growers would have a 1-week window to apply the control technologies against T. urticae of their choice.