Health effects of predatory beneficial mites and wasps in greenhouses
1 Introduction
The increasing awareness of the problems with pesticides used in greenhouses has led to a widespread use of biological control of pests both in the form of microbiological pest management and the use of beneficial arthropods. Beneficial arthropods as mites, parasitoids, ladybirds and nematodes have been introduced and used extensively in the production of vegetables and ornamental plants in Danish greenhouses during the last 30 years.
In parallel microbiological pesticides have been introduced in form of bacteria (i.e. Bacillus thuringiensis) for the control of larvae of butterflies and moths, ascarids etc., and fungi (i.e. Verticillium lecanií) for the control of different pests, aphids and whiteflies in environments with a high humidity. Trichoderma harzianum has been used as an antagonist for control of the pathogenic fungus Botrytis cinerea (grey mould). The health effects of these microbiological products have been investigated in the study which is the base of the actual project (Larsen & Bælum, 2002).
Introducing a new technology always gives rise to questions about the possible harmful effects on the health of the workers and consumers who may get in contact with the organisms or their waste products.
Toxicity, sensitization, and inflammatory airway diseases are the most likely adverse health effects of the arthropods. This is due to the protein content of these highly complicated animals, supported by the well known cases of allergy to closely related species as house dust mites, storage mites, and insects as cockroaches.
The possible health problems of exposure to beneficial arthropods (macrobiological pest controls) were anticipated in the planning of the study, and questions about handling of the animals have been included in the questionnaires, filled out at the annual examinations.
The present study is based on the material sampled during the follow-up study from 1997 to 2001 and supplemented with an inquiry in 2004 and field studies carried out in selected greenhouses in 2006.
1.1 Health of greenhouse workers
Greenhouse workers are a group of unskilled and skilled workers. In Denmark mostly women work in this trade.
According to mortality and general morbidity this group does not differ from other comparative working groups. They might actually be healthier when living in rural areas with lower morbidity than seen in the more densely populated urban areas. In the Danish register of hospital admissions in different jobs and trades 1995-99 (Hernandez et al., 2005) the greenhouse workers were included in the group of “aids in agriculture, forestry, and fishing”. Morbidity rates were compared with all actively working persons stratified according to gender. A large difference between males and females was seen for asthma with an index of 172 (CI95%: 105-266) for males while the females had a below average index of 53 (CI95%: 32-82). The males may primarily be farmer’s hands, a group with a well known high prevalence of asthma. In a Finnish study of occupationally related incidence of asthma, gardeners had an incidence ratio of 1.62 (CI95%: 1.19-2.19), higher than the average of the working population but low in relation to other agricultural workers (Karjalainen et al., 2002).
Occupationally related allergy and respiratory diseases have been studied in the large European study of respiratory diseases (ECRHS) covering 34 centers in 15 different European and other industrialized countries. In the study, greenhouse workers were included among other agricultural workers having a prevalence rate ratio (PRR) for asthma of 1.79 (95%CI 1.02-3.16), the reference group being office workers. (Kogevinas et al., 1999). Comparable jobs were farmers (PRR =2.62), painters (PRR = 2.34), and cleaners (PRR=1.97).
Among growers in four European countries (Denmark, Germany, Sweden, and Spain) the prevalence of asthma was higher among those cultivating flowers (5.4 %) than among those cultivating other crops (grain, vegetables etc.) (3.1 %). In the crops which both were cultivated indoors and outdoors work in greenhouses was a risk factor for asthma (odds ratio 2.1 95%CI 0.9 – 4.5) but not for respiratory symptoms per se. (Monso et al., 2000). The greenhouse workers were mainly from Spain as the Danish group only included 20 persons corresponding to 1.1 %.
In a study of Spanish greenhouse workers included in the above mentioned study, 40 randomly selected workers were examined in details (Monso et al., 2002). Five of these (13 %) had asthma and three of these had impairment of the lung function during work indicating occupational asthma giving a prevalence of 7.6 %. Exposure measurements revealed high concentrations of different moulds, both the widespread types Cladosporium, Aspergillus, and Penicillium species to which the persons with occupational asthma were sensitized as well as Botrytis cinerea. Totally, 13 persons (33 %) were sensitized to work place allergens, 7 to moulds and 8 to flowers (Gladiolus, Narcissus, Solidago, Helianthus, and Chrysanthemum).
