2. Minute polyphagous wasps as biological pest control
2.2. Global perspective
2.3. Application in orchards
Mass releases of parasitic minute polyphagous wasps is a common biological pest control practice across the world. The application can reduce chemical insecticide use and therefore contribute to a more sustainable agriculture. Nearly all frequently used species derive from the genus Trichogramma. These wasps are released on several Million hectares of agricultural production, particularly in maize cultivation. This report is aiming to give an insight into a growing system, where Trichogramma practices are still under development. With regards to the application density, efficacy, applied species and environmental risks of Trichogramma in European ecological fruit cultivation was reviewed. The common application methods are lacking consistent results and pest control efficacy, since orchards differ greatly from the usual grain field cropping system. Three research directions to increase efficiency can be distinguished: new application methods (1); environmental attributes that favour Trichogramma (2) and suitable Trichogramma (mixtures) species (3). Development of equipment, suitable to disperse Trichogramma eggs more equally between rows is needed. A nozzle fan, spraying Trichogramma eggs, seemed to decrease costs and labour force by simultaneously increasing pest control consistency. Flower strips, particularly by containing buckwheat and mustard, can increase the longevity and fecundity of the wasps, leading to a better pest control performance. Local Trichogramma species should be preferred, as they are used to the climatic conditions and contribute to agroecological practices. The risk of mass releases on non-target insect species must be surveyed continuously. No significant effects were found, but off-field emigration does happen and needs to be monitored. The research implies the potential of Trichogramma to be used as an agroecological practice, being able to contribute to insecticide reduction. However, many results still need to be tested under commercial conditions.
Agroecology as a science and practice is aiming to decrease the negative environmental effects of agriculture. Synthetic fertilisers and pesticides for pest control are aimed to be reduced or even completely abandoned. This brings up the question and demand for alternatives. Agroecological farming is aiming to replace agro-chemicals by biological practices, including the beneficial interactions between different organisms within the farming system (Watts and Williamson 2015). Pest control, as a concrete action, is industrially performed by spraying pesticides. Obviously, agroecological farmers need a way to protect their harvest against pests and to reduce their yield losses too. To do so, farmers can apply biological pest control, e.g. biological insecticides or predatory insects. Augmentative (mass releasing) biological pest control is utilised on more than 10°Mio. ha worldwide (van Lenteren and Bueno 2003). One of the most commonly used biological pest controls are (polyphagous) parasitic wasps. Most polyphagous wasps attack their host’s eggs and initiate the larvae’s death. The wasps’ ability to control pests is recognised for more than 100 years, but only in the 1960’s and 70’s, their commercial value developed (Smith 1996). Nowadays, parasitic wasps are successfully used all around the world for inundative pest management. Parasitic wasps are categorised as natural enemies. Releasing parasitic wasps is an intervention in the relation between natural predator/prey relationships. In agricultural systems, parasitic wasps are often restoring the natural enemy balance of exotic insects, that have reached pest status in their new environment (Altieri 1995). The wasps attack eggs of different insect families but are dominantly released for the rapid reduction of Lepidoptera (particularly caterpillars) (Martin and Sauerborn 2013). The most prominent case in Europe, is the parasitisation of the European corn borer (Ostrinia nubilalis) in maize cultivation. Apart from mass releases to cope with massive pests, also applications in smaller scale have developed. Nowadays, minute egg wasps are e.g. often used in organic orchards (Jehle et al. 2014). The evaluation of economic success of this biological pest control measure is closely linked to its effectiveness. Nevertheless, Agroecological measures are not only evaluated by their effectiveness, but by their environmental compatibility. Literature is often giving different results on the effectiveness of parasitic wasps, depending on the type of crop and its pest, as well as on region and climate. It seems quite hard to figure out, whether the release of parasitic wasps is suitable as an effective and ecological practice. The literature review is aiming to investigate on the effectiveness of this alternative method for pest control, its application range, limitations, risks and to which extent the application is suitable as an Agroecological practice.
2. Minute polyphagous wasps as biological pest control
Minute polyphagous wasps are endoparasitoids, that develop within the eggs of their host. Polyphagous parasites do not depend on one single host but are able to vary in the choice of their host. This characteristic can be seen as the reason behind the success story of Trichogramma. Rearing is simplified, if the parasitized host can be selected from a wide range of species, as well as it increases the application range (Grenier et al. 2005). By consisting in approximately half a million species (most of them still unknown), the Trichogrammatidae is a diverse superfamily, of which the Trichogramma genus are the most commonly used wasps in agriculture. The wasps attack eggs of different insect families (butterflies, true bugs, beetles), but are mainly utilised against major moth and butterfly pests (Martin and Sauerborn 2013, Lindsey et al. 2018). To understand the whole concept of the biological pest control, the biological mechanism behind the parasitisation by Trichogramma will be reviewed in 2.1.
The wasp lays its eggs directly onto the host’s egg, by simultaneously killing it. The next generation Trichogramma wasps will hatch circa ten days after. In figure 1 constitutes the biology and parasitisation mechanism of the most common example of lepidopterous pests, the European corn borer (O. nubilalis) and its parasite (T. brassicae). The moth’s life cycle, as well as the parasitic interference of the Trichogramma wasp are shown. The (interfered) lifecycle will continue as long as host eggs are available.
