Research Article |
Corresponding author: M. Lukas Seehausen ( l.seehausen@cabi.org ) Academic editor: Jianghua Sun
© 2021 M. Lukas Seehausen, Catarina Afonso, Hervé Jactel, Marc Kenis.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Seehausen ML, Afonso C, Jactel H, Kenis M (2021) Classical biological control against insect pests in Europe, North Africa, and the Middle East: What influences its success? NeoBiota 65: 169-191. https://doi.org/10.3897/neobiota.65.66276
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Many factors can affect the success and failure of classical biological control. However, these factors have mainly been studied independently of each other, which leaves their relative importance within the complexity of classical biological control (CBC) programmes unknown. Therefore, we set out to take a more holistic view on the factors that may impact the outcome of CBC of insect pests by insect predators and parasitoids. To this end, we filtered the BIOCAT catalogue to extract entries for the Greater Western Palearctic ecozone and added 15 new explanatory variables. These mainly concerned traits of released biological control agents, target pests, and host plants of the target, but also included the number of introductions for specific agent-target combinations as a management aspect. We then analysed the data regarding three levels of success: agent establishment, impact on the target population, and complete control of the target. Between 1890 and 2010 a total of 780 introductions of insects for biological control were undertaken in the analysed area, constituting 416 agent-target combinations. Overall success of agent establishment was 32%, successful impact of single agents on their target was 18%, and success of complete control was 11%. The number of factors significantly influencing the outcome of CBC decreased with increasing level of success. Remarkably few agent-related factors influenced the success: insect predators as agents decreased the probability of establishment and using oligophagous parasitoids significantly decreased the chances of complete control. Other significant factors were related to traits of target pests or their host plants. For example, sap feeders and target pests attacking reproductive plant parts were more likely to be successfully controlled. The rate of success increased with the number of introductions of CBC agents, in particular against univoltine target pests. These findings suggest that a focus on agent-related traits to increase the chances of successful CBC is not fully justified and should be complemented with the consideration of lower trophic levels and other aspects of CBC, such as abiotic factors and management.
BIOCAT, importation biological control, introductions, invasive species
Classical biological control (CBC), i.e. the introduction of natural enemies from the region of origin, has proven to be an efficient and cost-effective tool to control invasive insect pests worldwide, including pests of forests and ornamental trees (
An impressive amount of theoretical and empirical evidence has been gathered to identify the specific factors that have an impact on the successful outcome of CBC programmes. They can be classified into five categories.
The first three categories are the biotic factors that are inherently bound to the three trophic levels involved in this type of biological control. They correspond to the life-history traits that are involved in the trophic interactions between the pest’s host plant, the pest itself, and its introduced natural enemy. When considering which variables within these categories should be addressed to increase the success of CBC, the focus often falls on the highest trophic level, the selection of the biological control agent that is to be introduced. For example, it has been repeatedly suggested that parasitoids are more successful in target control, compared to predators (
The fourth category of factors that can have an influence on the success of biological control introductions concerns abiotic factors of the environment where introductions are made. An obvious factor may be the existence of physical barriers to dispersal. However, the abiotic factor that is the most considered in relation to the success and failure of biological control is climate. It has been estimated that climate is responsible for about 35% of failures of natural enemy introductions against arthropods (
The fifth category of factors potentially affecting the success of CBC involves the management of biological control programmes, and in particular release procedures. Some examples for these factors are the timing of release; release location; the quantity, quality, and life-stage of individuals that are released; the number of repeated introductions of an agent species against the same target; or the number of agent species that are released against a specific target (e.g., reviewed by
From the above list of factors that can influence the success of biological control introductions, it becomes sufficiently clear that CBC is very complex and its success depends on proper methods and decisions at different steps of the process. Even if many individual factors have been shown to influence the outcome of introductions, their importance relative to all other factors remains unclear. The main objective of this study is therefore to analyse the relative importance of factors that possibly affect successes and failures of CBC of invasive pest insects using a holistic approach that reflects the complexity of biological control introductions. The analysis is done on the basis of the BIOCAT catalogue of biological introductions (
The basis for all analyses in this report is the BIOCAT catalogue of introductions of insects for the CBC of insect pests (
Agent voltinism was not considered, because too few agents that were introduced into the analysed area were exclusively univoltine (n = 18).
