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Citrus nematode, Tylenchulus semipenetrans Cobb, 1913 is a major economically important plant-parasitic nematode responsible for citrus slow decline worldwide. This study aimed to evaluate the efficacy of controlled mild water stress on priming defences in navel orange ( Citrus sinensis “Navel”) seedlings and to model nematode-host interactions under stress. Five greenhouse treatments were applied: negative control (C-, 100% field capacity), positive control (C+, 100%), pre-infection water stress (T1, 40%), post-infection water stress (T2, 40%), and continuous water stress (T3, 70%). Nematode reproduction criteria, plant growth and physiological and biochemical parameters were evaluated. Results indicate that the controlled mild water stress significantly ( p  ≤ 0.05) reduced nematode reproduction. The final nematode population (Pf) and reproduction factor decreased from 6.176 to 2.47 in positive control to 2.034 and 0.81 in continuous water stress. Shoot length and fresh weight declined from 31.47 cm to 29.22 g in negative control to 7.30 cm and 6.58 g in continuous water stress. Regression based model indicated that a strong positive correlation between nematode final population and orange seedlings growth performance (R 2  = 0.995–0.999), demonstrated that the controlled mild water stress plays a critical role in mediating the interaction between T. semipenetrans and its host plant. Stressed plants accumulated higher flavonoids (up to 4.563 mg QE g − 1 FW), phenolics (6.498 mg g − 1 FW min − 1 ), and proline (11.50 mg g − 1 FW), while chlorophyll a and b decreased to 0.607 and 0.207 mg g − 1 FW, respectively. These findings suggest that controlled mild water stress, particularly when applied as a pre- or post-infection, can reduce T. semipenetrans reproduction and enhance citrus defense mechanisms. However, continuous water stress limits plant growth, highlighting a balance between nematode suppression and host adaptation for sustainable management.

Plants use volatile terpene compounds as odor cues for communicating with the environment. Fleshy fruits are particularly rich in volatiles that deter herbivores and attract seed dispersal agents. We have investigated how terpenes in citrus fruit peels affect the interaction between the plant, insects, and microorganisms. Because limonene represents up to 97% of the total volatiles in orange (Citrus sinensis) fruit peel, we chose to down-regulate the expression of a limonene synthase gene in orange plants by introducing an antisense construct of this gene. Transgenic fruits showed reduced accumulation of limonene in the peel. When these fruits were challenged with either the fungus Penicillium digitatum or with the bacterium Xanthomonas citri subsp. citri, they showed marked resistance against these pathogens that were unable to infect the peel tissues. Moreover, males of the citrus pest medfly (Ceratitis capitata) were less attracted to low limonene-expressing fruits than to control fruits. These results indicate that limonene accumulation in the peel of citrus fruit appears to be involved in the successful trophic interaction between fruits, insects, and microorganisms. Terpene down-regulation might be a strategy to generate broad-spectrum resistance against pests and pathogens in fleshy fruits from economically important crops. In addition, terpene engineering may be important for studying the basic ecological interactions between fruits, herbivores, and pathogens.

Background Phloem-feeding insects are known to modulate the salicylic acid (SA) signaling pathway in various plant-insect interaction models. Diaphorina citri is a phloem feeding vector of the deadly phytopathogens, Candidatus Liberibacter americanus and Candidatus Liberibacter asiaticus, and the interactions of D. citri with its host that may modulate plant defenses are not well understood. The objectives of this study were to investigate the molecular mechanisms involved in transcriptional regulation of SA modification and activation of defense-associated responses in sweet orange ( Citrus sinensis ) exposed to various durations (7-, 14- and 150- days) of continuous feeding by D. citri . Results We quantified expression of genes involved in SA pathway activation and subsequent modification, as well as, associated SA metabolites (SA methyl ester, 2,3-DHBA, and SA 2-O-β-D-glucoside). NPR1 and PR-1 expression was upregulated in plants exposed to continuous feeding by D. citri for 14 days. Expression of BSMT-like , MES1-like and DMR6-like oxygenase , as well as, accumulation of their respective SA metabolites (SA methyl ester, 2,3-DHBA) was significantly higher in plants exposed to continuous feeding by D. citri for 150 days than in those without D. citri infestation. Concomitantly, expression of UGT74F2-like was significantly downregulated and its metabolite, SA 2-β-D-glucoside, was highly accumulated in trees exposed to 150 d of feeding compared to control trees without D. citri . Conclusions D. citri herbivory differentially regulated transcription and SA-metabolite accumulation in citrus leaves, depending on duration of insect feeding. Our results suggest that prolonged and uninterrupted exposure (150 d) of citrus to D. citri feeding suppressed plant immunity and inhibited growth, which may highlight the importance of vector suppression as part of huanglongbing (HLB) management in citrus.

