Abstract
Chrysoperla externa (Neuroptera: Chrysopidae) is one of the natural enemies of the corn leaf aphid Rhopalosiphum maidis (Hemiptera: Aphididae) and has potential application in the biological control of that pest. The effect of temperature on the development and the predatory capacity of this chrysopid fed with R. maidis nymphs was investigated. Fresh eggs of C. externa were maintained at 15, 20, 25 or 30 °C under 70% relative humidity and 12 h photophase, and the duration of the embryonic stage, as well as the duration and viability of the larval (first, second and third instars), prepupal, pupal and adult forms of the predator were evaluated. The duration of each of these stages decreased with increasing temperature, whilst the viabilities of all forms attained 100% at 20 and 25 °C. The threshold temperature and the value of the thermal constant K obtained for the complete life cycle (egg to adult) were, respectively, 10.7 °C and 377.8 degree-days. Independent of temperature, the consumption of R. maidis nymphs by C. externa increased as the larvae reached maturity. At 20 and 25 °C the average number of aphids consumed during the complete larval stage was maximal at approximately 350 specimens. It is concluded, therefore, that the development of the immature forms of C. externa fed on R. maidis as well as its viability and predatory capacity, were favored at temperatures between 20 and 25 °C.
Introduction
The corn leaf aphid Rhopalosiphum maidis (Hemiptera: Aphididae) is regarded as a specific pest of the Poaceae since it has been found to infest more than 30 genera belonging to that family (McColloch 1921; Robinson 1992; Kuo et al. 2006; Razmjou and Golizadeh 2010). Moreover, in Brazil, R. maidis is distributed mainly in regions where sorghum and maize ("safrinha" type) are cultivated (Fonseca et al. 2004). In maize, infestation begins in isolated plants but spreads to the whole crop during the vegetative phase, particularly through tassel emergence, at which stage the value of the cul-ture may not be reduced significantly (Gassen 1996). In con-trast, considerable economic loss can result when infestation occurs prior to inflorescence, especially if it is associated with water stress or with the period of fertilization and grain filling (Everly 1960; Brodbeck and Strong 1987; Honek 1991; Al-Eryan and El-Tabbakh 2004; Kuo et al. 2006). Additionally, transmission of viruses by the aphid may inflict considerable indirect damage to the culture (Farrell and Stufkens 1992; Huggett et al. 1999; Almeida et al. 2001; SO et al. 2010).
The continuous use of broad spectrum insecticides in the control of aphids has intensified the occurrence of these insect pests and the emergence of others (Gahukar 1993; Oliveira et al. 2012). In order to minimize the environmen-tal impact of such chemicals, alternative control measures have been employed including biological control. Amongst the natural enemies of R. maidis are larvae of the green lacewing Chrysoperla externa (Hagen, 1861) (Neuroptera: Chrysopidae) (Alburquerque et al. 1994; Tauber et al. 2000; Fonseca et al. 2001; Maia et al. 2004; Barbosa et al. 2008; Oliveira et al. 2012). Although such larvae appear to be very effective, information regarding the population dynamics of the predator is very limited and this constitutes an impedi-ment to its practical application in biological control. Indeed, many control programs crucially depend on an understanding of the biology and physical requirements (mainly tempera-ture) of the predators (Cividanes 2000).
In view of the relevance of C. externa in the biological control of R. maidis, the objective of the present study was to determine the effect of different temperatures on the develop-ment and predatory rate of these chrysopids when fed with their natural prey.
Materials and methods
Laboratory-reared C. externa adults (generation F4) were maintained in glass jars (20 cm high x 20 cm diameter) at 25 ± 2 °C, 70 ± 10% relative humidity and 12 h photophase, and fed with a mixture of beer yeast and honey (1x1 w/w). R. maidis aphids were reared on leaf segments obtained from sorghum plants (cultivar BRS303; 80 cm height) placed in plastic recipients (50 mL) containing 25 mL of water and fixed with acrylic.
The effect of temperature on the development of C. ex-terna was studied by placing 15 freshly oviposited eggs into glass dishes (8.5 cm high x 2.5 cm diameter) and incubating at different temperatures (15, 20, 25 and 30 ± 1 °C) under the conditions previously described. The duration of the embry-onic stage, as well as the durations and viabilities of the lar-val (first, second and third instars), prepupal, pupal and adult forms of the chrysopid were evaluated. The predatory capac-ity of C. externa was investigated by feeding first, second and third instars with an amount of the prey (in the form of 15, 30 and 120 third and fourth instars of aphid nymphs, respec-tively) that had been previously determined to be larger than the daily requirements of the individual lacewing instars. Each day, any aphid nymphs that remained in the dishes were removed and replaced with new individuals.