Summary statistics were calculated with various Microsoft Excel functions using two modes: (1) Considering all introductions included in the database regardless if the same species was introduced once or several times against the same target; and (2) collapsing the database to unique combinations of an agent species introduced against a target species (henceforth called agent-target combinations) and keeping track of the number of introductions per unique species combination as a variable (see above), as well as calculating the sums of successful establishment, impact, and control for each combination separately. For the description of the trends over time, a decade is defined as a ten-year period and named based on their shared tens digit, from a year ending in a 0 to a year ending in a 9. For example, the period from 1960 to 1969 is the 1960s.
To assess the effect of the above described factors on (1) agent establishment, (2) impact of the agent on the target, and (3) complete control of the target, three separate Generalised Linear Mixed-Effects Models (GLMMs) with a binomial frequency distribution (logistic regressions) were conducted using the glmmTMB function of the package with the same name (Brooks et al. 2017) in R (R Core Team 2019). Success was used as dependent binomial variable and each full model included nine single factors and four interactions (see Table
Independent variables used in the full models for CBC agent establishment, target impact (partial to complete control), and target control (no other control is needed).
Independent variable | Levels | Description |
---|---|---|
Agent feeding strategy | 2 | predator, parasitoid |
Agent host range | 4 | mono-, oligo-, polyphagous, unknown |
Life-stage killed by the agent | 5 | eggs, larvae, larvae & adults, all stages, other |
Parasitoid feeding behaviour1 | 2 | endoparasitoid, ectoparasitoid |
Parasitoid brood size1 | 3 | solitary, gregarious, unknown |
Parasitoid attack strategy1 | 3 | koinobiont, idiobiont, unknown |
Target feeding guild | 3 | borers, defoliators, sap feeders |
Target host range | 2 | (1) mono- and oligophagous, (2) polyphagous |
Target voltinism | 2 | univoltine, multivoltine |
Plant attacked by target | 3 | herbaceous, woody, both |
Plant parts attacked by target | 4 | shoot (stem and leaves), reproductive (flowers, fruits, seeds), shoot & reproductive, other (root, and other combinations) |
Number of (№) introductions | Continuous | Number of introductions within agent-target combinations (1–34) |
№ Introductions × Agent feeding strategy | NA | Interaction |
№ Introductions × Target feeding guild | NA | Interaction |
№ Introductions × Target voltinism | NA | Interaction |
№ Introductions × Plant attacked by target | NA | Interaction |
In Europe, North Africa, and the Middle East, the first recorded introduction of an insect as a biological control agent against another insect was done by Egypt in 1890 with the introduction of the coccinellid Rodolia cardinalis from Australia against the cottony cushion scale Icerya purchasi (Hemiptera: Margarodidae), an invasive species from Australasia damaging citrus. The introduction was successful in that the agent became established and led to a complete control of the target. From this first case until the year 2010, a total of 780 introductions of insects for biological control of insect pests were undertaken in the analysed area.
From the 1890s to the 1960s the number of introductions per decade steadily increased up to a maximum of 120 attempts (Fig.
