Maria L. Pappas

Spider mites suppress tomato defenses downstream of jasmonate and salicylate independently of hormonal crosstalk.

Plants respond to herbivory by mounting a defense. Some plant-eating spider mites (Tetranychus spp.) have adapted to plant defenses to maintain a high reproductive performance. From natural populations we selected three spider mite strains from two species, Tetranychus urticae and Tetranychus evansi, that can suppress plant defenses, using a fourth defense-inducing strain as a benchmark, to assess to which extent these strains suppress defenses differently. We characterized timing and magnitude of phytohormone accumulation and defense-gene expression, and determined if mites that cannot suppress defenses benefit from sharing a leaf with suppressors. The nonsuppressor strain induced a mixture of jasmonate- (JA) and salicylate (SA)-dependent defenses. Induced defense genes separated into three groups: ‘early’ (expression peak at 1 d postinfestation (dpi)); ‘intermediate’ (4 dpi); and ‘late’, whose expression increased until the leaf died. The T. evansi strains suppressed genes from all three groups, but the T. urticae strain only suppressed the late ones. Suppression occurred downstream of JA and SA accumulation, independently of the JA–SA antagonism, and was powerful enough to boost the reproductive performance of nonsuppressors up to 45%. Our results show that suppressing defenses not only brings benefits but, within herbivore communities, can also generate a considerable ecological cost when promoting the population growth of a competitor.

Development, Survival, and Reproduction of the Predatory Mite Kampimodromus aberrans (Acari: Phytoseiidae) at Different Constant Temperatures

Abstract Development, survival, and reproduction of the predatory mite Kampimodromus aberrans Oudemans were studied at constant temperatures in the range from 15 to 35°C under laboratory conditions. Larval developmental rate for both males and females increased gradually from 15 to 35°C and decreased at higher temperatures. Lactin’s nonlinear model described with adequate accuracy the relationship between developmental rate and temperature. The model predicted that lower and upper threshold temperatures for preimaginal development ranged from 9.8 to 11.8°C and from 37.2 to 39.8°C, respectively. The intrinsic rate of population increase (rm) at the different temperatures ranged from 0.0442 to 0.1575, with the highest value recorded at 25°C. At 33°C a negative rm value was estimated. The rm values determined at different temperatures were fitted to Lactin’s nonlinear model, and the lower and upper threshold and the optimal temperatures for population increase were 10.5, 32.4, and 27.6°C, respectively. These data indicate that K. aberrans may be better adapted to intermediate temperatures around 27°C and, therefore, could be a useful biocontrol agent of spider mites during spring and early summer when such temperatures are prevalent in northern Greece. The results could also be useful in developing a population model for K. aberrans under field conditions.

Effect of Relative Humidity on Longevity, Ovarian Maturation, and Egg Production in the Olive Fruit Fly (Diptera: Tephritidae)

ABSTRACT The effect of five constant relative humidity (RH) regimes (12, 33, 55, 75, and 94%) on ovarian maturation, adult longevity, and egg production of Bactrocera (Dacus) oleae (Gmelin) (Diptera: Tephritidae) was studied at 25 ± 1°C and a photoperiod of 16:8 (L:D) h under laboratory conditions. The experimental insects developed in the preimaginal stages at 16:8 (L:D) h and 25 ± 1°C, and the adults were maintained in plastic cages in sealed plastic containers with saturated water salt solutions in their base to control RH. In the experiments concerning longevity and egg production, the adults were maintained individually, whereas in those concerning ovarian maturation they were in groups of 10 females and 10 males. Ovarian maturation was much reduced and the number of mature oocytes in the ovaries was very low in 7-d-old females maintained at 12 and 33% RH. By contrast, a high percentage of females of that age maintained at 55, 75, and 94% RHs had mature oocytes in their ovaries. Longevity of adult females, egg production and egg hatch were low at 12% RH. They were significantly higher at 33% RH, and even more so at 55 and 75% RHs but declined at 94% RH. These results could be useful for the mass rearing of B. oleae and for better understanding its population dynamics under field conditions.