Data were submitted to analysis of variance and polyno-mial regression with the aid of SAEG 8.0 (Ribeiro Jr. 2001) and Windows Excel 98 software. The lowest temperature of development, or base temperature (bT), for the embryonic, larval and all of the juvenile stages of the predator were calculated using the method of Haddad et al. (1999) based on the duration/time hyperbola and its reciprocal. The ther-mal requirements of C. externa were determined using the equation K = D (T-Tb), in which K is the thermal constant (expressed in degree-days), D is the time taken (days) to complete development, and T is the temperature at which de-velopment occurred (Wigglesworth 1972).
Results and discussion
The duration of the embryonic stage was 15.1 days at 15 °C, whereas at higher temperatures this stadium was consider-ably reduced and lasted for only 3.0 days at 30 °C. The varia-tion of the duration of the embryonic stage with increasing temperatures is shown in Figure 1 together with the fitted sec-ond-degree equation. Similar responses have been reported by other researchers (Maia et al. 2000; Fonseca et al. 2001; Figueira et al. 2002), who observed longer embryonic stages for C. externa at lower temperatures.
The rates of development of the individual instars and, consequently, the total larval stage of C. externa were also highly influenced by increasing temperatures (Figs. 2A - D). Thus, in the interval between 15 and 20 °C, the duration of the larval stage diminished by 18.4 days, whilst between 25 and 30 °C, the duration was reduced by 2.4 days (Fig. 2D). These results corroborated previous findings (Maia et al. 2000; Fonseca et al. 2001; Figueira et al. 2002; Oliveira et al. 2010; Lavagnini and Freitas 2012) concerning the sensitivity of C. externa instars to low temperatures. The viabilities of all instars were 100% at 20 and 25 °C (Table 1), but were reduced to approximately 93% at 15 and 30 °C. Fonseca et al. (2001) also observed higher mortalities of C. externa when immature stages were maintained at 15 and 30 °C. High temperatures may cause protein denaturation or metabolic imbalance due to the accumulation of toxins, whereas low temperatures can induce the formation of crys-tals in the body of the predator leading to difficulties in lo-comotion and, ultimately, death (Chapman 1998). However, according to Campbell et al. (1974), the deleterious effects of extreme temperatures only occur when such conditions are applied constantly.
The lengths of the prepupal and pupal stages of C. externa were also reduced as the temperature increased (Figs. 2E, F). However, the prepupal stage was less sensitive to tempera-ture variation in comparison with the late instar form, since viability remained at 100% between 15 and 25 °C and only diminished to 98.2% at 30 °C (Table 1). With respect to pu-pae, viability was in the range 92.3 and 100% between 20 and 30 °C, but decreased remarkably to 41.7% at 15 °C. The sen-sitivity of pupae to low temperatures may be associated with the physiological problems described by Chapman (1998).
Duration of the embryonic stage of Chrysoperla externa shown as a function of temperature (70 ± 10% relative humidity, 12 h photophase).
Viabilities of individual instars, total larvae, prepupae and pupae of Chrysoperla externa fed on Rhopalosiphum maidis at different temperatures (70 ± 10% relative humidity, 12 h photophase).
Duration on the first (A), second (B) and third (C) instar; larval (D), prepupal (E) and pupal (F) stages of Chrysoperla externa fed on Rhopalosiphum maidis shown as a function of temperature (70±10% relative humidity, 12 h photophase).
The complete biological cycle of C. externa from egg to adult ranged from 92.8 days at 15 °C to 20.1 days at 30 °C, representing an overall reduction of almost 73 days (Fig. 3).
It is clear that the lacewing is particularly sensitive to temperatures between 15 and 20 °C, whilst between 25 and 30 °C the rate of development tended to be more stable (Fig. 3). The overall viabilities of C. externa during the biological cycle (egg to adult) were 41.7% at 15 °C, 100% at 20 and 25 °C, and 85.7% at 30 °C. The poor viability observed at the lowest temperature may be attributed to the high mortality observed during the pupal stage and to the reduced capacity for wing growth during the pharate stage). Indeed, the viabil-ity of C. externa was 50% higher at 30 °C compared with 15 °C, although the most appropriate conditions for the develop-ment of this predator would be 20 or 25 °C.
The predatory capacity of C. externa depended on the phase of development and the temperature (Fig. 4). At 25 °C, the daily consumption of R. maidis nymphs by third instar predators (78.4) was 18-fold higher than that of their first instar counterparts (4.4). This finding is in agreement with that of Fonseca et al. (2001) who observed that as C. externa developed (at 24 °C) the consumption of Schizaphis grami-num increased 22 fold. Figure 4 also reveals that the daily consumption of nymphs typically increased with increasing temperature. Thus, in the interval 15-30 °C, there was a pro-gressive increase in the number of prey consumed by C. ex-terna with respect to the total larval stage, such that, the daily consumption at 30°C was 4 times greater than at 15 °C. First instar predators, however, did not follow the general trend but rather showed a tendency to consume fewer nymphs at 30°C than at 25 °C.