Number and success of classical biological control (CBC) introductions. Upper two panels A number of introductions of CBC agents per decade introduced in Europe, North Africa, and the Middle East against pests on (full line) all plants, (dashed line) woody plants, and (dotted line) herbaceous plants B total number of introductions for all target host plants, woody plants, and herbaceous plants (black bars) including repeated introductions of agents against the same target and (grey bars) for unique agent-target combinations. Note that some targets feed on both woody and herbaceous plants. Lower six panels: Percentage of successful biological control introductions (left side) per decade and (right side) overall, against pests on C, D all plants E, F woody plants, and G, H herbaceous plants. Success was measured in terms of (blue line) agent establishment, (purple line) partial to complete control of the target (i.e., impact), and (red line) substantial to complete control (i.e., control). Repeated introductions within specific agent-target combinations are included in the calculation of the percentages per decade and in full-coloured bars. Light-coloured bars indicate percentages based on unique agent-target combinations.
The success of the introductions varied over time, but in contrast to the number of introductions, it does not exhibit an equally clear trend (Fig.
Relative frequency of independent variable classes related to (red bars) biological control agents, (blue bars) biological control targets, (green bars) host plants of targets, and (black bars) management A agent feeding strategy B agent host range C life-stage killed by the agent D parasitoid feeding behaviour E parasitoid brood size F parasitoid attack strategy G target feeding guild H target host range I target voltinism J plant attacked by target K plant parts attacked by target, and L number of introductions within agent-target combinations.
Eight countries were responsible for more than two thirds (70.5%) of all introductions: Israel (16.3%), Italy (14.0%), Former USSR (10.1%), France (7.3%), Greece (7.1%), Spain (6.0%), Egypt (5.3%), and Cyprus (4.4%). Within these countries, the percentage of complete target control was very variable, ranging from 19.1 in Spain to 5.5 in Israel (Table
Countries in Europe, North Africa, and the Middle East and their respective number of introductions of biological control agents, agents established, targets impacted, and targets controlled. Target impact is defined as partial (reduced pest status) to complete (no other control needed) control and target control is defined as substantial (other control needed occasionally) to complete control. Percentages (numbers in parentheses) are calculated for all countries for which at least 10 introductions are recorded in the BIOCAT database.
Country | Number of introductions | Agents established Number (%) | Targets impacted Number (%) | Targets controlled Number (%) |
---|---|---|---|---|
Israel | 127 | 43 (33.9) | 15 (11.8) | 7 (5.5) |
Italy | 109 | 48 (44.0) | 30 (27.5) | 20 (18.3) |
Former USSR | 79 | 20 (25.3) | 12 (15.2) | 6 (7.6) |
France | 57 | 24 (42.1) | 16 (28.1) | 4 (7.0) |
Greece | 55 | 20 (36.4) | 12 (21.8) | 8 (14.5) |
Spain | 47 | 30 (63.8) | 25 (53.2) | 9 (19.1) |
Egypt | 41 | 14 (34.1) | 7 (17.1) | 6 (14.6) |
Cyprus | 34 | 22 (64.7) | 5 (14.7) | 3 (8.8) |
Former SFR Yugoslavia | 21 | 0 (0.0) | 0 (0.0) | 0 (0.0) |
Former Czechoslovakia | 19 | 3 (15.8) | 0 (0.0) | 0 (0.0) |
Turkey | 17 | 10 (58.8) | 7 (41.2) | 2 (11.8) |
United Kingdom | 15 | 4 (26.7) | 3 (20.0) | 2 (13.3) |
