Complementary roles of two classes of defense chemicals in white spruce against spruce budworm

MAIN CONCLUSION

Monoterpenes and phenolics play distinct roles in defending white spruce trees from insect defoliators. Monoterpenes contribute to the toxicity of the foliage, deterring herbivory, whereas phenolics impede budworm growth. This study demonstrates the complex interplay between monoterpenes and phenolics and their collective influence on the defense strategy of white spruce trees against a common insect defoliator. Long-lived coniferous trees display considerable variations in their defensive chemistry. The impact of these defense phenotype variations on insect herbivores of the same species remains to be thoroughly studied, mainly due to challenges in replicating the comprehensive defense profiles of trees under controlled conditions. This study methodically examined the defensive properties of foliar monoterpenes and phenolics across 80 distinct white spruce families. These families were subsequently grouped into two chemotypes based on their foliar monoterpene concentrations. To understand the separate and combined effects of these classes on tree defenses to the eastern spruce budworm, we conducted feeding experiments using actual defense profiles from representative families. Specifically, we assessed budworm response when exposed to substrates amended with phenolics alone or monoterpenes. Our findings indicate that the ratios and amounts of monoterpenes and phenolics present in the white spruce foliage influence the survival of spruce budworms. Phenotypes associated with complete larval mortality exhibited elevated ratios (ranging from 0.4 to 0.6) and concentrations (ranging from 1143 to 1796 ng mg-1) of monoterpenes. Conversely, families characterized by higher phenolic ratios (ranging from 0.62 to 0.77) and lower monoterpene concentrations (ranging from 419 to 985 ng mg-1) were less lethal to the spruce budworm. Both classes of defense compounds contribute significantly to the overall defensive capabilities of white spruce trees. Monoterpenes appear critical in determining the general toxicity of foliage, while phenolics play a role in slowing budworm development, thereby underscoring their collective importance in white spruce defenses.

First report of root knot nematodes Meloidogyne incognita and M. javanica parasitizing sweet potato, Ipomoea batatas L., in Portugal

Sweet potato, Ipomoea batatas L., is a tuberous root vegetable rich in low glycemic sugars, vitamins and fibers (Galvão et al., 2021). Although it is widely cropped and consumed in tropical regions, in Europe consumer demand is growing exponentially (CBI, 2021). In Portugal, the production area of sweet potato increased from 588 ha in 2011 to 954 ha in 2017, and exports increased from 2404 tons in 2011 to 13412 tons in 2019 (FAOSTAT, 2021). During a survey carried out in August 2019, sweet potato plants were collected in Almada (38°39'40"N 9°10'54"W) and Belmonte (38°39'40"N 9°10'54"W), South and Centre regions of Portugal, respectively. No symptoms were observed on leaves, however, roots presented numerous galls and/or small spots (females and respective egg masses) were observed in the tuberous root flesh, suggestive of root knot nematodes (RKN, Meloidogyne spp.) infection. At least 8 individual females and respective egg masses were handpicked from roots of each sample and characterized biochemically by electrophoretic analysis of esterases (Pais & Abrantes, 1989). Phenotypes I2 and J3, attributed to M. incognita and M. javanica, respectively, were present in samples from Almada, whereas only phenotype I2 was found from Belmonte sample (Santos et al., 2019). Pure RKN cultures were established on tomato cv. Coração-de-Boi to obtain inoculum for molecular characterization and host suitability assays. Molecular characterization was performed by DNA amplification with M. incognita (Mi-F/Mi-R) and M. javanica (Fjav/Rjav) species-specific primers (Zijlstra et al., 2000; Meng et al., 2004). DNA amplification resulted in unique bands of ≈900 bp and ≈650 bp, respectively, confirming the RKN species identification. The host suitability of sweet potato cvs. Lira (local variety, purple skin, yellow flesh) and Murasaki (purple skin, white/pale to yellow flesh) to M. javanica (Almada) and M. incognita (Belmonte) isolates was assessed. Sweet potato slips with ≈10 cm roots were transplanted to 500 cm3 pots (one slip/pot) and after 2 weeks, each plant was inoculated with 5000 eggs + second-stage juveniles (Pi, initial population density) and maintained in a growth chamber (25±2°C; 12:12 h photoperiod). Tomato cv. Coração-de-Boi was included as a positive control. Each RKN species-plant germplasm combination was repeated 6 times. At 60 days after inoculation, host suitability was evaluated on the basis of root gall index (GI) and reproduction factor (Rf=final population density/Pi) (Sasser et al., 1984). Sweet potato cv. Lira was susceptible (GI=5; Rf=111.8) to M. incognita and resistant (GI=2; Rf=0.11) to M. javanica; while cv. Murasaki was hypersusceptible (GI=5; Rf=0.9) to M. incognita and susceptible (GI=5; Rf=5.5) to M. javanica. Although cultivars varied in their response to M. incognita and M. javanica isolates and variation in the final population density was high, both RKN isolates reproduced in these sweet potato cultivars. In previous studies, cv. Murasaki was considered resistant to M. enterolobii and to M. incognita (La Bonte et al. 2008; Schwarz et al., 2021). Depending on the RKN species, cultivation of cvs. Murasaki and Lira may thus benefit succeeding crops, but they should be combined with other management strategies to further reduce RKN populations in the field. In Portugal, M. incognita and M. javanica have been found associated with economically important horticultural crops, such as tomato and potato, trees and weeds (Santos et al., 2019; Maleita et al., 2021). To our knowledge, these species are reported for the first time parasitizing sweet potato in Portugal and this is the first report on the occurrence of M. incognita and M. javanica infecting sweet potato in Europe. Although findings were not totally unexpected due to the wide distribution and host range of these RKN species, they are of crucial importance since the sweet potato production in Europe has almost doubled from 50 (2011) to 97 thousand tons (2017), with Spain, Portugal, Italy and Greece being the largest producers (FAO, 2021). Our findings also reveal that sweet potato cropped in Portugal have different susceptibility levels to these common RKN species, reinforcing the importance of cultivar selection in RKN management.

