1. Introduction
Tomato is an economically important crop in the Mediterranean region, in both protected and open field conditions. It is affected by several pests such as aphids (Hemiptera: Aphididae), leafminers (Diptera: Agromyzidae), mites (Acari: Tetranychidae and Eriophyidae), whiteflies (Hemiptera: Aleyrodidae), thrips (Thysanoptera), and with great importance, since its arrival in Spain in 2006, by
Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) [
1].
The biological control of pests has been used for a long time in tomato crops. For instance, the whitefly parasitoid
Encarsia formosa Gahan (Hymenoptera: Aphelinidae) is mass-produced and released since the 1920s [
2]. More recently, tomato crops have benefited from the use of other biological control agents (BCAs) like dicyphine mirids (Heteroptera: Miridae: Bryocorinae: Dicyphini). The wide use of dicyphines is due to the fact that some of these species are zoophytophagous, which allows them to endure periods of prey scarcity by feeding on host plants, and are particularly well adapted to plants with glandular trichomes like tomato [
3,
4].
In the Mediterranean region, several dicyphine species in the genera
Dicyphus,
Macrolophus, and
Nesidiocoris naturally occur on tomato crops [
5,
6,
7,
8], and their role against pests is widely recognized [
5,
9,
10,
11].
Nesidiocoris tenuis (Reuter) and
Macrolophus pygmaeus (Rambur) are currently mass-produced and commercialized for augmentation, whereas European
Dicyphus species provide biological control services, mostly through conservation strategies [
8,
12]. Despite their importance and broad use, currently commercialized dicyphines can present unfavorable aspects that limit their usefulness to biological control. Plant feeding by
Nesidiocoris tenuis, in particular, can cause severe damage to tomato [
13,
14,
15], while
M. pygmaeus may take a long time to establish plentiful populations on crops [
16,
17]. Because of these limitations, it is important to evaluate other European dicyphines as candidate BCAs of tomato pests, as is demonstrated by the increasing research interest in species of the genus
Dicyphus [
18,
19,
20,
21].
In Europe, the genus
Dicyphus has 14 known species. Among them,
Dicyphus cerastii Wagner is distributed along the Mediterranean region [
21,
22,
23] and, in Portugal, it is commonly found in low pesticide pressure tomato greenhouses [
24,
25,
26]. Like other dicyphines,
D. cerastii can feed on different prey species and has been observed preying upon pests like leafminers [
24], whiteflies [
25,
26],
T. absoluta [
25], mealybugs (Hemiptera: Pseudococcidae) and aphids [
27]. However, the extent to which this predator contributes to biological control on tomato crops is not fully understood, particularly on key indicators like prey preference, predation rate, numerical and functional response.
The functional response (FR) describes how the individual predation rate changes with increasing prey availability and it is a major element when it comes to assessing the predatory efficacy of a BCA [
28]. Three types of FR were proposed by Holling [
29]: type I describes a linear increase of prey consumption with increasing prey density until a maximum is reached, and is mostly associated to predators like filter feeders [
30]; type II expresses a negatively density-dependent relation in which the predation rate decreases with increasing prey density and is represented by a hyperbolic curve; in type III FR, a positively density-dependent is described, in which the predation rate first increases at lower prey density and then decreases at higher prey density resulting in a sigmoidal curve. Despite some records of type III FR [
31,
32,
33], dicyphine predators are more commonly reported to have type II FR [
19,
32,
33,
34,
35,
36,
37,
38].
Type II FR is associated with unstable predator–prey dynamics [
39,
40], since at lower densities, there is a risk of prey extinction, as predators are able to consume most prey available. Differently, at higher densities, predators may not consume enough prey, and thus provide limited control over prey populations. This unstable dynamic leads to associating predators exhibiting type II FR to inundative biological control programs for direct pest population reduction [
41], such as mirid releases in seasonal crops like tomato, rather than long-term biological control.
Functional response type and its parameters, attack rate (
a) and handling time (
h), are influenced by abiotic factors including spatial complexity [
42] and temperature [
33], and also biotic factors like the presence of alternative prey [
43], prey distribution [
44], and prey type and size [
45].
The aim of this study was to further understand the predation capacity of
D. cerastii, by evaluating the influence of prey size and mobility on the shape and magnitude of its FR, and on predation rate. In laboratory bioassays,
D. cerastii females were exposed to different densities of immobile
Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) nymphs and
T. absoluta eggs, given the economic importance of these species as pests [
46,
47]. We also evaluated predation on, also immobile,
Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) eggs, as these are widely used as factitious prey in mirid mass rearing. And finally, predation was also evaluated for a mobile prey species,
Myzus persicae (Sulzer) (Hemiptera: Aphididae).