Germany | 14 | 5 (35.7) | 5 (35.7) | 0 (0.0) |
Morocco | 13 | 8 (61.5) | 3 (23.1) | 2 (15.4) |
Poland | 13 | 1 (7.7) | 1 (7.7) | 0 (0.0) |
France (Corsica) | 11 | 10 (90.9) | 8 (72.7) | 3 (27.3) |
Oman | 11 | 8 (72.7) | 2 (18.2) | 2 (18.2) |
Portugal | 8 | 6 | 3 | 3 |
Switzerland | 8 | 6 | 6 | 2 |
Georgia | 8 | 4 | 4 | 2 |
Malta | 6 | 4 | 4 | 3 |
Tunisia | 6 | 5 | 3 | 2 |
Ukraine | 6 | 2 | 0 | 0 |
Iran | 5 | 1 | 0 | 0 |
Russia | 5 | 1 | 1 | 0 |
Syria | 5 | 5 | 0 | 0 |
Austria | 4 | 3 | 1 | 1 |
Algeria | 3 | 2 | 0 | 0 |
Belgium | 3 | 1 | 1 | 1 |
Spain (Canary) | 3 | 2 | 1 | 0 |
Czech Republic | 2 | 2 | 0 | 0 |
Hungary | 2 | 0 | 0 | 0 |
Ireland | 2 | 2 | 2 | 2 |
Portugal (Azores) | 2 | 0 | 0 | 0 |
Spain (Balearic) | 2 | 2 | 1 | 0 |
Sweden | 2 | 2 | 2 | 1 |
Yemen | 2 | 1 | 0 | 0 |
Afghanistan | 1 | 1 | 0 | 0 |
Azerbaijan | 1 | 0 | 0 | 0 |
Croatia | 1 | 0 | 0 | 0 |
Denmark | 1 | 1 | 1 | 0 |
Greece (Crete) | 1 | 1 | 1 | 1 |
Greece (Rhodes) | 1 | 0 | 0 | 0 |
Italy (Cuneo) | 1 | 0 | 0 | 0 |
Jordan | 1 | 1 | 1 | 1 |
Lebanon | 1 | 0 | 0 | 0 |
Portugal (Madeira) | 1 | 0 | 0 | 0 |
Saudi Arabia | 1 | 1 | 1 | 1 |
Slovenia | 1 | 0 | 0 | 0 |
Uzbekistan | 1 | 0 | 0 | 0 |
Grand Total | 780 | 326 (41.8) | 184 (23.6) | 94 (12.1) |
CBC attempts were conducted for a total of 416 agent-target combinations. From these combinations, 78% of the agents were parasitoids (Fig.
CBC agents released in the Greater Western Palearctic ecozone for the described time period belonged to 33 insect families. Within the agent-target combinations, four insect families made up 72% of all agents, with the three most abundant ones (Aphelinidae, Encyrtidae, and Coccinellidae) being agents attacking targets belonging to the insect order Hemiptera (Fig.
Descriptive statistics of (red) biological control agent families, (blue) biological control target families, and (green) plant groups. Upper row: Relative abundance of A agent and D target families, and G crops represented in introductions for biological control against insects in the analysed area (Europe, North Africa, and the Middle East). Middle row: Relationship between the relative abundance of B agents E targets, and H crops in all introductions and successful complete control of the target. Lower row: Relationship between the percentage of agent establishment and the percentage of partial to complete control (impact) of the target for the most abundant C agent and F target families, and I crops. Repeated introductions of agent-target combinations are not considered.
With 36 insect families, the biological control targets were similarly diverse as the agents. However, the top 72% of targets comprised eight families, which is twice the amount of the top agent families. Not surprisingly, the six most abundant target families (Diaspididae, Pseudococcidae, Coccidae, Aleyrodidae, Tephritidae, and Aphididae) belong to the order Hemiptera, the insect order in which the above described majority of agent families are specialised (Fig.
Almost one third (31.7%) of crops attacked by the target pests were citrus, followed by olive (8.7%), potato (4.6%), mulberry (3.9%), various fruits (3.6%), various fruit trees (3.4%), and avocado (2.9%; Fig.
The explanatory variables ‘number of introductions × plant attacked’, ‘number of introductions × target feeding guild’ and ‘life-stage killed by the agent’ were removed from all but one of the regressions due to high collinearity. The variable ‘life-stage killed by the agent’ remained in the regression analysing the success of establishment of parasitoids as CBC agents.