The carambola fruit fly, Bactrocera carambolae, is a tephritid native to Asia that has invaded South America through small-scale trade of fruits from Indonesia. The economic losses associated with biological invasions of other fruit flies around the world and the polyphagous behaviour of B. carambolae have prompted much concern among government agencies and farmers with the potential spread of this pest. Here, ecological niche models were employed to identify suitable environments available to B. carambolae in a global scale and assess the extent of the fruit acreage that may be at risk of attack in Brazil. Overall, 30 MaxEnt models built with different combinations of environmental predictors and settings were evaluated for predicting the potential distribution of the carambola fruit fly. The best model was selected based on threshold-independent and threshold-dependent metrics. Climatically suitable areas were identified in tropical and subtropical regions of Central and South America, Sub-Saharan Africa, west and east coast of India and northern Australia. The suitability map of B. carambola was intersected against maps of fruit acreage in Brazil. The acreage under potential risk of attack varied widely among fruit species, which is expected because the production areas are concentrated in different regions of the country. The production of cashew is the one that is at higher risk, with almost 90% of its acreage within the suitable range of B. carambolae, followed by papaya (78%), tangerine (51%), guava (38%), lemon (30%), orange (29%), mango (24%) and avocado (20%). This study provides an important contribution to the knowledge of the ecology of B. carambolae, and the information generated here can be used by government agencies as a decision-making tool to prevent the carambola fruit fly spread across the world.

Key message Carrizo transgenic plants overexpressing methionine-gamma-lyase produced dimethyl sulfide. The transgenic plants displayed more resistance to nematode attacks ( Tylenculus semipenetrans ) and may represent an innovative strategy for nematode control. Tylenchulus semipenetrans is a nematode pest of many citrus varieties that causes extensive damage to commercial crops worldwide. Carrizo citrange vr. ( Citrus sinensis L. Usb ×  Poncirus trifoliate L. Raf) plants overexpressing Brevibacterium linens methionine-gamma-lyase (BlMGL) produced the sulfur volatile compound dimethyl sulfide (DMS). The aim of this work was to determine if transgenic citrus plants expressing BlMGL showed increased tolerance to T. semipenetrans infestation and to determine the effect on the content of key amino acids. While transgenic lines emitted dimethyl sulfide from leaves and roots, no sulfur-containing volatiles were detectable in wild-type Carrizo in the same tissues. Significant changes detected some key amino acids from leaves of transgenic plants such as aspartate, lysine, glycine, leucine and threonine with no changes in the amounts of methionine and α-ketobutyrate. In roots only glycine showed significant changes across all transgenic lines in comparison to wild-type plants. Transgenic plants expressing BlMGL and emitting DMS had less T. semipenetrans aggregation and more biomass than infected WT control plants, indicating that they may represent an innovative management alternative to pesticide/nematicide-based remedies.

In 2003, the pest species Bactrocera dorsalis (Hendel) was reported for the first time in Kenya, Africa, and subsequently on many other African countries. In this work, 20 locations along the Rusitu Valley (Zimbabwe) were sampled in 2014 during the sweet oranges fruiting seasons, to verify the fruit fly taxonomy, invasion source, population dynamics, and fruit damage. The trapped fruit flies were identified using morphological traits and molecular techniques, as B. dorsalis . The haplotype network analysis revealed that Zimbabwe COI sequences were identical to other African B. dorsalis sequences. Fruit fly trappings per day varied during the year, although it remained always high. The same applies to fruit damage, most likely due to the permanent availability of cultivated and wild fruit varieties during the year. Rusitu Valley was invaded by B. dorsalis , most likely from neighbouring countries. Ten years after the first report in Kenya, the complete or near complete invasion of Africa has been achieved by B. dorsalis . In northern Africa the distribution is clearly limited by the Sahara desert. The large population size, the polyphagous nature of the species, and the continuous availability of suitable host fruit species during the year complicates the eradication of this species.

