Identifying the two mirids
N. tenuis was detected for the first time in France in 1986, in the Rhône valley . M. pygmaeus has been used as a beneficial insect to control many tomato pests since the early 1990s .
N. tenuis and M. pygmaeus are two predatory plant bugs that belong to the family Miridae, tribe Dicyphini. They have a very similar morphology and feed on some of the same host plants and insect prey. The two bugs can therefore easily be mistaken one for the other. Some morphological characteristics, visible to the naked eye or with a hand lens, make it possible to distinguish one from the other (see photo below). Phytophagous activity can also be used to differentiate the two species. Indeed, in the absence of prey, N. tenuis causes damage that is highly detrimental to the tomato crop, which is not the case for M. pygmaeus. This is why N. tenuis is considered as an important pest of tomato crops, especially in soilless cultivation, in the South of France.
Colonization of a tomato plant
Several studies have been conducted in France and in Europe to determine which part of the plant constitutes the main habitat of these two mirid bugs. The observations focused on the top (from the apex to the 5th leaf), and the middle of the plant (from the 5th to the 10th leaf). This work showed that N. tenuis preferred to colonize the upper part, more precisely from the apex to the 4th-5th leaf, while M. pygmaeus preferred the area between the 5th and 7th leaf as well as the lower area but to a lesser extent , . These results also suggest that the bugs are not in direct competition for their prey.
Population monitoring conducted in 2018 confirms these results (Figures 1 and 2). Indeed, measurements made during the period of June to September 2018 show that the majority of the N. tenuis population is concentrated at the top of the plant. Levels vary between 60 and 90% of the total N. tenuis population depending on the observation date (Figure 1). This proportion is the total opposite for M. pygmaeus populations that are mostly concentrated in the middle and at the bottom of the plant. The results indicate that 80 to 95% of the total population is observed in this stratum (Figure 2). On average, 93% of N. tenuis and 7% of M. pygmaeus were observed at the top of the plant. In the middle and lower parts of the plant, M. pygmaeus is in the majority with 62% of the total mirid population, and N. tenuis represents 38%. This work was conducted by APREL within the framework of the IMPULsE project in 2018, in a commercial soilless tomato crop in the South-East of France. The observations carried out concerned the upper part (from the apex to the 5th leaf), as well as the middle and lower parts of the plant (from the 5th leaf to the basal leaf removal zone).
Macrolophus moves little on the plant during the day
However, this close monitoring could hide changes in the spatial distribution of predatory bugs during the day. This is why a trial was conducted in 2019 in an experimental greenhouse at the CTIFL center in Balandran. The objective was to qualify the movements of mirid populations on tomato plants, within three different zones: the top (from the apex to the 5th leaf), the middle (from the 5th to the 10th leaf), and the bottom of the plant (from the 10th leaf to the basal leaf removal zone). During the monitoring period, N. tenuis was observed very little, so the study only focused on M. pygmaeus.
During the whole observation period, the population of M. pygmaeus is always higher in the middle of the plant, whatever the date or time of observation (Figure 3). However, we observe at the beginning of this period (April 2019) a migration of individuals from the top to the middle of the plant during the day. The number of individuals at the top being lower at the end of the day compared to the beginning. During the season (May, June), with the progressive increase of temperatures in the greenhouse, we also observed that the proportion of individuals decreases at the top and increases in the middle and at the bottom of the plant, without being related to the time of the observations.
M. pygmaeus seems to move very little between the different zones and prefers to colonize the middle of the plant as shown previously. Some movements take place at the beginning of the season when the temperature at the top of the plants is still optimal for M. pygmaeus (around 20°C).
Targeting treatments at the top of the plant to control Nesidiocoris tenuis
Observations suggest that N. tenuis and M. pygmaeus occupy different strata on the plant. These results suggest that a targeted strategy should be implemented at the top of the plant to improve the control of N. tenuis while minimizing the impact as much as possible on M. pygmaeus. Warm periods favor the presence of N. tenuis at the top of the plant while M. pygmaeus colonizes the lower levels more. Thus, by intervening at warmer times of the day (when compatible with the treatment), it seems possible to reduce the undesirable effects of treatments (physical, biological or insecticide) on M. pygmaeus. During production, different action thresholds are used. As an indication, we can cite the observation of 1 individual per plant and/or three necrotic rings (feeding damage) per plant on average. However, it will be important to limit treatments at the top of the plant before the hot period begins (June), because the impact on M. pygmaeus populations can be significant. Finally, other studies have shown that when both mirids were present in high densities on the tomato plant, each species tended to colonize the other's stratum. M. pygmaeus is more present at the top and N. tenuis, tends to go lower down . Work conducted under controlled conditions («arena») also showed predation by N. tenuis on young M. pygmaeus larvae in the absence of alternative prey. This may impact the development of M. pygmaeus populations later on.