With respect to the goodness of fit of the regression lines shown in Fig. 4, Fonseca et al. (2001) also observed that the variation in the determination coefficient (rβ) increased pro-gressively from the first to third instar as a function of devel-opment (Figs. 4A-C). Although the value of rβ for the first instar was only 0.74, when the three instars were analyzed in conjunction (as larval stage) the value of rβ was 0.99 (Fig. 4D). On this basis it is predicted that, under field conditions, the predatory capacity of C. externa might be favored by higher temperatures. Knowledge regarding the effect of tem-perature on the consumption of prey by C. externa can also contribute to the management of laboratory-reared predators and therefore, to the increased effectiveness of biological control programs, particularly when the availability of prey is limited.
The total numbers of prey consumed by the individual instars of C. externa were highly influenced by temperature (Table 2). For example, the maximum consumption of the first instar (18.1 nymphs in total) was observed at 15 °C, and this was probably due to the lengthy duration of this stage under such conditions, i.e. the larvae were fed for a longer period. Such a hypothesis does not explain the unexpectedly large reduction in consumption observed at 20 °C, however. With respect to the second instar, highest consumption (42 - 44 nymphs) occurred at 15 and 30 °C. Whilst the explanation given previously for the first instar at 15 °C applies here, the high consumption of nymphs by the second instar at 30 °C can be justified only by the intensification of metabolism and predatory activity, since the duration of this stage was much shorter at the higher temperature (Table 2). The highest num-ber of prey (301-302 nymphs) consumed by the third instar of C. externa was at 20 and 25 °C.
The overall consumption of prey by the larval stage as a whole was essentially determined by the predatory capacity of the third instar, which was typically 20 times greater than that of the first and approximately seven times greater than that of the second instar. Whilst the total consumption of prey by the first instar at 20 °C was half of that observed at 25 °C, the difference was compensated by the increased consump-tion of the second instar at 20 °C. Additionally, there were no differences in the numbers of prey consumed by the third instar at 20 and 25 °C. Thus, the highest consumption of prey by the larval stage (348 - 351 nymphs) occurred at 20 and 25 °C, demonstrating that such conditions would the most favor-able for biological control of R. maidis by the predator.
The thermal requirements of C. externa, as represented by Tb and K values determined in the present study, varied according to the developmental stage of the predator (Table 3; Fig. 5), and were similar to those reported by Maia et al. (2000) for C. externa fed on S. graminum. The rate of devel-opment of C. externa was affected by temperature, with the duration of the immature stages being significantly reduced at increasing temperatures (Fig. 5). C. externa was particular-ly sensitive to temperature changes between 15-20 °C, since an incremental temperature increase in this range had a much greater effect on the duration of the immature stages than a similar variation at 25 °C or above. The determination coeffi-cients of the regression equations were large at all stages, and varied from 98.8 to 99.7% demonstrating that the duration of development of these chrysopids is intimately correlated with the variation in temperature (Fig. 5).
In conclusion, this study has demonstrated that the most favorable temperatures for the development of the immature forms of C. externa are 20 and 25 °C, conditions in which the consumption of R. maidis is also at a maximum. The benefi-cial effect of higher temperatures on immature forms of C. externa was confirmed by the bT and K values, which varied according to the development stages of the predator.
Duration of the biological cycle (egg to adult) of Chrysoperla externa fed on Rhopalosiphum maidis shown as a function of tempera-ture (70 ± 10% relative humidity, 12 h photophase).
Total numbers of Rhopalosiphum maidis consumed by individual instars and during the whole larval stage of Chrysoperla externa at different temperatures (70 ± 10% relative humidity, 12 h photophase).
Daily consumption of Rhopalosiphum maidis by the first (A), second (B) and third (C) instars, and larval stage (D) of Chrysoperla externa shown as a function of temperature (70 ± 10% relative humidity, 12 h photophase).
Threshold temperature (Tb), and thermal constant (k) of Chrysoperla externa fed on Rhopalosiphum maidis (70±10% relative humidity, 12 h photophase).
Relation between temperature, duration of development (days) and development rate (1/duration) for the embryonic (A), larval (B) and juvenile (C) stages of Chrysoperla externa fed on Rhopalosiphum maidis shown as a function of temperature (70 ± 10% relative humidity, 12 h photophase).