1.2 Beneficial arthropods
1.2.1 Production and use in the greenhouses
1.2.1.1 Suppliers of beneficial arthropods
The greenhouses included in this study used predatory animals supplied by three major suppliers of predatory animals. Most products were imported from two large Dutch and Belgian suppliers, Koppert and Biobest (Information from importers (GARTA and Borregaard)), while two major suppliers have production of Amblyseius cucumeris in Denmark. Some of the larger enterprises had in some periods cultured their own Hypoaspis miles.
1.2.2 Health effects of predatory animals
The hypothesis is that the greenhouse workers are exposed to the animals or their debris by inhalation either during mixing and applying the products (direct exposure) or when handling the plants or plant products to which predatory animals have been applied (indirect or reentry exposure).
The development of a disease is normally a complicated interaction between one or more external factors and the person’s individual capacity to react. Different pathogenic mechanisms may be involved at the same time.
Figure 1.1 shows a model of the possible respiratory effects of predatory animals. The different factors are included in the figure. For each factor the methods of estimation in the actual study are included.
The arrows show the relations between the different factors. The exposure may give rise to allergy and the allergic response may elicit symptoms or physiological changes. On the other hand exposure may give rise to symptoms or psysiological changes via another pathway not involving the mechanisms of allergy.
Personal factors, such as sex, age, and tendency to react against normally occurring allergens may have an influence on all the relations in the figure.
Exposure is ideally estimated by individual measurements during the whole period. However, normally indirect information about time and method of use, annual consumption etc. has to substitute. This estimation is normally the most difficult and critical factor in revealing a cause effect relationship. The exposure assessment is described in chapter 4.
In the present study allergy was defined as a biochemical estimation of a sensitization to one of the actual beneficial species. It was estimated by two different methods. One was the identification and measurement of the concentration of a specific antibody within the type IgE (typical the allergy antibodies) against an extract of the animals.
The other type of method (Platzer et al., 2005) was a functional test of basophile leucocytes response to an antigen, in this case extract from the animals. In the assay the liberation of histamine from the basophiles were visualized as a result of the allergic response. In the original form the test has to be made on fresh whole blood. However, in a new method, the serum of the person was incubated with leucocytes from donor blood. This modification makes it possible to make the analyses on frozen plasma or serum as in the present study.
An allergic response is in itself not a disease, but in many respects a pathogenic pathway to development of health effects: symptoms, measurable physiological responses, or manifest diseases. These diseases may be asthma, other lung diseases, or upper airway diseases as rhinitis. These conditions were detected (diagnosed) by a combination of symptoms and functional tests.
Figure 1.1. A model for the health effects of beneficial arthropods. The figure shows the different factors of interest in the study (Exposure, allergy, health effects, symptoms, and individual factors) and their interrelationship. Below each factor the type of measurements used in the study are shown. The arrows show the different relations which will be tested in the study. The individual factors will be modifiers in all the possible relations.
1.2.2.1 Amblyseius cucumeris and Amblyseius californicus
Only a single Dutch study has addressed the health effects of A. cucumeris (Groenewoud et al., 2002a) in relation to their study of allergy to bell pepper. No information about A. californicus could be found in the literature.
In the Dutch cross sectional study 472 greenhouse workers, all potentially exposed to A. cucumeris, were tested with skin prick test with an extract of the mite (Groenewoud et al., 2002a).
Of the 472 persons in the study, 109 (23 %) were prick positive, defined as a veal of 3 mm or more. Additionally an allergen-specificIgE (RAST) against A. cucumeris was made, which were only reported as positive or negative. Of the 100 prick positive persons where samples were available, 54 (54 %) had a positive specific IgE while among the about 350 persons with negative prick test for A. cucumeris, only 10 (ca. 3 %) had a positive IgE.
