Mutation in the ace‐1 gene of the tomato leaf miner (Tuta absoluta) associated with organophosphates resistance

The tomato leaf miner, Tuta absoluta (Lepidoptera: Gelechiidae), is a major invasive pest that has spread throughout many countries in the Mediterranean basin and parts of Asia over the last decade. The control of T. absoluta has relied heavily on the use of chemical insecticides, a strategy that has led to the evolution of resistance. In this study, biological and molecular methods were used to determine the susceptibility of five strains of T. absoluta to the organophosphate chlorpyrifos and to investigate the molecular mechanisms underlying resistance to this class of insecticides. High levels of resistance to chlorpyrifos were observed in all five strains tested. Cloning and sequencing of the gene encoding the organophosphate target site, ace‐1, of T. absoluta revealed the presence of an alanine to serine substitution at a position that has been previously linked with organophosphate resistance across a range of different insect and mite species. The presence of this mutation at high frequency in T. absoluta populations originating from various countries further supports the suggestion that the rapid expansion of this species is, in part, mediated by the resistance of this pest to chemical insecticides.     

Organophosphate and pyrethroid resistances in the tomato leaf miner Tuta absoluta (Lepidoptera: Gelechiidae) from Iran

The tomato leafminer, Tuta absoluta (Meyrich) (Lepidoptera: Gelechiidae), is a serious pest of tomato crops worldwide. The intensive use of chemical pesticides to control it has led to the selection of resistant populations. This study investigated the resistance of T. absoluta populations to pyrethroid and the organophosphate insecticides from ten regions of Iran. The resistance ratios at LC₅₀ for chlorpyrifos and diazinon varied among populations from 4.3 to 12 and from 1.4 to 9.0, respectively. The resistance ratios of the pyrethroids cypermethrin, deltamethrin and permethrin varied from 1.3 to 3.7, 2.7 to 13 and 1.2 to 4.3, respectively. Inclusion of synergists in toxicological bioassays and the variation observed in the activity of esterases, glutathione S‐transferase and cytochrome P450‐dependent monooxygenase suggest the existence of metabolically based resistance. Esterase and P450 biochemical assays were positively correlated with deltamethrin, and cypermethrin tolerance and diazinon tolerance correlated with esterase activity. The genes encoding the organophosphate and pyrethroid target sites acetylcholinesterase (ace1) and sodium channel (kdr) were partly sequenced. The genotyping revealed mutations in high frequencies in all populations leading to an A201S substitution in ace1 and three substitutions in the sodium channel gene L1014F, M918T, T929I. In summary, our results indicate the presence of organophosphate and pyrethroid resistance in Iranian T. absoluta populations with involvement of both detoxification enzymes and target site alterations. Most likely the populations of T. absoluta imported to Iran were resistant upon arrival.     

Population growth potential of Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) on tomato,potato, and eggplant

The influence of host plant on population dynamics of an invasive pest, Tuta absoluta was studied on three economically important solanaceous crops. Experiments were conducted in laboratory (29 ± 0.5°C, 75 ± 5% RH and a photoperiod of 14:10 hr [L:D]) using tomato (Solanum lycopersicum L.), potato (Solanum tuberosum L.) and eggplant (Solanum melongena L.). Results indicated that intrinsic rate of increase (r), finite rate of increase (λ) and net reproductive rate (R₀) were higher, and mean generation time (T) was the shortest on tomato. Results suggested that T. absoluta developed on all the three plants, and tomato plant was most preferred one. Results suggested that T. absoluta has a potential to become a serious pest on potato and even on eggplant under favourable conditions. We used the life tables of 0.025th and 0.975th percentiles of bootstraps to project the uncertainty of population growth, a new concept.     

