1. Formulation of Entomopathogenic Nematodes for Crop Pest Control

The excessive use of chemical pesticides in agriculture causes serious damage to soil, air, water, flora, fauna and human beings. Therefore, it is necessary to develop environmentally friendly alternatives to control soil pests, such as entomopathogenic nematodes (EPNs). The EPNs are obligate parasites of insects and have demonstrated to pose a high potential for the agricultural pest control. Also, they have been exempted from registration in the USA, are compatible with many chemical pesticides and amenable to genetic selection. These EPNs possess a symbiotic association with pathogenic bacteria of the genera Xenorhabdus and Photorhabdus, which are associated with Steinernema and Heterorhabditis, respectively.

The infective juveniles (IJs) are the only free-living stage and serve three main functions: dispersal, host finding, and survival under environmental detrimental conditions. When the IJs enter the body cavity of a host-insect, the symbiotic bacteria are released, multiply and the host death occurs within two days. The IJs complete their development and reproduce for two or three generations inside their host. When food is depleted, a new generation of IJs exits from the host cadaver to search new hosts.

Currently, the factors that affect the application of EPNs in the field include: markets, crops and target insects, formulation and shelf life, usage directions, technical support, cost and others.

However, the formulation is one of the most important factors for the successful use of EPNs as microbial insecticides. The EPN formulation is a process of the transformation of living entities into a product that can be applied by practical methods. The EPNs are formulated to facilitate their storage, transport and application . The formulations are based on the fundamental principle of energy conservation: restricting their movements (physical restraint) or reducing oxygen consumption to induce an anhydrobiotic partial state. An optimal formulation should show high and consistent quality, easy handling and transport, high effectiveness and an easy way to be applied in the field. In the last 15 years, several reviews have pointed out the progress of the EPN formulation and application technology.

However, the most detailed review on the EPN formulation was conducted by Grewal. Therefore, we consider necessary to perform a new review of the latest developments in EPN formulations. In this review, we present a critical analysis of the progress and perspective of the EPN formulations in the last years, emphasising a systemic approach to include disciplines such as invertebrate physiology, materials science, and systems modelling.

2. Key Scale Insects (Hemiptera: Coccoidea) of High Economic Importance in a Mediterranean Area

Scale insects (Hemiptera: Coccoidea) are small phytophagous arthropods widely distributed in different ecosystems around the world. The superfamily Coccoidea contains nearly 8000 species of plant-feeding hemipterans belonging to 32 families. The most commonly encountered families are those with the most species, namely the Diaspididae, Pseudococcidae, and Coccidae. Among the 8000 scale insect species described worldwide, 2000 are known to occur in the Palearctic region. Scale insect females are typically wingless and are often protected by waxy protective secretions or covers. The adult females may exhibit reduction or loss of appendages, depending on the family and instar, and are often sedentary or sessile, in contrast to adult males which are usually winged and inconspicuous, do not feed and live a few days. Scale insects have diverse egg-protecting methods, a range of chromosome behaviours (including several methods of sex determination), marked sexual dimorphism, more rarely sexual dichronism, and a wide range of genetic systems. Generally, there are three to four instars in the female and five instars in the male. Crawlers (first instar nymphs) are generally the main agents of dispersal for these insects. Scale insects have been reported as serious pests attacking a huge number of host plants around the world. These insects are found on various parts of their hosts, and may infest leaves, twigs, branches and roots, and some live inside plant domatia. They feed almost exclusively on the phloem of their host plants to which they cause direct damage, but they can also cause indirect damage by transmitting plant pathogens through injection or through the build-up of honeydew, promoting the attack of plant pathogens. However, some species feed on the parenchyma tissue by directly feeding on the contents of parenchyma cells. Identification of scale insects is mainly based on morphological characters of adult females. Overall, scale insects that have worldwide been reported as economically important belong to the families Diaspididae, Coccidae, and Pseudococcidae. Research studies exploring the most effective pest management approaches have been performed worldwide to avoid serious phytosanitary problems caused by some key species of scale insects, which is also the case for Tunisia, a Mediterranean country. In Tunisia, the number of research studies dealing with analyses of the status of scale insect fauna has been still progressively increasing during the last decade after a relatively long period of quasi-absence (especially between the 1950’s and the 1980’s) of studies related to this group of hemipterans. Based on literature, several scale insect species can be found on various host plants grown in Tunisia. According to Garcia Morales et al. (2016), 68 scale insect species have been reported to occur in Tunisia, but in addition other scale insect species have also been recorded in Tunisia. The great majority of these species found in Tunisia are of minor economic importance since they have no negative impact on either crop health or yield, but a few other species are considered as having high economic importance, implying that they are capable of seriously damaging host plants and causing noteworthy economic losses.