Additionally a nasal challenge test was performed on 23 persons with positive prick test to only A. cucumeris, to A. cucumeris and Tyrophagus putrescentiae, or a strong reaction in prick test to A. cucumeris. The results showed that persons with reported rhinitis had a much stronger response to the extract than persons, although sensitized, but without reported rhinitis symptoms.
The sensitization to A. cucumeris was followed by a high number of rhinitis and asthma symptoms, and there were a high proportion of persons with positive prick test to common allergen (defined as atopics) and sensitization to bell pepper as well as T. putrescentiae.
The study shows a high prevalence of sensitization to A. cucumeris, very often in relation to other allergies to other greenhouse related allergens. According to the effect of exposure to A. cucumeris on respiratory symptoms, the study only gives limited information as all persons were exposed to bell pepper which had a substantial effect (Groenewoud et al., 2002b), and there was no control group without exposure to the mites in the study.
1.2.2.2 Phytoseiulus persimilis
Two recent articles from a Swedish group is the only available information of health effect of this predator in the medical scientific literature (Johansson et al., 2003; Kronqvist et al., 2005).
The first article reports on a pilot study of 31 greenhouse workers working in houses where Phytoseiulus persimilis and Hypoaspis miles were used (Johansson et al., 2003). A method of SDS-PAGE and immunoblotting was used and 10 persons (31 %) had IgE binding in immunoblotting to P. persimilis but no relation was seen between years working in greenhouse and sensitization. Of the 31 workers, 6 (19 %) were atopics defined as a positive reaction to a screening test for common allergens (Phadiatop®, Pharmacia). Five of these were sensitized to mites.
The aim of the study was mainly to describe the possibility of detecting sensitization to the mites and no health effects were recorded.
In the following study 96 greenhouse workers were investigated (Kronqvist et al., 2005). All the persons worked in greenhouses where mites were used for pest control but neither the method of recruiting, the individual level of exposure to the mites nor the working conditions were described in the article.
For measurements of specific IgE a UNICAP® (Pharmacia Diagnostics, Sweden) method was used and targets were P. persimilis, H. miles as well as the spider mite Tetranychus urticae, the storage mite Tyrophagus putrescentiae, and the two house dust mites Dermatophagoides pteronyssinus and Dermatophagoides farinae.
Reports of respiratory work related symptoms as well as lung function were included. Sensitization to mites was related to the occurrence of asthma as well as rhino-conjunctivitis. In the crude analyses, sensitization to predatory mites was related to asthma. However, in the multiple regression analysis where several factors were included, only sensitization to mites in general was related to asthma and to rhino-conjunctivitis.
In the group, 76 persons (79 %) handled mites themselves and among the 15 persons sensitized to mites, 13 (88 %) handled predatory mites. There was no information about the methods of application, the frequency of the use of mites, or the actual culture. Handling of mites was neither related to asthma nor to rhino-conjunctivitis.
The study demonstrated a higher frequency of symptomatic persons among those sensitized to mites in general. This is not surprising as sensitization to one biological agent, naturally occurring or occupational, increase the probability of sensitization to more agents. The relation between exposure to P. persimilis or H. miles and health effects can not be revealed from the article.
1.2.2.3 Hypoaspis miles
Only the above mentioned Swedish articles report about H. miles (Kronqvist et al., 2005). Sensitization was seen in 14 of the 96 workers. However these persons were also sensitized to P. persimilis.
Therefore no specific effects of this widespread mite can be revealed.
1.2.3 Health effects of Tetranychus urticae and Tyrophagus putrescentiae
In contrast to the scarce information about the beneficial species more information is available about these two mites known as spider mites and mould mites, respectively. They are naturally occurring in greenhouses as well as used as prey fodder for the beneficial mites.
In an Italian prevalence study of Tetranychus urticae including 960 farmers and greenhouse workers, 58 (6 %) were sensitized to T. urticae measured by skin prick test (Astarita et al., 2001). Of these persons, 38 had positive tests to a mixture of standard and food allergens. In 20 persons (26 %) an isolated positive test to T. urticae was seen.
