Literature review

Two databases (BIOSIS Previews and Web of Science), were searched using the key words: “biological control” and either “release rate”, “release density”, or “number released.” Thirty-nine studies of augmentative biological control were identified where a biological control agent was released at various densities and the effectiveness of the agent as a function of release rates was measured.

All studies included in the review were required to have used an appropriate experimental design (Collier and van Steenwyk 2004), which included: (1) treatment(s) in which natural enemies were released at two or more release rates in replicated experimental units and (2) a control treatment in which no natural enemies were released in other replicated experimental units. Studies were excluded from the review if they used an inappropriate experimental design. Studies were also excluded from the review if the natural enemy had no significant effect on the target pest at any release rate compared to the control treatment. Thus, in all studies included in the analysis, the augmentative biological control agent was effective in suppressing the target pest with at least one release rate.

Of the 39 reviewed studies, 31 met the conditions for inclusion. These studies analyzed the impact of 35 biological control agents on 42 pests with various release rates (Table 1). A separate analysis was done for cases in which parasitoids or predators were utilized as the biological control agent. In addition, the effect of release rates was examined when targeted pests were grouped by order. In this analysis, results are presented by pest order regardless of whether the biological control agent utilized was a parasitoid or predator.

The studies varied in the effects measured (Table 1). Despite this, the same approach was used for evaluating whether release rates significantly affected augmentative biological control across all studies. This approach was based on whether the author(s) indicated that pest populations or damage were suppressed below some specified target density or damage level, or that parasitism rates increased above a target level, in the highest density release treatments but not in lower density release treatments. A similar approach has been used in other review studies of augmentative biological control (Collier and van Steenwyk 2004).

Table 1a.

Impact of release rates of biological control agents on selected pests (Parasitoids)

Comparison of parasitoids and predators

Fifteen of the biological control agents (43%) were parasitoids. All of the parasitoids were from the order Hymenoptera (Table 1). Nineteen pests (45%) were targeted with parasitoids. Of the pests targeted with parasitoids, the most common order was Hemiptera (8 pests; 42%), followed by Diptera (6 pests; 32%), and Lepidoptera (5 pests; 26%) (Table 1).

Twenty of the biological control agents (57%) were predators. The order most commonly used as a predatory biological control agent was Acari (11 agents, 55%), followed by Hemiptera (6 agents, 30%), Coleoptera (2 agents, 10%) and Neuroptera (1 agent, 5%) (Table 1). Twenty-three pests (55%) were targeted with predators. The most common order targeted with predators was Acari (11 pests; 48%), followed by Hemiptera (7 pests, 30%), Coleoptera (2 pests, 9%), Thysanoptera (2 pests, 9%), and Lepidoptera (1 pest, 4%) (Table 1).

Impact of release rates relative to other factors

Several studies analyzed the impact of release rates compared to other factors that impacted the effectiveness of an augmentative biological control agent. Five studies compared the effects of release rates relative to the method and timing of augmentative biological control applications (Daane and Yokota 1997; McDougall and Mills 1997; Campbell and Lilley 1999; Clark et al. 2001; Jung et al. 2004). Two studies analyzed the impact of release rates when insecticides were used in conjunction with an augmentative biological control agent (Hough-Goldstein and Whalen 1993, van Driesche et al. 2001). For each study, the relative impact of release rates compared to these factors is discussed.

Table 1b.

Impact of release rates of biological control agents on selected pests (Predators)

Impact of release rate when parasitoids were used for biological control

Of the 19 pests targeted with a parasitoid, in 12 cases (63%), increasing the release rate of the parasitoid did not significantly affect the density or mortality of the pest or the rate of parasitism (Del Bene 1990; Peterson and Cawthra 1995; Hoddle et al. 1997, 1998; Fernandes et al. 1998; Hommay et al. 2002; Arnold and Sengonca 2003; Stansly et al. 2005; Bellows Jr. et al. 2006) (Table 1, Figure 1). All of the parasitoids used were Hymenoptera. Thus, release rates of Hymenopteran parasitoids did not significantly impact the effectiveness of biological control in 63% of studies reviewed (Table 1, Figure 2).