Focus on Nesidiocoris tenuis
Influence of food sources (prey)
N. tenuis is considered as an insect pest in the South of France. Even if it consumes important tomato pests like whitefly and Tuta absoluta, its phytophagous behavior rapidly makes it a pest. Indeed, the feeding damage causes a reduction in plant growth and yield, in quantity as well as quality. This balance between entomophagy and phytophagy is regulated by the quantity of prey available. The less prey available (e.g., Tuta absoluta eggs), the more the feeding damage on plants increases , . Plant feeding damage also increases as the N. tenuis population increases , . The presence of nets at the openings could have an aggravating effect by preventing N. tenuis from migrating outside the greenhouse to seek prey when it is insufficient inside .
The diet (type of prey available) significantly impacts the development of beneficial insects. For example, the development of M. pygmaeus is much faster when feeding on whitefly Bemisia tabaci than on Tuta absoluta . M. pygmaeus and N. tenuis share the same prey. However, the nutritional quality is different for the two mirids which results in differences in population growth and dynamics . Therefore, differences in colonization of tomato crops may be linked to the food available to the mirids.
Influence of temperature
N. tenuis is found in areas with warmer climates. This means it tolerates high temperatures very well, but its performance is strongly reduced at low temperatures (< 15 °C) . Indeed, the development cycle is very rapid at 35 °C. For example, larval development (L1 to L5) decreases from about 56 days at 15 °C to only about 10 days at 30 °C. In comparison, the larval development of M. pygmaeus is about 14 days at 30 °C. Egg hatching time is also reduced with increasing temperature. It averages only 7.5 days at 30°C while it averages 10.1 days for M. pygmaeus at the same temperature. These differences could explain the ability of N. tenuis to rapidly colonize tomato crops in summer, and to overtake M. pygmaeus whose populations decline with high temperatures.
As this is an invasive species, work has been done on the potential risks of introduction and establishment as a function of temperature . This work has also made it possible to characterize the tolerance of this insect to low temperatures. In the laboratory, at a constant temperature of 5°C, it takes nine days to reach 50% mortality in the study population. In the field, 100% mortality is achieved after less than four weeks of exposure to the winter cold. In addition, N. tenuis does not appear to be able to enter diapause, so there is no overwintering form. Finally, the threshold for development was calculated at 12.9°C in the laboratory, below which no development is possible. These results suggest that N. tenuis would have difficulty surviving outdoors, especially in northern areas. On the other hand, mild winters in the Mediterranean area could improve its survival rate.
Improving control of N. tenuis
Work conducted at the CTIFL showed that early releases of M. pygmaeus that are well-established seemed to have a negative impact on the establishment of N. tenuis as it takes much longer to do so. However, this observation was made in a test where the availability of prey was lower than in the other tests in the trial . These observations suggest that it would be possible to slow down the establishment of N. tenuis by improving the competitive ability of M. pygmaeus. This hypothesis is being studied in the ACOR¹ project, financed by the Casdar and carried out by the CTIFL, which started in the summer of 2020.
¹ ACOR (améliorer l'utilisation des punaises prédatrices et concevoir des pratiques Agroécologiques pour le COntrôle des Ravageurs aériens en cultures maraîchères)
Using trap plants for control
Two plants: sesame (Sesamum indicum) and slimy inula (Dittrichia viscosa) were identified as interesting relay plants in biological control strategies in Spain . This discovery could benefit the control of N. tenuis in France, by using these plants as trap plants. Indeed, N. tenuis seems to be more attracted to S. indicum and D. viscosa than to tomato crops. Under controlled conditions, less feeding damage is observed on tomato in the presence of these trap plants. The technical problem to be solved remains the management of these trap plants: how to eliminate N. tenuis once it has moved to them? Vacuuming could be an interesting technical approach to experiment with, provided that a system is developed that is compatible with professional use in production conditions.