Sensitization was correlated with respiratory symptoms at work with a mean time from start of farm work to onset of symptoms of 3 years. Greenhouse workers had a higher prevalence of sensitization to T. urticae than open field farmers and their time to onset of symptoms was shorter.
A Spanish study of 246 greenhouse workers and 227 controls showed a positive skin prick test to T. urticae in 25 % of the greenhouse workers compared to 8 % in the controls (Navarro et al., 2000). In the greenhouse workers, 16 (26 %) of the positive values were solitary while in the rest and in all controls, positive values were seen in persons who responded to other common allergens. In 48 % of the workers, but none of the controls with positive skin prick test, specific IgE antibody (RAST) against T. urticae were detected. There was a five fold relative risk of respiratory symptoms in greenhouse workers with a positive prick test to T. urticae. However the high prevalence of multiple allergies in this group makes it difficult to conclude.
In the above mentioned Swedish study sensitization to T. urticae was seen in 23 persons (24 %), the highest sensitization rate in the study (Kronqvist et al., 2005). This mite might then be of importance in the development of respiratory symptoms, but the study does not permit any firm conclusions.
A Korean study of 725 apple-cultivating farmers found a prevalence of sensitization to T. urticae in 26 % and there was some relation to occupationally related symptoms (Kim et al., 1999). In the study, however, the prevalence of sensitization to an European red mite (Panonychus ulmi) was 40 % and there was a considerable correlation between the sensitizations to the two mites. Older case studies have demonstrated both allergy and exposure related symptoms from T. urticae (Reunala et al., 1983; Delgado et al., 1997) and a study has shown that the allergenicity of T. urticae from different sources in greenhouses can not be distinguished (Orta et al., 1998).
Regarding Tyrophagus putrescentiae a number of studies have addressed the prevalence in farmers (Hage-Hamsten et al., 1985), bakers (Revsbech & Dueholm, 1990), grain elevator workers (Revsbech & Andersen, 1987) as well as in a general population in a humid area in Northern Spain (Vidal et al., 2004). In the general population a very high rate of sensitization (28 %) was seen. In comparison the rate for house dust mites was 14 %. A considerably higher prevalence of sensitization was seen among the younger persons while it was lower in farming occupation than in others. In Swedish farmers a prevalence of sensitization was 6,8 %, often in combination with other storage mites. In the Dutch study of A. cucumeris mentioned above, 44 % of the persons sensitized to A. cucumeris had a positive prick test to T. putrescentiae (Groenewoud et al., 2002a).
1.2.4 Conclusions from the studies of predatory mites
From the literature it can be concluded that in greenhouse workers sensitization to A. cucumeris, P. persimilis, and H. miles was seen. Sensitization was seen in atopic persons as part of multiple allergies, but also as solitary allergies. Respiratory symptoms are less well documented, but still a probable effect in analogue with allergies to other biological agents.
A considerable problem is the cross reactions between the different mites and the predators. The naturally occurring mites T. urticae and T. putrescentiae present a special problem as they show high sensitization rates in greenhouses, they are both naturally occurring pests, and they are used in the production of predators as prey.
The studies have serious limitations as they are cross sectional and only describe exposed groups. Therefore the prevalence of sensitization of the normal population without occupational exposure to mites and the time course of the sensitization and development of related symptoms are not known.
1.2.5 Health effects of the beneficial arthropods
According to the general literature, no information on the human health effects of the beneficial insects Aphidius colemani, Aphidius ervi, or Encarsia formosa is available.The literature mainly concerns the biology, control efficiency and the effects of different pesticides on the insects.
On the other hand health effects of the insects by occupational exposure are possible, if not probable. Allergy to cockroaches is one of the most prevalent allergies in several countries (Arlian et al., 1997) and occupational allergies in persons working with breeding of insects (Ephestia kuehniella and Orius laevigatus) are well known (Belisario et al., 2001; Cipolla et al., 1997), also in Scandinavia (Nielsen, 2000).
In greenhouses the exposure is probably lower but the number of sensitive persons much higher, why a sensitization may be seen.
Version 1.0 August 2007, © Danish Environmental Protection Agency