Unlike the studies involving predators described later, in the 7 cases where release rates did affect augmentative biological control with a parasitoid, the highest release rate tested in each study did not reduce pest densities to near O (Wen and Grower 1994; Peterson et al. 1995; Norton and Welter 1996; McDougall and Mills 1997; Hommay et al. 2002; Stansly et al. 2005; Li et al. 2006). Thus, in these studies, an optimal release rate threshold where further increases were unlikely to increase control could not be identified. This result may be attributable to the observation that reductions in pest abundance are typically much greater when predators are used as augmentative biological control agents compared to parasitoids (Stilingand Cornelissen 2005).

Figure 1.

Number of pests affected by release rate with either a predator or a parasitoid as the natural enemy utilized for augmentation biological control.

Impact of release rate when predators were used for biological control

Of the 23 pests targeted with a predator, in 15 cases (65%), increasing the release rate of the predator did not significantly affect the target pest density (Oatman et al. 1976; Krishnamoorthy and Mani 1989; Duso et al. 1991; Kawai 1995; Lambdin and Baker 1996; Daane and Yokota 1997; Grundy and Maelzer 2002; Arnold and Sengonca 2003; Jung et al. 2004; Opit et al. 2004; Liu and Stansly 2005; Alomar et al. 2006) (Table 1, Figure 1).

Of the 11 pests managed with predatory mites (Acari), in 7 cases release rates did not impact control of the pest (Oatman et al. 1976; Krishnamoorthy and Mani 1989; Duso et al. 1991; Fan and Petitt 1994; Campbell and Lilley 1999; Lesna et al. 2000; Nachman and Zemek 2003; Jung et al. 2004; Opit et al. 2004). Of the 7 pests managed with Hemipteran predators, 4 were not affected by release rate and 3 were affected by release rate (Lambdin and Baker 1986; Hough-Goldstein and Whalen 1993; Gabarra et al. 1995; Kawai 1995; Grundy and Maelzer 2002; Alomar et al. 2006) (Table 1, Figure 2). Two pests managed with a Coleopteran were affected by release rate and one was not (Arnold and Sengonca 2003; Liu and Stansly 2005) (Table 1, Figure 2). Both pests managed with Neuropterans were not affected by release rate (Daane and Yokota 1997).

In all 8 cases where release rates of predators affected augmentative biological control (Table 1), the author(s) demonstrated a threshold where further increases in release rates would not have improved control. For example, control of the sweetpotato whitefly, Bemisia tabaci (Gennadius), on hibiscus with the lady beetle, Nephaspis oculatus (Blatchley), with the highest release ratio of 1:4 beetle: whitefly resulted in no visible damage to any hibiscus plants, indicating that higher release rates would not have improved whitefly control. In another study, elimination of the bulb mite, Rhizoglyphus robini (Claparede), with releases of a predatory mite, Hypoaspis aculeifer (Canestrini), was possible with a release rate of 3:1 predators:prey in field and greenhouse experiments but not with lower rates (Lesna et al. 2000). Results in the other six cases were similar, as in each study the highest release rate tested reduced pest populations, or damage, to negligible levels, indicating that further increases would not have significantly increased control (Hough-Goldstein and Whalen 1993; Fan and Petitt 1994; Gabarra et al. 1995; Campbell and Lilley 1999; Lesna et al. 2000; Nachman and Zemek 2003).

Figure 2.

Number of pests affected by release rate with various orders of natural enemies utilized for augmentation biological control.

Impact of release rate on targeted pests

Hemiptera was the most common pest order, regardless of whether the biological control agent was a parasitoid or predator (15 pests, 36%), followed by Acari (11 pests, 26%), Diptera (6 pests, 14%), Lepidoptera (6 pests, 14%), Coleoptera (2 pests, 5%), and Thysanoptera (2 pests, 5%) (Table 1, Figure 3). The control of Hemipteran pests was not affected by release rate in 11 out of 15 cases. The control of mites (Acari) was not affected by release rate in 7 of 11 cases. In 5 out of 6 cases the control of Dipteran pests was not affected by release rate, while the control of Lepidopteran pests was not affected by release rate in only 2 out of 6 cases. In 1 out of 2 cases the control of either Coleopteran or Thysanopteran pests was not affected by release rate (Table 1, Figure 3).