Beyond Predation: The Zoophytophagous Predator Macrolophus pygmaeus Induces Tomato Resistance against Spider Mites

Many predatory insects that prey on herbivores also feed on the plant, but it is unknown whether plants affect the performance of herbivores by responding to this phytophagy with defence induction. We investigate whether the prior presence of the omnivorous predator Macrolophus pygmaeus (Rambur) on tomato plants affects plant resistance against two different herbivore species. Besides plant-mediated effects of M. pygmaeus on herbivore performance, we examined whether a plant defence trait that is known to be inducible by herbivory, proteinase inhibitors (PI), may also be activated in response to the interactions of this predator with the tomato plant. We show that exposing tomato plants to the omnivorous predator M. pygmaeus reduced performance of a subsequently infesting herbivore, the two-spotted spider mite Tetranychus urticae Koch, but not of the greenhouse whitefly Trialeurodes vaporariorum (Westwood). The spider-mite infested tomato plants experience a lower herbivore load, i.e., number of eggs deposited and individuals present, when previously exposed to the zoophytophagous predator. This effect is not restricted to the exposed leaf and persists on exposed plants for at least two weeks after the removal of the predators. The decreased performance of spider mites as a result of prior exposure of the plant to M. pygmaeus is accompanied by a locally and systemically increased accumulation of transcripts and activity of proteinase inhibitors that are known to be involved in plant defence. Our results demonstrate that zoophytophagous predators can induce plant defence responses and reduce herbivore performance. Hence, the suppression of populations of certain herbivores via consumption may be strengthened by the induction of plant defences by zoophytophagous predators.

Effect of cold exposure and photoperiod on diapause termination of the predatory mite Euseius finlandicus (Acari : Phytoseiidae)

Abstract The effect of cold storage and photoperiod on diapause termination of the predatory mite Euseius finlandicus was studied under laboratory conditions. Females in diapause that developed under a short-day photoperiod of L:D 8:16 h and 20°C were exposed for 1–13 wk to 5°C and subsequently maintained under L:D 8:16 h and 20°C, after which the percentage of females terminating diapause was recorded. After 1–4 wk at 5°C, the number of days required for diapause termination in 50% of females (TA50) was similar to that of the control mites maintained continuously at L:D 8:16 h and 20°C. This indicates that cold is not a prerequisite for the completion of diapause development. Furthermore, when mites were maintained for longer periods (7, 10, or 13 wk) at 5°C, TA50 was significantly longer than in the control mites. This may indicate an adverse effect of cold storage on the completion of diapause development or on postdiapause development. However, the longer exposures to cold resulted in synchronous diapause termination. When diapause females were maintained under a series of different photoperiods and 20°C without previous cold exposure, diapause was terminated rapidly under the photoperiods of L:D 16:8, 14:10, and 13:11 h. In contrast, under the shorter-day photoperiods of LD 8:16, 10:14, and 12:12 h, diapause was maintained and eventually ended spontaneously after a long period of time. These results may be useful for the long-term storage of this predatory mite, which could be required for use in biological control programs.

Induced plant-defenses suppress herbivore reproduction but also constrain predation of their offspring

Inducible anti-herbivore defenses in plants are predominantly regulated by jasmonic acid (JA). On tomato plants, most genotypes of the herbivorous generalist spider mite Tetranychus urticae induce JA defenses and perform poorly on it, whereas the Solanaceae specialist Tetranychus evansi, who suppresses JA defenses, performs well on it. We asked to which extent these spider mites and the predatory mite Phytoseiulus longipes preying on these spider mites eggs are affected by induced JA-defenses. By artificially inducing the JA-response of the tomato JA-biosynthesis mutant def-1 using exogenous JA and isoleucine (Ile), we first established the relationship between endogenous JA-Ile-levels and the reproductive performance of spider mites. For both mite species we observed that they produced more eggs when levels of JA-Ile were low. Subsequently, we allowed predatory mites to prey on spider mite-eggs derived from wild-type tomato plants, def-1 and JA-Ile-treated def-1 and observed that they preferred, and consumed more, eggs produced on tomato plants with weak JA defenses. However, predatory mite oviposition was similar across treatments. Our results show that induced JA-responses negatively affect spider mite performance, but positively affect the survival of their offspring by constraining egg-predation.