3. Results
Prey mass was directly related to its body size.
Tuta absoluta eggs were the lightest prey (
Table 2) as, on average, each egg is 0.36 mm long and 0.22 mm in diameter [
60]. The remaining three prey species had similar masses (
Table 2) and body sizes, as
E. kuehniella eggs are 0.58 mm long and 0.33 mm wide [
61],
Bemisia tabaci 4th instar nymphs are approximately 0.63 mm long and 0.39 mm wide [
62], and
Myzus persicae 1st instar nymphs are 0.78 mm long and 0.33 mm wide [
63].
Dicyphus cerastii readily accepted all prey species tested. Average consumption increased with prey availability, reaching a maximum of 88.8
B. tabaci 4th instar nymphs at a density of 200, 134.4
E. kuehniella eggs at a density of 300, 37.3
M. persicae 1st instar nymphs at a density of 100 and 172.3
T. absoluta eggs at a density of 200 (
Table 3).
The results of fitting Rogers’ random predator equation (type II FR) and Hassel’s extension for type III are presented in
Table 4 and
Table 5, respectively. The type II FR model, showed lower AIC values than type III, for each of the four prey. However, ∆AIC values indicate that both models could describe the data well (
Table 6). The highest ∆AIC was found for
B. tabaci, and coherently, a plot of attack rate, as fitted by Hassel’s extension for type III FR, reveals that the attack rate of
B. tabaci quickly tends to the asymptote of the hyperbolic function (
b/
c) that is very close to the value of attack rate obtained from fitting Rogers’ random predator equation (
Figure 1,
Table 4). In the remaining prey species (with lower ∆AIC values), this tendency is also observed, although not as fast as in
B. tabaci (
Figure 1). Despite this, the logistic regression of the proportion of prey consumed derived significant linear coefficients for all prey species (
Table 7), which indicates that a type II FR should be preferable in our case, as presented in the fitted curves (
Figure 2).
Estimated parameters for the Rogers’ random equation (
Table 4) reveal that attack rate (
a) was highest on
T. absoluta eggs, followed by
B. tabaci nymphs,
E. kuehniella eggs and
M. persicae nymphs. Handling time (
h) was highest on
M. persicae nymphs, followed by
B. tabaci nymphs,
E. kuehniella eggs, and
T. absoluta eggs. From handling time, it was possible to calculate the theoretical maximum predation rate (1/
h), which was 225.9
T. absoluta eggs day
−1, 165.1
E. kuehniella eggs day
−1, 104.2
B. tabaci nymphs day
−1, and 47.1
M. persicae nymphs day
−1. Fitting Hassel’s extension for type III also resulted in similar estimates for handling time compared to type II (
Table 5).
According to the bootstrapped 95% CIs of parameter estimates (
Figure 3,
Table S1), attack rate did not differ between
B. tabaci nymphs,
E. kuehniella eggs and
T. absoluta eggs as the correspondent 95% CIs overlapped, and the attack rate of
M. persicae only overlapped with that of
E. kuehniella. Handling times were different for all prey, except for
E. kuehniella and
T. absoluta eggs, in which 95% CIs briefly meet at 0.005 (
Table S1).
4. Discussion
In this study,
D. cerastii females were offered prey belonging to different species, with different characteristics of mobility and size. We found that
D. cerastii females were able to consume the majority of prey individuals at lower densities, but predation rate decelerated as prey density increased, thus showing a type II FR for all prey tested (
Table 3,
Figure 2). This agrees with previous records of other dicyphine species feeding on
E. kuehniella [
34] and
T. absoluta eggs [
19,
32,
34], whitefly nymphs [
33,
35,
36] and aphids [
37,
38,
64].
The FR parameters, attack rate (
a) and handling time (
h), determine the slope and the height of the FR curve, respectively [
65]. The attack rate is a measure of a predator’s efficiency in finding prey at low prey densities, whereas handling time is defined as the time a predator stops searching for prey after a capture [
65]. In this study, prey mobility greater than prey size may have negatively affected the predator’s efficiency at lower densities, as the mobile prey,
M. persicae, had the lowest attack rate and the values of 95% CIs for attack rate overlapped among immobile prey (
E. kuehniella,
B. tabaci,
T. absoluta). However, mobility could not explain why the attack rate of
M. persicae marginally overlapped with that of
E. kuehniella (
Figure 3,
Table S1).