For all CBC agents together, the number of explanatory variables significantly influencing the outcome of CBC decreased with an increasing level of success, i.e. from establishment to impact to control (Table
In contrast to models for all CBC agents together, for parasitoids the number of significant explanatory variables remained the same at all three levels of success (Table
Analysis of Deviance Tables (Type II Wald chi-square tests) for fixed effects of the logistic regressions testing the factors influencing biological control agent establishment, impact of the agent on the target species, and complete control of the target species, considering A all biological control agents (predators and parasitoids) and B parasitoids as biological control agents only.
Variable | Establishment | Impact | Control | ||||||
---|---|---|---|---|---|---|---|---|---|
χ2-value | df | P | χ2-value | df | P | χ2-value | df | P | |
(A) All agents (Predators and parasitoids) | |||||||||
Number of (№) introductions | 17.9290 | 1 | <0.0001 | 12.5121 | 1 | 0.0004 | 0.7041 | 1 | 0.4014 |
Agent feeding strategy | 6.3983 | 1 | 0.0114 | 1.6525 | 1 | 0.1986 | 0.1924 | 1 | 0.6609 |
Agent host range | 2.8100 | 3 | 0.4219 | 4.0226 | 3 | 0.2590 | 5.3605 | 3 | 0.1472 |
Target feeding guild | 17.3652 | 2 | 0.0002 | 8.9079 | 2 | 0.0116 | 7.6286 | 2 | 0.0221 |
Target host range | 0.2938 | 1 | 0.5878 | 0.6057 | 1 | 0.4364 | 1.0514 | 1 | 0.3052 |
Target voltinism | 0.4109 | 1 | 0.5215 | 0.1767 | 1 | 0.6743 | 0.0001 | 1 | 0.9915 |
Plant attacked by target | 1.5464 | 2 | 0.4615 | 2.4780 | 2 | 0.2900 | 1.7736 | 2 | 0.4120 |
Plant parts attacked by target | 12.5481 | 3 | 0.0057 | 0.5769 | 3 | 0.9017 | 0.6256 | 3 | 0.8906 |
№ Introductions × Agent feeding strategy | 0.0556 | 1 | 0.9491 | 0.0556 | 1 | 0.8136 | 1.0702 | 1 | 0.3009 |
№ Introductions × Target voltinism | 5.2701 | 1 | 0.0217 | 4.5016 | 1 | 0.0339 | 3.2968 | 1 | 0.0694 |
(B) Parasitoids only | |||||||||
Number of introductions | 11.9774 | 1 | 0.0005 | 9.0146 | 1 | 0.0027 | 0.0037 | 1 | 0.9518 |
Agent host range | 2.1136 | 3 | 0.5492 | 5.6368 | 3 | 0.1307 | 8.9607 | 3 | 0.0298 |
Agent feeding behavior | 0.0299 | 1 | 0.8627 | 0.0153 | 1 | 0.9017 | 0.1627 | 1 | 0.6867 |
Agent brood size | 1.3036 | 2 | 0.5211 | 0.4971 | 2 | 0.7799 | 0.0119 | 2 | 0.9941 |
Agent’s attack strategy | 1.3689 | 2 | 0.5044 | 2.0629 | 2 | 0.3565 | 1.1576 | 2 | 0.5606 |
Target’s life-stage killed by agent | 1.7470 | 3 | 0.6265 | NA | NA | NA | NA | NA | NA |
Target feeding guild | 6.4020 | 2 | 0.0407 | 12.7859 | 2 | 0.0017 | 11.6543 | 2 | 0.0029 |
Target host range | 0.0110 | 1 | 0.9164 | 1.3492 | 1 | 0.2454 | 1.0008 | 1 | 0.3171 |
Target voltinism | 0.7702 | 1 | 0.3802 | 1.9263 | 1 | 0.1652 | 1.3506 | 1 | 0.2452 |
Plant attacked by target | 1.5567 | 2 | 0.4592 | 3.4705 | 2 | 0.1764 | 3.3887 | 2 | 0.1837 |
Plant parts attacked by target | 5.9048 | 3 | 0.1163 | 0.6848 | 3 | 0.8768 | 0.3072 | 3 | 0.9587 |
№ Introductions × Target voltinism | 4.8460 | 1 | 0.0277 | 4.9852 | 1 | 0.0256 | 4.4323 | 1 | 0.0353 |
The models’ estimates for predators and parasitoids together revealed that keeping all other variables constant, the odds for agent establishment significantly increased with the number of introductions against specific agent-target combinations, when the target was a borer (endophagous) and when it was a sap feeder, as well as by an interaction of the number of introductions and the target being univoltine. But agent establishment significantly decreased when the agent was a predator, and when the target fed on reproductive plant parts (Fig.