Bacterial symbionts are broadly distributed among insects, influencing their bioecology to different degrees. Aphids carry a number of secondary symbionts that can influence aphid physiology and fitness attributes. Spiroplasma is seldom reported as an aphid symbiont, but a high level of infection has been observed in one population of the tropical aphid Aphis citricidus . We used sister isolines of Spiroplasma -infected (Ac-BS) and Spiroplasma -free (Ac-B) aphids reared on sweet orange (optimum host) and orange jasmine (suboptimum host) to demonstrate the effects of Spiroplasma infection in the aphid proteome profile. A higher number of proteins were differently abundant in aphids feeding on orange jasmine, indicating an impact of host plant quality. In both host plants, the majority of proteins affected by Spiroplasma infection were heat shock proteins, proteins linked to cell function and structure, and energy metabolism. Spiroplasma also induced changes in proteins involved in antimicrobial activity, carbohydrate processing and metabolism, amino acid synthesis and metabolism in aphids feeding on orange jasmine. We discuss on how the aphid host proteome is differentially affected by Spiroplasma infection when the host is exploiting host plants with different nutritional values.

• Background and Aims Potassium, sulphur and zinc contents of mistletoe leaves are generally higher than in their hosts. This is attributed to the fact that chemical elements which are cycled between xylem and phloem in the process of phloem loading of sugars are trapped in the mistletoe, because these parasites do not feed their hosts. Here it is hypothesized that mutant albino shoots on otherwise green plants should behave similarly, because they lack photosynthesis and thus cannot recycle elements involved in sugar loading. • Methods The mineral nutrition of the mistletoe Scurrula elata was compared with that of albino shoots on Citrus sinensis and Nerium oleander. The potential for selective nutrient uptake by the mistletoe was studied by comparing element contents of host leaves on infected and uninfected branches and by manipulation of the haustorium-shoot ratio in mistletoes. Phloem anatomy of albino leaves was compared with that of green leaves. • Key Results Both mistletoes and albino leaves had higher contents of potassium, sulphur and zinc than hosts or green leaves, respectively. Hypothetical discrimination of nutrient elements during the uptake by the haustorium is not supported by our data. Anatomical studies of albino leaves showed characteristics of release phloem. • Conclusions Both albino shoots and mistletoes are traps for elements normally recycled between xylem and phloem, because retranslocation of phloem mobile elements into the mother plant or the host is low or absent. It can be assumed that the lack of photosynthetic activity in albino shoots and thus of sugars needed in phloem loading is responsible for the accumulation of elements. The absence of phloem loading is reflected in phloem anatomy of these abnormal shoots. In mistletoes the evolution of a parasitic lifestyle has obviously eliminated substantial feeding of the host with photosynthates produced by the mistletoe.

Antennal sensillae of male and female peach fruit flies, Bactrocera zonata (Saunders) (Diptera: Tephritidae), obtained from three different host fruit species (guava, Psidium guajava L. (Myrtales: Myrtaceae); peach, Prunus persica (L.) Stokes (Rosales: Rosaceae); and orange, Citrus sinensis (L.) Osbeck (Sapindales: Rutaceae)), were studied with scanning electron microscopy. This study was carried out to describe the different types of sensillae present on the three antennal segments (scape, pedicel, and flagellum or funiculus) of both sexes of B. zonata on different host fruit. The antennal segments of females tended to be larger than those of males feeding on peach and guava fruit. On orange, both sexes were similar (no significant differences were found). The first two antennal segments, scape and pedicel, are reinforced by some bristles and have different types of sensillae, including trichoid I, II, S; basiconic II; and sensilla chaetica in different numbers on different host fruit species. Numerous microtrichia, as well as trichoid (I, II), basiconic (I), clavate, and coeloconic (I, II) sensillae were observed on the funiculus with a great variation in number and length. As a result of feeding on different hosts, differences were found between sexes and some plasticity in size, number, distribution, and position of some sensillae, including trichoid, basiconic, chaetica, and clavate on the antennae of the female B. zonata. These sensillae were significantly larger in females. Also, some morphological and morphemetric differences have been found according to their feeding on different host fruit.

Tetranychus urticae is an important pest of citrus, especially lemon and mandarin, under Mediterranean climate. Factors leading to this problem are poorly understood, but saline stress is suspected to contribute to spider mite outbreaks. In this study, the effect of NaCl concentration in nutritive solutions used to water potted young mandarin trees on population growth of T. urticae reared on leaf discs obtained from these plants was investigated. Although the differences observed between treated (5, 10, 30 and 60 mM NaCl) and control groups were in most cases not significant, when all biological parameters calculated were combined to obtain Ro, T and r(m), remarkable differences appeared, and a concentration-dependent effect was detected. Although high salt concentrations negatively affected T. urticae, at the lowest concentration tested the r(m) value was significantly higher than at the water control and this may contribute to the observed field explosions of T. urticae.