Prey size influences handling time, as bigger prey may require more time for manipulation and feeding [
66]. Accordingly, the lowest handling time was found on the smaller prey that we offered;
T. absoluta eggs. Despite the size difference, and similarly to attack rate, the 95% CI estimate for handling time of
T. absoluta overlapped with that of
E. kuehniella (
Figure 3,
Table S1). However, even when 95% CIs of parameter estimates overlap, parameters may still combine to produce differences in predicted consumption as a function of prey density [
52], as was observed for these two prey species (
Figure S1). Handling time was different among
B. tabaci,
E. kuehniella and
M. persicae, despite their similarities in size and mass (
Figure 3,
Table S1). Other factors, besides prey size, can determine the feeding capacity of a predator [
45] and, in our case, prey mobility could also explain the lower predation found on
M. persicae nymphs. We observed that, at higher densities, aphid dispersion in the arenas at the end of experiments was also higher. Even though the majority was found on the leaflets, consumed aphids were found dispersed throughout the arena at higher densities. In this case, predators could have spent more time searching for prey compared to the other immobile prey offered. In addition to mobility, aphids can also present defense behaviors, such as exudate secretion, which can affect predation [
67] and thus increase predator handling time. Despite both being immobile and size equivalent,
E. kuehniella eggs and
B. tabaci nymphs also had different maximum predation rates. This could be due to other prey features, that may limit predatory capacity, such as integument hardness [
45], nutritional content [
68] and prey digestion [
69].
Type II FR is associated with unstable predator–prey dynamics [
39,
40]. However, in more natural setups, predators displaying type II FR may be under the pressure of stabilizing elements such as temperature [
33], host plant species [
70], presence of alternative prey [
43], prey distribution [
44], prey species [
71], prey size [
72], and spatial complexity [
42], which may approach their predatory activity to a type III FR. In our case, although the combination of the AIC method with the logistic regression indicated that a type II FR model was preferable, the low ∆AIC between type II and III models suggests that the type III model could also fit well, particularly on
E. kuehniella,
M. persicae and
T. absoluta (
Table 6,
Figure 1). This could mean that our setup may have hampered a more evident distinction between type II and III FR, as either due to its small size, or low spatial complexity, predators could have found prey unrealistically easily, particularly at low densities.
Although traditionally more associated to vertebrates, type III FR has been increasingly reported for arthropods including dicyphine predators such as
M. pygmaeus and
N. tenuis [
31],
Engytatus varians (Distant) and
Macrolophus basicornis (Stål) females [
32] feeding on
T. absoluta eggs, and for
N. tenuis on
B. tabaci [
33]. In more natural setups, like greenhouse crops, it is possible that the presence of stabilizing elements could drive dicyphine predators to display an FR closer to type III. One of such effects could be that, in more complex habitats, predation may be reduced at lower prey densities, since the ability to find prey can be affected by the availability of refuge [
42,
73]. Additionally, horticultural crops are often colonized by different arthropods that may be preyed upon by dicyphine predators. This abundance of alternative prey can also stabilize predator–prey systems if the predator is able to switch between available prey [
43], which dicyphines, in general, do [
74]. Furthermore, glandular trichome bearing plants, such as tomato, provide abundant entrapped arthropod cadavers, which may serve as lower effort prey on which these predators also feed [
24]. Phytophagy may also stabilize predator-prey dynamics by helping dicyphines avoid bottom-up effects of prey scarcity. However, this effect may be more important on adult predators, as plant feeding alone may affect immature development in some species [
75,
76]. Additionally, some plant resources may reduce excessive top-down effects on prey and stabilize predator-prey dynamics, as was demonstrated for
M. pygmaeus, which reduced its predation rate on
M. persicae when eggplant flowers or pollen were available [
77].
Functional response outcomes, although important in understanding predator–prey dynamics, can be difficult to interpret, and mostly give a theoretical contribution to the assessment of the biological control potential of a natural enemy. Predation rate, however, provides a concrete measure of the feeding capacity of a BCA and allows for direct comparison with other predators.
We observed that, for all prey,
Dicyphus cerastii females were highly voracious and predation often started when Petri dishes were still being sealed.
Dicyphus cerastii was able to consume an average of 172.3
T. absoluta eggs, and we estimated a maximum predation rate of 225.9 eggs day
−1. These are higher values than those found for similar sized dicyphines like
D. bolivari and
D. errans that can feed on more than 130
T. absoluta eggs when exposed to 350 prey items, but have estimated maximum predation rates of 188.52 and 197.24 eggs for
D. bolivari and
D. errans females, respectively [
19].
