1. Introduction
Tuta absoluta (Lepidoptera, Gelechiidae), known as the South American tomato leafminer or pinworm, is one of the most devastating pests for tomato crops (
Solanum lycopersicum L.), both in greenhouse and open-field locations in different parts of the world [
1]. It originates from South America and was first described in Peru in 1917 as
Phthorimaea absoluta (Meyrick, 1917) [
2]. Recently,
T. absoluta has also become a serious threat to tomato production in the Mediterranean region [
3]. In Europe, this pest was first detected at the end of 2006 in the northern part of Castellón de la Plana in eastern Spain [
4]. Since then, it has rapidly invaded other European countries and spread throughout the Mediterranean basin, including parts of North Africa and the Middle East where it immediately reached damaging levels [
2,
5,
6].
This pest attacks leaves, flowers, stems, and fruits at any developmental stage, from seedlings to mature plants. In the absence of control strategies, yield losses can reach 80%–100% [
2]. The damage is caused by the larvae mining the leaves and, sometimes, also the tomato fruits. The larvae feed on the mesophyll, which affects the photosynthetic capacity of the crop, decreases the production, and makes the tomatoes unsuitable for the market [
7,
8].
Nowadays, biological control, based on the predators
Nesidiocoris tenuis (Hemiptera: Miridae) and
Macrolophus pygmaeus (Hemiptera: Miridae), has been used to regulate the
T. absoluta population [
4]. Nevertheless, in many countries (South America, Italy, Spain, etc.) chemical control still is the main method to control
T. absoluta. In order to decrease the damage caused by
T. absoluta, horticultural growers applied insecticides [
1,
7]. They applied the chemical products more than twice a week during a single cultivation period, which not only resulted in food and environmental contamination, but also increased the cost of production and reduced the number of natural enemies of the pest [
9]. Furthermore, extensive use of insecticides may lead to the development of resistance in insect populations. Development of resistance in
T. absoluta populations was previously reported in South America [
7,
10,
11,
12]. Studies have shown that
T. absoluta can develop resistance to many classes of insecticides when resistance management strategies are not properly established. Accordingly, high risks are involved in the use of insecticides based on spinosyns, one of the few classes of insecticides still effective against
T. absoluta in South America [
13,
14].
Subsequently, the bioinsecticide “emamectin benzoate” is a new macrocyclic lactone insecticide derived from the avermectin family of natural products. These products have been developed for the control of Lepidoptera pests on a variety of vegetable crops worldwide, with a particular efficacy against
T. absoluta [
15]. Mortality rates of 90% were observed, which was similar to the results obtained by Lopez et al. [
15,
16].
However, Campos et al. [
17] have already detected low levels of resistance of
T. absoluta to spinosad. Notwithstanding the good results of emamectin benzoate, farmers should be careful using this product when natural enemies are involved in the control of the pest [
15]. Subsequently, the introduction of the European Directive on the sustainable use of pesticides (2009/128/EC) requires that all professional users of pesticides follow the general principles of IPM (integrated pest management). Thus, the implementation of environmentally safe alternatives, reducing the use of chemicals, should contribute to the sustainability of tomato production.
One of the alternatives is the use of zeolites as an insecticide. Zeolites represent a broad range of natural or synthetic microporous, crystalline aluminosilicates. Generally, their structure is built of [SiO
4]
4− and [AlO
4]
5− tetrahedra, linked by the sharing of the oxygen atoms [
18]. Zeolites are used for a great number of applications in different domains, including agriculture [
19]. The use of particle films on plant surfaces is intended to prevent most of the negative effects that occur with the current application of chemical plant protection products. Moreover, such particle films might also provide a number of beneficial effects in terms of reduction in pesticide use, control of pests and diseases, water efficiency, increase in crop yield, and tolerance to abiotic stress [
19].
In addition to the applied product, insecticide susceptibility also varies with the life stage of an insect [
20]. Although the egg stage is sometimes perceived as the most vulnerable, it is a difficult target for insecticide application because the sessile condition of the eggs are often at concealed sites [
21,
22,
23,
24]. In addition, egg structure and physiology protect the developing embryos and may minimize insecticide penetration [
21,
24,
25]. The insect chorion is a compound set of envelopes, remarkably effective in providing the oocyte with protection against possible disadvantageous environmental influences like desiccation, water loss, bacterial infection, and physical destruction. On the other hand, the egg shell enables gas exchange and maintenance of the water balance [
26,
27]. However, this does not apply to female insects that lay their eggs inside the leaf or that protect their eggs with scales and other materials.
This study aimed to assess the insecticidal effect of zeolites on T. absoluta. The treatments were mainly targeted against eggs and larvae. Ovicidal properties were studied by spraying the eggs directly (topically) or by treating leaves before oviposition (residually). A final experiment was done to assess whether the used products had repellent or attractant properties to adult females. This oviposition behavior of females was determined by choice tests.
5. Conclusions
Based on the results, it can be derived that the tested products, BEA, FAU, LTA and their formulations, had no real insecticidal activity against the eggs of T. absoluta. Nevertheless, egg exposure to zeolites seemed to affect the development process by weakening the first instar larvae and increasing their mortality. Consequently, zeolites can be applied as a preventive control measure, but cannot control a pest that is already established. To be effective, a continuous coverage of plant material with a zeolite particle film is needed. This requires multiple applications and a better coverage than that obtained in these tests, as newly expanding foliage needs to be covered as well. However, particle films are less effective in suppressing T. absoluta compared with spinosad. Nevertheless, they may be an important supplement to other biological and chemical control measures in future T. absoluta management strategies.
Subsequently, based on the choice test, no significant difference was observed between the number of eggs laid on the treated leaves and control leaves.