Two current projects on plant bug control
The data in this article are from the following two projects:
- The LABEL² project (Evaluation of Physical Control by Suction for the Control of Pests and Diseases in protected Vegetable crops) 2018-2020, led by the CTIFL and funded by FranceAgriMer. The objective of the project is twofold: (i) firstly, to identify the level of effectiveness and validate the effect of regular physical intervention on pest populations, but also on the beneficial insects of crops grown using Integrated Biological Control strategy through precise counts, then secondly, to determine the most effective modes of intervention in an Integrated Biological Protection strategies and to evaluate the necessary frequencies of passage; (ii) then to evaluate the costs of this control method so as to give the necessary information to growers on the technico-economic feasibility of physical control by suction as well as avenues for improvement.
- The IMPULsE³ project (Development and Integration of Innovative Methods for the control of Bugs in Vegetable crops) 2017-2020, led by the CTIFL and co-financed by AFB (French Agency for Biodiversity) and Casdar and accredited by GIS PIClég. The aim of this project is to propose global strategies of protection against phytophagous bugs to professionals. The objectives are: i) improving knowledge on the biology of these pests and on the severity of the damage caused; ii) evaluating current protection methods (insect-proof nets, detection trapping, mechanical control methods); iii) developing new biocontrol solutions (use of native beneficial insects, other biocontrol products, use of relay plants); iv) combining and integrating these methods into control strategies in organic agriculture and integrated pest management in a viable technico-economic context.
² LABEL (évaluation de la Lutte physique par Aspiration pour le contrôle des BioagrEsseurs en cultures Légumières sous-abri)
³ IMPULsE (développement et Intégration de Méthodes innovantes pour la maîtrise des PUnaises en cultures LEgumières)
What you need to know
Plant feeding by Nesidiocoris tenuis can cause serious damage to tomato crops. N. tenuis is similar to Macrolophus pygmaeus in appearance but some characteristics that are visible to the naked eye allow them to be distinguished one from the other. N. tenuis prefers to colonize the top of the plants, whereas M. pygmaeus prefers the middle. This difference is all the more significant when the climate is hot in the greenhouse and this can be used to control N. tenuis populations with localized treatments at the top of the plant to reduce the impact on M. pygmaeus.
Mirides prédateurs M. pygmaeus et N. tenuis : comportement et répartition sur la culture de la tomate
Nesidiocoris tenuis est difficile à contrôler sans impacter Macrolophus pygmaeus, du fait de sa proximité biologique avec cet auxiliaire. Cet article fait le point sur les connaissances acquises sur la biologie et le comportement de ces deux punaises mirides, avec un focus sur N. tenuis. Les deux punaises ne partagent pas les mêmes strates sur la plante de tomate. N. tenuis est davantage inféodé à la partie haute et M. pygmaeus au milieu de plante. Cette différence s'accentue avec l'augmentation des températures dans la serre. Ce comportement peut être mis à profit pour appliquer des méthodes de gestion localisées afin de gérer N. tenuis en impactant au minimum M. pygmaeus. N. tenuis est favorisé par un climat chaud, au détriment de M. pygmaeus dont l'optimum thermique est inférieur, ce qui pourrait expliquer la baisse d'effectif de M. pygmaeus observée en été.
The mirid predators M. pygmaeus and N. tenuis : behaviour and distribution on tomato crops
Nesidiocoris tenuis is difficult to control without affecting Macrolophus pygmaeus, as it is very similar in appearance to this beneficial insect. This article reviews the knowledge acquired on the biology and behavior of these two mirid bugs, particularly on N. tenuis. The two bugs do not forage on the same strata of the tomato plant, as N. tenuis is mainly found on the upper part, and M. pygmaeus in the middle of the plant. This difference becomes more pronounced when temperatures increase in the greenhouse. This behavior can be useful for applying localized control treatments to N. tenuis with minimal impact on M. pygmaeus. A warm climate favors the development of N. tenuis, contrary to M. pygmaeus that has a lower temperature threshold, which could explain the decline in M. pygmaeus populations observed in summer.
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