Organ‐specific responses of tomato growth and phenolic metabolism to nitrate limitation

Phenolic compounds are secondary metabolites involved in plant innate chemical defence against pests and diseases. Their concentration varies depending on plant tissue and also on genetic and environmental factors, e.g. availability of nutrient resources. This study examines specific effects of low (LN) and high (HN) nitrogen supply on organ (root, stem and leaf) growth and accumulation of major phenolics [chlorogenic acid (CGA); rutin; kaempferol rutinoside (KR)] in nine hydroponically grown tomato cultivars. LN limited shoot growth but did not affect root growth, and increased concentrations of each individual phenolic in all organs. The strength of the response was organ‐dependent, roots being more responsive than leaves and stems. Significant differences were observed between genotypes. Nitrogen limitation did not change the phenolic content in shoots, whereas it stimulated accumulation in roots. The results show that this trade‐off between growth and defence in a LN environment can be discussed within the framework of the growth–differentiation balance hypothesis (i.e. GDBH), but highlight the need to integrate all plant organs in future modelling approaches regarding the impact of nitrogen limitation on primary and secondary metabolism.     

Compensation to simulated insect leaf herbivory in wild cotton (Gossypium hirsutum): responses to multiple levels of damage and associated traits

• Identifying the mechanisms of compensation to insect herbivory remains a major challenge in plant biology and evolutionary ecology. Most previous studies have addressed plant compensatory responses to one or two levels of insect herbivory, and the underlying traits mediating such responses remain elusive in many cases.
• We evaluated responses associated with compensation to multiple intensities of leaf damage (0% control, 10%, 25%, 50%, 75% of leaf area removed) by means of mechanical removal of foliar tissue and application of a caterpillar (Spodoptera exigua) oral secretions in 3‐month‐old wild cotton plants (Gossypium hirsutum). Four weeks post‐treatment, we measured plant growth and multiple traits associated with compensation, namely: changes in above‐ and belowground, biomass and the concentration of nutrients (nitrogen and phosphorus) and non‐structural carbon reserves (starch and soluble sugars) in roots, stems and leaves.
• We found that wild cotton fully compensated in terms of growth and biomass allocation when leaf damage was low (10%), whereas moderate (25%) to high leaf damage in some cases led to under‐compensation. Nonetheless, high levels of leaf removal (50% and 75%) in most cases did not cause further reductions in height and allocation to leaf and stem biomass relative to low and moderate damage. There were significant positive effects of leaf damage on P concentration in leaves and stems, but not roots, as well as a negative effect on soluble sugars in roots.
• These results indicate that wild cotton fully compensated for a low level of leaf damage but under‐compensated under moderate to high leaf damage, but can nonetheless sustain growth despite increasing losses to herbivory. Such responses were possibly mediated by a re‐allocation of carbohydrate reserves from roots to shoots.

     

Butterflies and plants: preference/performance studies in relation to plant size and the use of intact plants vs. cuttings

Plants have evolved a number of defences to ameliorate herbivore attacks including chemicals induced by mechanical wounding. Such changes in plant chemical composition are potential confounding factors in experiments on plant – insect interactions, which often present cuttings of potential host plants to phytophagous insects. In particular, this could affect studies of female egg‐laying preference and larval performance, because the same plant chemicals that deter certain generalist insects can elevate attacks from more specialized insects. Furthermore, plant cuttings are by definition smaller than intact plants, and any female host size preference could thus affect experiments using plant cuttings. We first assessed female preference and larval performance of a specialist herbivore, Pieris napi (L.) (Lepidoptera: Pieridae, Pierini), confronted with either intact plants or leaf‐cuttings of four Brassicaceae host plants, Alliaria petiolata (Bieb.) Cavara & Grande, Barbarea vulgaris (L.) WT Aiton, Berteroa incana (L.) DC., and Brassica napus (L.). Egg and larval survival did not differ between intact plants and leaf‐cuttings, whereas larval growth was slightly, but significantly, faster on leaf‐cuttings. Females, however, significantly preferred to lay eggs on intact plants of all four hosts, although the preference hierarchy for the intact plants was largely mirrored by that for leaf‐cuttings. We then tested the female preference for different size‐classes of intact B. napus plants. Small individuals received more eggs than larger individuals, and follow‐up experiments showed that this difference was largely generated by a strong female preference for cotyledon leaves; there was no significant difference in female preference for large and small individuals when both carried cotyledons, and females landing on cotyledons were more likely to oviposit compared to when landing on a true leaf. Our study concludes that plant cuttings can serve as adequate proxies for live plants for preference/performance studies, but that experimentalists should be aware of the variation imposed both by plant handling and plant phenology for female oviposition preference.