Back-transformed (odds ratios) significant estimates and 95% confidence intervals from the logistic regression determining the factors negatively (<1, on the left of the dotted line) or positively (>1, on the right of the dotted line) influencing the success of biological control introductions, for (left panels) all biological control agents and (right panels) parasitoids as biological control agents. Success was measured as (upper panels (A, B)) establishment of the agent, (middle panels (C, D)) impact on the target (partial to complete control), or (lower panels (E, F)) substantial to complete control of the target. Estimates with numbers followed by an asterisk (*) are statistically significant at α=.05, those without an asterisk at α=.1.
The impact on targets was also positively influenced by the number of introductions against specific agent-target combinations, by sap feeders as targets, and by the interaction of the number of introductions and univoltine targets, but only had the tendency to be negatively influenced (.05>α<.10) when agents were oligophagous and targets univoltine (Fig.
The odds to control a target significantly decreased when the agent was oligophagous but significantly increased with sap feeders as targets. It had also the tendency to be positively influenced by the interaction of ‘number of introductions × univoltine targets’ (Fig.
A graphical analysis of the interaction between the number of introductions and target voltinism for each level of success separately (establishment, impact, and control) shows that for both univoltine and multivoltine targets the probability of success increased with the number of introductions. However, for univoltine species an asymptote at 100% probability of success was reached at 10–20 introductions, while for multivoltine species the relationship was rather linear, with a decreasingly steep slope at increasing levels of success (establishment>impact>control; Fig.
Model predictions for the influence of the interaction between number of introductions within agent-target combinations and target voltinism (red=univoltine, blue=multivoltine) on the probability of (upper panels) agent establishment, (middle panels) impact on target, and (lower panels) target control, by (left panels) all biological control agents and (right panels) only parasitoids as biological control agents.
The overall success of biological control introductions of insect predators and parasitoids against herbivorous insects in Europe, North Africa, and the Middle East is comparable to the success of CBC worldwide (
Until the 1990s, it was common practice to collect several agent species in the area of origin of the target and release them with no or only a minimum of studies in the invasive range of the pest. Our analysis shows that in Europe, North Africa, and the Middle East these rather uninformed and hasty introductions led to a slight increase in agent establishment but less frequently to a sufficient impact on the target. Furthermore, these introductions were often only attempted once for any given agent species, which can explain the high percentage of one-time introductions for agent-target combinations (74%). In fact, as has also been shown in a global analysis of the BIOCAT catalogue (
On the other hand, we show here that for the agent-target combinations for which introductions have been repeated several times, a positive correlation between the number of introductions and success was found. There are several possible reasons why repeated introductions of the same agent against one target may lead to a higher probability of success. For example, if several introductions were made from different source locations, the more diverse genetic sources of the agents may lead to a higher possibility that at least one of the strains can well adapt to the new environment in the location of release (
Regardless of the underlying mechanisms discussed above, we found that as few as 10 introductions increased the mean probability of agent establishment to 75% for univoltine species, and with 20 introductions success of CBC increased on all levels (establishment, impact, and control) to near 100%. However, those results were biased by the facts that in only 2.6% of agent-target combinations more than 10 introductions have actually been done, and that only 15.4% of targets were univoltine species. Nevertheless, it emphasizes the importance of the number of introductions for specific agent-target combinations for the success of CBC.