Incidence of resistance to neonicotinoid insecticides in greenhouse populations of the whitefly, Trialeurodes vaporariorum (Hemiptera: Aleyrodidae) from Greece

The greenhouse whitefly, Trialeurodesvaporariorum Westwood, is an important pest of field and greenhouse crops of horticultural and ornamental plants. In integrated pest management programs its control is mainly based on the release of biological control agents and application of chemical insecticides. Neonicotinoids are relatively new chemicals currently applied for the chemical control of T. vaporariorum. However, cases of development of insecticide resistance to neonicotinoids have already been reported. The state of resistance to neonicotinoid insecticides for populations of the greenhouse whitefly in Greece is currently unknown. The objective of our study was to screen a number of whitefly populations for resistance to the neonicotinoids imidacloprid and thiacloprid. Seven whitefly populations were collected from tomato greenhouse crops from different areas of central and northern Greece. LC50 values were estimated for all populations following the method proposed by the Insecticide Resistance Action Committee (IRAC). The development of resistance to both neonicotinoids was confirmed for all tested populations with resistance ratios ranging from 1.5 to 4.4-fold and from 1.4 to 12.2-fold for imidacloprid and thiacloprid, respectively. We discuss our results with regard to the development of neonicotinoid resistance in T. vaporariorum populations and its implications for whitefly control.

Effect of heat stress on survival and reproduction of the olive fruit fly Bactocera (Dacus) oleae

The olive fruit fly Bactrocera (Dacus) oleae Gmelin is a major olive pest in Greece and other Mediterranean countries. Its population density and respective olive infestation is usually low in many areas of northern Greece during summer months. To some extent, this may be due to the prevailing high temperature and low relative humidity conditions. In the present work the effects of short term exposure to high temperatures on the survival and egg production of B. oleae pre‐imaginal stages and adults were studied under laboratory conditions. Different larval instars within infested green olive fruits, adults and pupae and were exposed for 2 h to a series of different high constant temperatures ranging from 34 to 42°C. Subsequently, survival percentages of pre‐imaginal stages and adults as well as the number of eggs laid by females previously exposed to high temperatures were determined. At temperatures up to 38°C high survival percentages of larvae and adults were observed, whereas pupae displayed a relatively increased heat tolerance up to 40°C. Female longevity and egg production were substantially reduced after heat stress. Prior acclimation at 33°C for 1 and 3 days resulted in increased adult survival following heat stress. We discuss the results with respect to the ability of the fly to survive and reproduce under high summer temperatures.

The role of phytophagy by predators in shaping plant interactions with their pests

ABSTRACT Zoophytophagy is common among predacious arthropods, but research on their role in plant-herbivore interactions is generally focused on predation effects whereas their phytophagy is largely neglected. Our recent study revealed the ability of zoophytophagous predators to induce defense related traits and to affect herbivore performance apart from predation through the plant. Additionally, we show here that predator-exposed plants suffer less damage compared to unexposed plants. Thus, zoophytophagous organisms likely shape community structure by both their predation on herbivores and their phytophagy. Here, we consider zoophytophagous predators as plant vaccination factors and outline how their dual role in affecting herbivores may impact their use in biological pest control. Because plant responses to phytophagy and phytopathogens are known to interact, zoophytophagous predators may also affect plant-pathogen interactions. When we consider these indirect interactions with different plant pest organisms, we will likely better understand the ecology of the complex relationships among plants, herbivores and predators. Moreover, a comprehensive knowledge on the effects of the phytophagy of predators in these ecological interactions will potentially allow us to enhance sustainability in pest control.

Induced plant defences in biological control of arthropod pests: a double-edged sword

Abstract Biological control is an important ecosystem service delivered by natural enemies. Together with breeding for plant defence, it constitutes one of the most promising alternatives to pesticides for controlling herbivores in sustainable crop production. Especially induced plant defences may be promising targets in plant breeding for resistance against arthropod pests. Because they are activated upon herbivore damage, costs are only incurred when defence is needed. Moreover, they can be more specific than constitutive defences. Nevertheless, inducible defence traits that are harming plant pest organisms may interfere with biological control agents, such as predators and parasitoids. Despite the vast fundamental knowledge on plant defence mechanisms and their effects on natural enemies, our understanding of the feasibility of combining biological control with induced plant defence in practice is relatively poor. In this review, we focus on arthropod pest control and present the most important features of biological control with natural enemies and of induced plant defence. Furthermore, we show potential synergies and conflicts among them and, finally, identify gaps and list opportunities for their combined use in crop protection. We suggest that breeders should focus on inducible resistance traits that are compatible with the natural enemies of arthropod pests, specifically traits that help communities of natural enemies to build up.