Dicyphus cerastii also showed higher predation than
M. pygmaeus and
N. tenuis which are reported to consume approximately 50
T. absoluta eggs daily [
78]. Our results also indicate higher predation compared to neotropical mirid species:
Tupiocoris cucurbitaceus (Spinola) could prey on an average of 147.45 eggs day
−1 [
79];
Campyloneuropsis infumatus (Carvalho),
Engytatus varians (Distant) and
Macrolophus basicornis (Stål) females consumed an average of 51.0, 91.1 and 100.8
T. absoluta eggs, respectively, although these experiments were carried on tomato seedlings [
32].
Predation on aphids was also high, as
D. cerastii females could consume up to an average of 37.3
M. persicae 1st instar nymphs day
−1 whereas
D. tamaninii and
M. pygmaeus can feed on 22.8 and 21.7 1st instar nymphs of
M. persicae, respectively [
80].
D. cerastii also surpassed
T. cucurbitaceus that preyed on 19.75
M. persicae nymphs [
79], although these authors used mixed nymphal instars of the aphid (1st−3rd). In another study,
D. maroccanus (
syn.
D. bolivari [
21]) and
N. tenuis females only preyed on approximately 15 1st instar nymphs of
M. persicae, whereas
M. pygmaeus fed on roughly 10 [
81]; however, only 20 aphids were offered initially. Despite the previously mentioned lower predation rates, in experiments with different aphid species,
D. tamaninii females fed on 46.2 young nymphs of
Aphis gossypii Glover on cucumber, and 43.6
Macrosiphum euphorbiae (Thomas) on tomato [
38], which suggests that predation rate may depend on aphid species.
Dicyphus cerastii females could prey upon an average of 88.8
B. tabaci 4th instar nymphs when 200 individuals were offered, which is a much higher predation rate than that found for most other dicyphine species.
Dicyphus tamaninii is reported to prey on an average of 12
B. tabaci 4th instar nymphs day
−1, whereas
M. pygmaeus could consume 5 [
82].
Tupiocoris cucurbitaceus females can prey on an average of 38.2
B. tabaci nymphs (3rd–4th instar) [
79], and
N. tenuis on up to 42.1 and 45.1
B. tabaci 4th instars day
−1 at 25 °C and 35°C respectively [
33]. Finally, in a study with the greenhouse whitefly
Trialeurodes vaporariorum (Westwood),
D. errans females were reported to have an estimated maximum predation rate of 114 4th instar prey [
36], which is similar to what we found here for
D. cerastii on
B. tabaci (104.2 nymphs day
−1).
The predation rate on
E. kuehniella eggs is important for mass rearing dicyphine predators, as it is widely used as factitious prey. We found that
D. cerastii females fed on an average of 134.4
E. kuehniella eggs which agrees with the predation rate previously reported for
D. hesperus that can consume approximately 139
E. kuehniella eggs in 24h [
83]. A lower predation has been reported for
N. tenuis, which is able to consume 58
E. kuehniella eggs day
−1 [
84].
As mentioned above, our setup may have been too simple, as Petri dishes represent very simplified versions of what predators may encounter in nature. In the future, FR should be evaluated under more complex arenas, with multiple prey in order to assess the effect of stabilizing elements like spatial complexity and prey switching.
Besides FR and predation rate, there are other factors that may be important to address in future research regarding
D. cerastii. Among these, the numerical response, or how the predator population changes with prey density [
28], is of major importance to fully understand the potential of
D. cerastii to regulate pests. In the case of type II predators, population size is crucial for the success of biological control, particularly at higher prey densities, when individual predation capacity may be limited. Moreover, numerical response is influenced by biological parameters that drive population dynamics such as reproductive and developmental thermal thresholds, and further information about these parameters is required for
D. cerastii. Prey suitability also impacts predator populations, since different prey may have distinct impact on predator performance, as shown for
M. pygmaeus [
85,
86] and
N. tenuis [
86] females that have lower fertility when feeding on
T. absoluta eggs compared to those of
E. kuehniella. Determining prey preference is also essential in the case of generalist predators like dicyphines, which may disperse their predation through prey switching behavior [
74]. Therefore, an insight into predator preferences may help to understand and predict the efficacy of
D. cerastii in multiple prey situations. Dicyphines can also present cannibalistic and intraguild predatory interactions [
87,
88]. Both these direct, and indirect interactions such as competitive displacement [
31], may also affect the success of biological control programs with these predators. Finally, it is important to note that despite their services as BCAs, dicyphines can also damage crops through plant feeding [
13,
14,
15], and the severity of plant damage is related to mirid species [
14]. So far, the phytophagy of
D. cerastii has been studied on tomato plantlets, on which it produced necrotic spots on leaves [
25]. Therefore, a larger assessment on the impact of phytophagy of this species should also be considered in the future.