Interestingly, remarkably few agent-related factors significantly influenced the success of CBC. The odds of establishment decreased when agents were predators, a finding that has been repeatedly confirmed empirically by comparing the success of predators and parasitoids used in CBC programmes (e.g.,
Furthermore, oligophagy of agents was negatively related to the impact and control of targets, which holds true when considering all CBC agents together or only parasitoids. Considering agent host range as a single factor, the overall success of target control was 21%, 8% and 10% (or 28%, 7%, 11% when considering repeated introductions) for mono-, oligo-, and polyphagous agents, respectively, which would suggest that rather monophagy is an advantageous characteristic over oligo- and polyphagy. Given the relatively low number of agents that were considered monophagous in this study (13.5%), the effect of agent host range on the success of CBC should be further assessed in a multiple regression context including data from additional regions of the world. Interestingly, oligo- and polyphagy have been mentioned in the literature as a desirable trait for CBC agents, e.g., to survive periods when target pest densities are low by switching to other prey/hosts (
The factor with the strongest influence on the chances of success of CBC was related to the trophic guild of the target insect: sap feeders were the target feeding guild most likely to be successfully controlled. This result is consistent in our analyses through all levels of success and for both parasitoids and all agents combined. It is also consistent with results from previous analyses, where ‘Homoptera’ were repeatedly found to be the group of insect pests with the highest number of agent releases, establishments, and successful control (
Our analysis only comprises a limited number of factors from the various aspects of CBC that have been reviewed so far. Many other factors may significantly influence the outcome of biological control introductions. In our opinion, especially climate matching and management factors such as the timing, the quantity, and quality of CBC agents being released deserve more attention to increase the success of CBC programmes. However, for the majority of the introductions that have been done, the principal challenge might be the limited availability of information about many of the potentially important factors explaining their success or failure. To further advance in this direction, more data have to be gathered, for which a more rigorous documentation of CBC programmes and a wider availability of these data to scientists and biological control practitioners are paramount. Additionally, our analysis was restricted to data from Europe, North Africa, and the Middle East. Because results may change among ecozones, BIOCAT data from other ecozones or for the whole world should also be analysed.
The finding that only few CBC agent-related factors significantly influenced the success of CBC suggests that the reoccurring focus on agent-related traits is not justified and should be redirected to include lower trophic levels and other aspects of CBC, such as abiotic factors (i.e., climate) and management (e.g., release procedures). Indeed,
The results from this study should be understood as a first step to give the incentive for a holistic, rather than an independent consideration of factors affecting the success of CBC. The analysis of the entire BIOCAT catalogue or an updated version including the more recent introductions should lead to further insights and help to develop decision support tools to increase the success of CBC at all levels.
We thank Matthew Cock for granting us access to BIOCAT, Bob Douma for help with the statistical analysis, and Alessandro Racca for help with the data assembly. The research leading to these results received funding from the European Union’s Horizon 2020 Program for Research & Innovation under grant agreement no. 771271 (HOMED). MLS and MK were supported by CABI with core financial support from its member countries (see https://www.cabi.org/about‐cabi/who‐we‐work‐with/key‐donors/). CA was supported by a PhD grant (SFRH/BD/135845/2018) from Fundação para a Ciência e a Tecnologia (FCT).
Supplementary tables and figures
Data type: Tables and figures
Explanation note: Table S1. Model coefficients (estimate ± standard error) and their level of significance in the logistic regressions for all biological control agents (predators and parasitoids) analysing variables impacting agent establishment, agent’s impact on the target, and target control. Table S2. Model coefficients (estimate ± standard error) and their level of significance in the logistic regressions for parasitoids as biological control agents, analysing variables impacting agent establishment, agent’s impact on the target, and target control. Figure S1. The Greater Western Palearctic ecozone (sensu