Abbildung in dieer Leseprobe nicht enthalten
Figure 1: Life-cycle and mechanism of the Trichogramma wasp and its host (AMW Nützlinge GmbH 2014)
Several generations of the Trichogramma wasp can develop within one year. After mass releases, the half a millimetre measuring wasp usually does not survive the winter season. In nature, Trichogramma usually survives as a prepupae, the final larvae stadium before pupation. In the new season, increasing temperatures are necessary to complete the pupation development. When temperature rises, development is completed and the first slip of Trichogramma occurs. The emergence takes place temporally offset to the first warm days of the year. The annual hatching in Europe differs between mid-April and the beginning of May. In Southern regions, the emergence will happen slightly earlier (Barnay et al. 2001, Kursch- Metz 2014). Trichogramma in Europe is active at temperatures above 15 °C, while its optimum is between 23 and 28 °C. The activity decreases at higher temperatures. Over 32 °C, no more eggs are laid and the parasitisation activity is inhibited (AMW Nützlinge GmbH 2011, Ksentini et al. 2011).
2.2. Global perspective
Apart from the genus Trichogramma, only few other species are commercially used for lepidopterous pest control. Annually, Trichogramma wasps are applied on nearly 10 Mio. ha worldwide, being the major utilised natural enemy in biological pest control (van Lenteren and Bueno 2003, Vinson et al. 2015). On the contrary, microbial biocontrol agents such as nematodes, fungi, bacteria and viruses are estimated to be utilised on approximately 1.5 Mio. ha worldwide.
The five most common species of Trichogramma are: T. evanescens, T. dendrolimi,
T. pretiosum, T. brassicae, and T. nubilale. In smaller scale of application and crop type, the species diversity is higher. Smith (1996) states four issues for the commercial appropriate application of Trichogramma species for biologic pest control. These are the “selection of the appropriate population to release, a system for mass rearing, distribution of the parasitoid, and a strategy for field release”. In globally common crops, such as sugar cane, wheat, maize parasitisation rates of 60-80% can be achieved. Therefore, yield losses can be substantially reduced in a natural way. For a successful parasitisation, two applications of approximately 100,000 individuals per hectare are recommended (Wajnberg and Hassan 1994 in Martin and Sauerborn 2013). However, the right application of Trichogramma is depending on several factors, considering the approach (inoculative or inundative), timing and frequency. Climate, weather and season are also playing an important role. Local species should therefore be preferred, thanks to a better adaption to local climate and habitat. Using species native to their environment is the basis of the inundative theory (Smith 1996). All five most commercially relevant species are used on a global scale. T. nubilale and T. pretiosum are native to the United States, T. dendrolimi to China and T. evanescens and T. brassicae are native to Europe. Exotic species are used because of their commercial success and effectiveness. The use of exotic species should be limited to situations where no local species are available and a proper environmental screening is implemented prior to its application. The Agroecological idea of promoting natural enemies for pest control (Altieri 1995), is correlating with the concept of using local (natural) predators as biological pest control. The correlation is unbalanced, if exotic species are utilised. Due to exotic species and the heavy reduction of natural enemies, the predator/prey balance is often not given. In the long run, the restorage of the natural predator biodiversity is much more promising, than introducing one single predator species. Altieri (1995) is mentioning several opportunities which can lead to pest control (by parasitic wasps) in a natural and local way. Non-selective insecticides have put pressure on the parasitoids, while several studies (Hagen et al. 1971 and Stephens 1984 in Altieri 1995) proved the restorage of natural predator/prey balances, by reducing the insecticide and therefore increase natural parasitoid activity. Increasing the host diversity, either by biodiversity favouring measures (crop & vegetational diversity) or by releasing host populations, can stimulate the natural parasitoid count to increase ten-fold throughout the whole season (Parker and Pinnell 1972, Stephens 1984 in Altieri 1995). However, instead of reviewing appropriate measurse to naturally restore natural enemy biodiversity, this report will mainly focus on the agricultural practice of releasing parasitic wasps. The compatibility of Trichogramma species for the application in orchards and the environmental risks of mass releasing native as well as exotic species will be reviewed in the continuing.
2.3. Application in orchards
The use of polyphagous wasps in orchards is recognised for thirty years (Hassan et al. 1988). In Europe, the most prominent example of biological pest control in orchards is the protection against the Codling Moth (Cydia pomonella) and the Plum Moth (Grapholita funebrana) by T. cacoeciae and T. dendrolimi (Jehle et al. 2014). C. pomonella is considered as one of the main pests in the worldwide fruit growing, dedicated to several fruits, e.g. apple, peach, plum and pear cultivation (Blomefield 1989 in Samara et al. 2008). Research and commercial range of application in Central Europe has been conducted increasingly for 20 years (Zimmermann 2004). Because of upcoming resistances against common control practices, like granulovirus (CpGV) and scab-controlling Sulphur products, a diversification of pest control methods is urgently needed. The interest for commercial alternatives in organic fruit growing is increasing and drawing attention to polyphagous wasps (Kienzle et al. 2012, Jehle et al. 2014). The application of polyphagous wasps in fruit cultivation is more intense than in crop cultivation. Instead of 100,000 eggs (Wajnberg and Hassan 1994 in Martin and Sauerborn 2013) the rates of wasps released in orchards count several Millions per hectare and application. Methods depend on the size of the orchards.