Bottom‐up forces of abiotic factors on a tritrophic plant‐aphid‐parasitoid system

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Biological control of pests in the ecological context

Then God said, “I give you every seed-bearing plant on the face of the whole earth and every tree that has fruit with seed in it. They will be yours for food.” Genesis 1:29 | NIV |

The importance of biological control

Crop losses caused by pests challenge crop protection efforts. Pests throughout the world are destroying about 35% of all crop production (Pimentel et al., 1991; Oerke 2006). Among them, insect pests cause an estimated 13% crop loss, plant pathogens 12%, and weeds 10% (Cramer, 1967). Crop protection measures could play essential roles in preventing both pre‐ and post‐ harvesting loss. Since 1942, the discovery of new chemical pesticides gave rise to their golden era where they integrate into many foods, fiber, and fuel production systems. Agricultural producers spent around 40 billion‐worth for pesticides per year worldwide for estimated 2.5 x 106 tons of pesticides; these numbers increase annually (Pimentel 1991; Popp et al. 2012). At the first time, pesticides (in this context understood as chemical ones) were efficiency against pests as they protected up to 70% of potential crop yields (Oerke 2006). However, intensive overuse of chemical products places a heavy burden on ecology, environment, and human health in the long term and threaten the overall future of agriculture. Pesticide residues were present in surface waters, aquatic sediments, and groundwater in more than 2500 sites in 73 countries (Stehle and Schulz, 2015). They diminish soil biodiversity and spoil soil function by killing beneficial organisms, and indirectly cause soil erosion (FAO and ITPS, 2017). Residues of pesticides can also be found in a tremendous variety of everyday foods and beverages, including cooked meals, water, wine, fruit juices, refreshments, and animal feeds (Nicolopoulou‐Stamati et al., 2016). Adverse effects associated with chemical pesticides upon animal or human health include dermatological, gastrointestinal, neurological, carcinogenic, respiratory, reproductive, and endocrine effects (Bonner and Alavanja, 2017; Kim and Jahan, 2017). To confront these deleterious effects, the governments of the countries like USA, EU, Canada, India, China have regulated or restricted the use of several types of pesticides (United Nations, 2009). Since the publication of “Silent Spring” (Rachel Carson, 1962), the growing public concern increases the pressure on replacing chemical measures by more sustainable and “green” methods.
Most of all, pesticide application is not omnipotent, as many pest problems in agriculture root in drastic changes in the crop agroecosystem (Pimentel, 1991). Despite an apparent increase in pesticide use, crop losses have not significantly decreased during the last 40 years (Oerke 2006). In the years of ’90 in the US, corn losses to rootworm pests average 12% (Pimentel, 1991) despite the application of 14 x 106 kg insecticide per year. However, in 1945, before the invention of synthesized pesticides, corn losses to these insects averaged only 3.5% (USDA, 1954). Different mechanisms could explain the inefficiency of chemical methods. Firstly, the overuse of pesticides eliminates essential ecosystem services such as natural enemy populations, resulting in secondary pest outbreaks. Secondly, non‐selective application of pesticides jeopardizes resistance among pests (R4P network, 2016; FAO, 2017), which cause the cycle of new pesticide generations ‐ further pest resistance. Thirdly, traditional crop rotations are replaced by continuous and monocultures, resulting in the carry‐over of the pest infestation from one year to the next. Fourthly, the introduction of exotic crops to a new geographic region might render plants susceptible to local pests in the absence of defensive mechanisms and cause pest outbreaks (Pimentel, 1991). Therefore, pest problems and crop protection must be approached by holistic strategies.
Under such circumstance, the concept of Integrated Pest Management (IPM) is developed and implemented at the global scale for over 20 years (FAO, 2017). IPM is a knowledge‐ intensive process of decision making that combines various strategies such as biological, cultural, physical, and chemical methods and regular field monitoring of the crops. IPM focuses on the reduction of pesticide use to sustainably manage dangerous pests (FAO 2017). As an ecological approach, IPM promotes biological control (BC) methods in pest prevention and suppression among other strategies (Figure 1).
BC or the use of natural enemies to control pests has been a traditional agriculture practice up to now (van den Bosch et al. 1982). However, BC practices are still limited. Over 67000 pest species in the world, 300 pest species targeted to BC; among which only around 120 pest species have been controlled efficiently (Pimentel, 1991). Different types of natural enemies used as biological control agents (BCA) include predators, parasitoids (insects that lay eggs inside or on pests and eventually kill them), pathogens (such as fungi and bacterial toxins) and soil predatory nematodes (Heimpel and Mills, 2017) (Figure 2). BC has several advantages in compares to the chemical approach. Firstly, BC associate with few risks on environmental and public health (FAO, 2017). Secondly, as BCA are highly specific to few targeted pests in compare to pesticides, the efficiency of BCB are promising. Furthermore, BC could have sustainable effects as the released agents are self‐powered, self‐sufficient, and self‐regulating and could permanently establish in the new environment. In such cases, further investments in BC are not required in contrast to the obligate annual applications of pesticides. Examples of other pest species that have been permanently controlled are numerous (Sweetman, 1958; DeBach, 1964; Huffaker, 1980). For example, the introduction of beetle predators in California (USA) has provided effective control of the cottony scale for over 100 years (Simmonds et al., 1976; Pimentel, 1991). Three main strategies of BC are (1) importation or classical BC consists of the introduction of an exotic BCA to regulate a pest species. Over the past 120 years, more than 2000 exotic BCAs were released worldwide to try to control pests in a new area (Bale et al. 2008). (2) Augmentative BC consists of inundative or inoculative releases of BCA and their genetical enhancement; (3) Conservation BC includes in managing the agricultural landscape to favor BCA (Debach and Rosen. 1991) (Figure 3). BC contribute up to US$5.4 billion per year in the USA (Losey and Vaughan, 2006) merely for pest control, not to mention the value of controlling disease vectors. Recently, BC contributes more than US$239 million per year in four US states in managing a single invasive pest, the soybean aphid, Aphis glycines, (Landis et al., 2008; Schowalter, 2016). BC within the IPM practice is nowadays a post‐pesticide‐era approach to counteract the increase of pesticide resistance on targeted species and are a path toward sustainable agriculture.
Figure 3. Spatial and temporal scales of the four main approaches of the BC process (from Klinken 1999)