Bottom-up regulation of a tritrophic system by Beet yellows virus infection: consequences for aphid-parasitoid foraging behaviour and development

Effects of plants on herbivores can cascade up the food web and modulate the abundance of higher trophic levels. In agro-ecosystems, plant viruses can affect the interactions between crops, crop pests, and natural enemies. Little is known, however, about the effects of viruses on higher trophic levels, including parasitoids and their ability for pest regulation. We tested the hypothesis that a plant virus affects parasitoid foraging behaviour through cascading effects on higher trophic levels. We predicted that the semi-persistent Beet yellows virus (BYV) would influence plant (Beta vulgaris) quality, as well as aphid host (Aphis fabae) quality for a parasitoid Lysiphlebus fabarum. We determined amino acid and sugar content in healthy and infected plants (first trophic level), lipid content and body size of aphids (second trophic level) fed on both plants, as well as foraging behaviour and body size of parasitoids (third trophic level) that developed on aphids fed on both plants. Our results showed that virus infection increased sugars and decreased total amino acid content in B. vulgaris. We further observed an increase in aphid size without modification in host aphid quality (i.e., lipid content), and a slight effect on parasitoid behaviour through an increased number of antennal contacts with host aphids. Although the BYV virus clearly affected the first two trophic levels, it did not affect development or emergence of parasitoids. As the parasitoid L. fabarum does not seem to be affected by the virus, we discuss the possibility of using it for the development of targeted biological control against aphids.

Life cycle of Amblyomma mixtum (Acari: Ixodidae) parasitizing different hosts under laboratory conditions

Amblyomma mixtum is a tick species in the Amblyomma cajennense complex. The known geographic range of A. mixtum extends from Texas in the USA to western Ecuador and some islands in the Caribbean. Amblyomma mixtum is a vector of disease agents of veterinary and public health importance. The objective of this study was to describe the life cycle of A. mixtum under laboratory conditions. Bovines, rabbits and sheep were infested with larvae, nymphs, and adult ticks under controlled conditions to assess several biological parameters. Eggs, larvae, nymphs and adults were kept in an incubator (27 °C temperature and 80% relative humidity) when they were off the host. The average life cycle of A. mixtum was 88 and 79 days when fed on rabbits and cattle, respectively. Sheep were found to be unsuitable because no ticks attached. The rabbit is a more practical host to maintain a colony of A. mixtum under laboratory conditions. The data from this study can be considered as an example for the life cycle of A. mixtum. However, caution must be exercised when making comparisons to the biology of A. mixtum in its natural habitat.

Host suitability and diet mixing influence activities of detoxification enzymes in adult Japanese beetles

Induction of cytochrome P450, glutathione S transferase (GST), and carboxylesterase (CoE) activity was measured in guts of the scarab Popillia japonica Newman, after consumption of single or mixed plant diets of previously ranked preferred (rose, Virginia creeper, crape myrtle and sassafras) or non-preferred hosts (boxelder, riverbirch and red oak). The goal of this study was to quantify activities of P450, GST and CoE enzymes in the midgut of adult P. japonica using multiple substrates in response to host plant suitability (preferred host vs non-preferred hosts), and single and mixed diets. Non-preferred hosts were only sparingly fed upon, and as a group induced higher activities of P450, GST and CoE than did preferred hosts. However, enzyme activities for some individual plant species were similar across categories of host suitability. Similarly, beetles tended to have greater enzyme activities after feeding on a mixture of plants compared to a single plant type, but mixing per se does not seem as important as the species represented in the mix. Induction of detoxification enzymes on non-preferred hosts, or when switching between hosts, may explain, in part, the perceived feeding preferences of this polyphagous insect. The potential consequences of induced enzyme activities on the ecology of adult Japanese beetles are discussed.

Host Suitability of Selected Hybrids, Varieties and Inbreds of Corn to Populations of Meloidogyne spp

Rates of reproduction of root-knot nematodes on corn varied with Meloidogyne species, with different populations of certain species, and with corn cultivars. M. arenaria, M. incognita and M. javanica reproduced at varying rates on all corn cultivars tested. None of the three selections of M. hapla reproduced on corn. Most of the Meloidogyne populations increased more rapidly on 'Coker' and 'Pioneer' hybrids than on 'McNair' hybrids or on open-pollinated varieties or inbreds. Nematodes often reduced root growth, but the differences within given nematode-cultivar treatments were not usually significant. Root growth of 'Coker 911,' which supported a high rate of reproduction, was affected less than 'Pioneer 309B' which supported a low rate of nematode reproduction.