“The species can build easily extremely high population densities, that are devastating for the commodity. It some countries, it is particularly abundant on wheat and barley, but it can be frequently found in non-preferred commodities, such as rice. Apart from the infestation per se, it produces a lot of feces and cast skins, causing additional qualitative degradations to the products. The presence of cast skins, hair etc. from the larvae can cause serious allergenic reactions and respiration problems.”
Prof. Christos G. Athanassiou
University of Thessaly, Greece
Distribution of the khapra beetle
Trogoderma granarium is a Coleopteran species, that belongs to the family Dermestidae. Globally, it is known as “khapra beetle” and it is a quarantine pest for many countries, such as Canada, US, Australia and many more. The khapra beetle is currently considered as the most important quarantine post-harvest insect species, globally (Myers and Hagstrum 2012, EPPO 2013). The pest originates from India and is currently present in Western Africa and the Middle East, as well as in some countries of Asia and Europe (EPPO 2013). Moreover, apart from the records of established populations, the number of interceptions of T. granarium in non-khapra beetle countries has been notably increased during the recent years (Myers and Hagstrum 2012, Day and White 2016, EPPO 2016). The increased number of interceptions in wider geographical areas constitutes T. granarium an emerging pest of major importance. The species is classified among the 100 most important invasive species globally (Lowe et al. 2000).
Key biology characteristics
The khapra beetle is generally a very successful invader, and has a very wide range of food preferences, as it has been found infesting approx. 100 commodities, of both plant and animal origin (Hagstrum and Subramanyam 2009). In a recent study, Kavallieratos et al. (2017) found that this species could be successfully compete with other major grain colonizers, such as the rice weevil, Sitophilus oryzae and the lesser grain borer, Rhyzopertha dominica. At the same time, when conditions prevailing are not suitable, its population does not collapse and remains at low levels. After the increase of population to 30 oC or more it has an enormous population growth, outcompeting easily other major stored product insects (Kavallieratos et al. 2017). Its ability to survive, even in low numbers in unfavorable commodities, gives an extra advantage to T. granarium, regarding geographical spread. Apart from being a generalist species, T. granarium takes advantage of its ability to feed on products with low moisture content (up to 2%), when most storage insects require a grain moisture content of at least 8%. In this way, T. granarium can compete successfully the other major stored-product insect species at low humidity conditions, where it is usually the only species that prevails. The key characteristic of its biology is its long diapause, which can last for years, and can be interrupted for “foraging excursions”. The diapause occurs at the larval stage, which is usually the most difficult to kill life stage. Still, morphologically, the species cannot be distinguished easily from other relative species of the same genus, which are not quarantine species. For this purpose, experts are needed for accurate identification.
Control of the khapra beetle
The presence of khapra beetle in imported commodities, such as wheat and rice, triggers the phytosanitary legislation and procedures. Nevertheless, interestingly, this species is the stored product insect that is extremely tolerant to heat and cold at the same time (Wilches 2016). At the same time, there are many reports that clearly indicate that the species has developed a considerable level of tolerance to several insecticides, but also to non-chemical methods, which is mostly related to a natural phenomenon and not due to previous exposure. Apparently, diapausing larvae are considered to be the least susceptible life stage to various insecticides and fumigants, including phosphine. However, resistance that is related with previous exposure to certain insecticides has been also reported. Paradoxically, most of the reports available for this species are from old references that have been published during the 50s and 60s, while there are disproportionally few data from newer publications. Still, this can be partially attributed to the difficulties in some countries to establish colonies of the khapra beetle for experimentation, due to phytosanitary/quarantine restrictions. Until recently, non-chemical control of this species has not been investigated in detail. In this context, for example, there are very few data available for the predators and parasitoids that can be utilized for the control of this species. Moreover, little is known for the use of controlled and modified atmospheres, such as nitrogen, carbon dioxide etc. Many studies illustrate that, for many of these methods, adults are much more susceptible than immature stages; despite this, most of the studies available are for adults, probably due to the easiness in handling in laboratory scale.
Importance and future needs
The species can build easily extremely high population densities, that are devastating for the commodity. It some countries, it is particularly abundant on wheat and barley, but it can be frequently found in non-preferred commodities, such as rice. Apart from the infestation per se, it produces a lot of feces and cast skins, causing additional qualitative degradations to the products. The presence of cast skins, hair etc. from the larvae can cause serious allergenic reactions and respiration problems.
Recent guidelines clearly indicate that there are important endemic zones for T. granarium within the EPPO zone, i.e. several zones within Europe or the Mediterranean basin (EPPO 2013). These starts from Morocco to the west and ends in Israel to the east, and covers also Tunisia, Egypt, Cyprus and Turkey. Moreover, there are other EPPO countries in which T. granarium has been recorded but not established, such as Austria, Belgium, Hungary, Luxemburg, Italy, the Netherlands and Spain. At the same time, there are serious concerns that currently this species is likely to be established in certain parts of Southern Spain and Southern Greece, but this has not been officially confirmed yet (EPPO 2013). The “alien” and “invasive” nature of this species, along with the change in global temperature and the international trade of durable food, constitutes its further expansion to the north likely in the near future. In this regard, there is an urgent need for better knowledge of key biology issues of T. granarium, as well as the development of effective management strategies for its control. Moreover, further work is needed to train phytosanitary inspectors to accurately identify this species, as misidentification may lead to its further spread. As a “cryptic” species, especially at its immature life stages, improved detection methods are needed for its monitoring and presence estimation. Finally, given its reduced susceptibility in many chemicals as well as in non-chemical methods, additional work is needed to design “targeted” protocols for this species.