The risks of BC and requirements for BC practices

Despite all outstanding advantages, BC does not come without risks. The use of BCA also implies ecological risks, e.g. the extinction of non‐target native species, or the unpredictable evolution of the BCA in a new environment (Kaser and Heimpel 2015; Roderick et al. 2012). Most of all, invasive alien species are considered among the most significant threats to global biodiversity. Exotic species once released could benefit from the lack of their natural enemies or the defensive systems of their food species and become dominant in the ecosystem through intra‐guild predation (when one of two species (or both) competing for the same host or prey also consumes its competitor) (Roy and Wajnberg, 2008). As a result, the native biodiversity reduces. The rapid increase in introduced exotic species worldwide and the potential of these species to become invasive have ecological and evolutionary consequences (Olden and Poff 2004; Olden et al. 2006). Secondly, in many cases, the efficiency and the cost‐ effective of BC has been questioned. Many accidentally or intentionally introduced species fail to establish in their new range. Unlike chemical compounds, BCAs are sensible to many biotic and abiotic factors from the ecosystem and from the rearing conditions through ecological interactions. Therefore, the BCA efficiency is not thoroughly predictable. Thirdly, of those alien species that do manage to establish many have negligible effects and some species, often those introduced with agriculture and forestry, are even considered beneficial and desirable (Williamson 1999). Lastly, when the BCA are parasites or parasitoids, the
selective pressure upon pest hosts by parasite populations can cause rapid evolution in the pest hosts (Lamichhane et al. 2016). Therefore, BC practices require a deep understanding of the ecosystem.
BC comes with the concept of keeping pests at an economically acceptable threshold. Often, low levels of populations of some pests are needed to keep natural enemies in the field, and IPM aims to reduce pest populations to avoid damage levels that cause yield loss. Therefore, IPM strategies, mainly BC requires understanding the crop ecosystem as a basis for right crop management decisions and are specific to each crop variety, country, region, and location (FAO, 2017). The efficiency of the BCA depends on several factors including (1) their biological traits (2) interactions with their surrounding environmental conditions, the so‐called species niche, and (3) their specificity toward the ecosystem they are released in (Roderick et al. 2012). Also, risk assessment is an essential component in the development of any biological control strategy (Roy and Wajnberg, 2008). The prior study of a potential BCA focuses on its BC efficiency and safety. Both parameters could be predicted through the initial evaluation of the BCA host specificity.

Aphid parasitoids as biological control agents

Aphids: biology, agricultural importance, and natural enemies

Aphid biology

Aphids (Hemiptera: Aphididae) are soft‐bodied insects of less than 10 mm in length. Their body parts consist of a head, thorax, an abdomen, siphunculus, antenna, and cauda (Figure 4). They can secrete protective liquid, alarm pheromones, and honeydew. Aphids excrete honeydew, which are food resources for several insects such as ants and parasitoids to reduce the osmotic pressure of ingested phloem sap or host plant toxins (Auclair 1963). Among around 4300 aphid species, aphid pests belong mostly to the sub‐family Aphidinae of two tribes: Aphidini and Macrosiphini (Kim et al. 2011).

Table of contents :

Chapter 1. Host range testing: biological control practice requiring ecological understanding
1.1. Biological control of pests in the ecological context
1.2. Aphid parasitoids as biological control agents
1.2.1. Aphids: biology, agricultural importance, and natural enemies
1.2.2. Aphid parasitoids: biology and application in biological control
1.2.3. Co‐evolution of parasitoids and aphid hosts
1.2.4. The significance of parasitoid host specificity in biological control
1.3. Ecology of host specificity
1.3.1. Ecological specialization concept
1.3.2. Shades of ecological specialization: specialist vs. generalist species
1.3.3. The continuum of specialization patterns: specialist ‐ generalist
1.4. How to evaluate the host specificity of aphid parasitoids?
1.4.1. Host specificity index
1.4.2. Host specificity specter of Aphidiinae parasitoids
1.4.3. The optimal foraging behavior: correlation preference‐performance.
1.4.4. The host specificity testing process in biological control
Chapter 2. Bottom‐up and top‐down forces in ecological network
2.1. Ecological networks and interactions
2.2. Bottom‐up versus top‐down forces
2.3. Bottom‐up forces of abiotic factors on a tritrophic plant‐aphid‐parasitoid system
2.3.1. How could plant traits mediate bottom‐up and top‐down forces?
2.3.2. How could aphid traits mediate bottom‐up and top‐down forces?
2.3.3. How abiotic stresses level‐up to parasitoids through the tri‐trophic interaction?
2.3.4. Bottom‐up and top‐down forces in biocontrol context
Chapter 3. Objectives
Chapter 4. Aphid‐parasitoid host specificity
Article I. Preference‐performance relationship in host use by generalist parasitoids
Chapter 5. Water stress‐mediated bottom‐up effects on aphid parasitoid diet breath
Article II. Bottom‐up effect of water stress on the aphid parasitoid Aphidius ervi (Hymenoptera: Braconidae)
Article III. Water stress‐mediated bottom‐up effect on aphid parasitoid diet breadth


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