Impact of release rate relative to the method and timing of application

In some cases, the release rate of a biological control agent had a relatively small impact on the control of a pest compared to the method and timing of application of the agent. The method of application primarily affects the ability of a biological control agent to establish in the field, which is necessary for long-term control of a pest. The timing of release of a biological control agent affects its synchrony with the host insect, which can improve the chances of success. Several studies compared the effects of release rates relative to the method and timing of biological control applications (Daane and Yokota 1997; McDougall and Mills 1997; Campbell and Lilley 1999; Clark et al. 2001; Jung et al. 2004).

Figures 3.

Number of pests affected by release rate with various orders of pest targeted for augmentation biological control. Pests were grouped by order regardless of the type of natural enemy (parasitoid or predator).

For example, augmentative biological control of two leafhopper pests, Erythroneura variabilis (Beamer) and E. elegantula (Osborn), in vineyards with green lacewings, Chrysoperla spp., was not affected by release rates but was affected by the method and timing of application (Daane and Yokota 1997). Releases of green lacewings at densities from 6,175 to 1,235,000 eggs or larvae per ha provided similar levels of control. However, releases that were timed to approximately 50-70% leaf hopper egg hatch had a greater effect on densities than releases timed to peak leaf hopper nymphal densities. In addition, releases of green lacewing larvae were more effective than releases of lacewing eggs (Daane and Yokota 1997). In another study, releases of the predatory mite Phytoseiulus persimilis (Athias-Henriot) early in the season to control the two-spotted spider mite, Tetranychus urticae (Koch), on dwarf hops maintained populations at lower densities than releases later in the year regardless of the release rate (Campbell and Lilley 1999).

In another study, when the predatory mite Amblyseius womersleyi (Schicha) was used to control populations of the two-spotted spider mite, Tetranychus urticae, initial settlement of the predatory mites, which was aided by multiple releases, was significantly more important than release rate in determining the effectiveness of control (Jung et al. 2004). For control of spotted knapweed, Centaurea maculosa (Lamarck), many, smaller releases of two herbivores, Cyphocleonus achates (Fahraeus) and Agapeta zoegana (L.), was more effective than fewer, larger releases, while release rate did not affect establishment of the herbivores (Clark et al. 2001). Similarly, many releases of the egg parasitoid Trichogramma spp. were necessary for control of the codling moth, Cydia pomonella (L.), regardless of release rate (McDougall and Mills 1997).

Impact of release rate with insecticide usage

Two studies reviewed analyzed the impact of release rates when insecticides were used in conjunction with a biological control agent. When the parasitoid Eretmocerus eremicus (Rose and Zolnerowich) was used in combination with the insect growth regulator buprofezin to control two whitefly species, Trialeurodes vaporariourum (Westwood) and Bemisia argentifolii (Bellows and Perring), lower release rates provided effective control when used in combination with insecticides (van Driesche et al. 2001). A release rate of one parasitoid per plant per week in combination with a mid-season application of buprofezin reduced pest densities as effectively as releasing 3 parasitoids per plant per week without insecticides or 2 parasitoids per plant per week with a mid-season application of buprofezin. The low release treatment with buprofezin cost an average of $0.38 per plant, while the other two treatments cost $1.18 and $0.75, respectively, indicating that using a low release rate along with insecticides was most beneficial from an economic perspective (van Driesche et al. 2001).

As mentioned earlier, the Colorado potato beetle was managed more effectively with high release rates of predatory stink bugs than low release rates (Hough-Goldstein and Whalen 1993). However, when a low release rate was combined with an application of the toxin Bacillus thuringiensis (Bt), the level of control achieved was not significantly different from that with a high release rate and was significantly greater than either a low release rate alone or Bt alone (Hough-Goldstein and Whalen 1993). The results of these two studies indicate that lower release rates of biological control agents are often as effective as higher rates when biological control is used in conjunction with insecticides.