Biological control is important in Integrated Pest Management (IPM) in different crops in Brazil (Pires et al. 2011; Soares et al. 2011, 2012; Bueno et al. 2012). Parasitoids of the family Trichogrammatidae can suppress populations of agricultural and forest pests, and reduce the excessive use of insecticides (Soares et al. 2007; Oliveira et al. 2011; Goulart et al. 2012). Development of large scale production and release technologies for these natural enemies is necessary (van Lenteren 2012). Improved mass rearing techniques can provide parasitoids at times of high demand with reduced costs. Trichogrammatidae demand may be low when there are surplus of host (rearing) eggs (Lohmann & Santos 2007). By contrast, shortage in host eggs during periods of high demand can hinder biological control programs and require the use of insecticides for pest control. Storage at low temperatures can increase the lifetime of parasitoids and hosts. This technique allows synchronizing releases of natural enemies with pest outbreaks (Pitcher et al. 2002; Colinet & Boivin 2011).

Eggs of Anagasta kuehniella (Zeller) (Lepidoptera: Pyralidae) may be kept in a refrigerator for up to 30 days without losing their viability as hosts for Trichogrammatidae (Pratissoli et al. 2003). Parasitism rates of stored Trichogramma ostriniae (Pang & Chen) (Hymenoptera: Trichogrammatidae) were similar to controls after 2 to 4 weeks' storage at 9–12 °C but declined with storage periods longer than 4 weeks (Pitcher et al. 2002). However, parasitoids reared on refrigerated eggs may have reduced performance because long exposure to low temperatures reduces host quality (Flanders 1938; Kostal et al. 2004, 2006). Eggs of Riptortus pedestris (F.) (Hemiptera: Alydidae) maintained at 2 °C were less parasitized (34%) by Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae) than eggs kept under natural conditions (62%) (Alim & Lim 2011). Storage at 4 ± 1 °C reduced fecundity and longevity of Trichogramma brassicae Bezd. (= T. maidis n. sp.), Trichogramma cacoeciae Marchal and Trichogramma evanescens Westwood (Hymenoptera: Trichogrammatidae) (Özder 2008). Studies to determine ideal temperatures and storage times of host eggs for parasitoid rearing are needed.

The objectives of this study were to determine the emergence and parasitism potentials of 10 Trichogrammatidae species reared in eggs of A. kueniella stored at 5 °C for different time periods.

Material and Methods

Results

Parasitism by all Trichogrammatidae species decreased as the storage periods increased (Fig. 1). Trichogrammatoidea annulata, T. acacioi, T. brasiliensis, T. bruni, T. pretiosum, and T. soaresi parasitized eggs stored up to 24 days; T. atopovilia parasitized eggs stored up to 16 days, T. demoraesi and T. bennetti parasitized eggs stored up to 15 days, and T. galloi eggs stored up to 13 days (Fig. 1).

The storage period for maximum parasitoid emergence was 23 days for T. acacioi, T. annulata, T. brasiliensis, T. bruni, and T. pretiosum, 15 days for T. atopovilia, T. benneti, T. demoraesi, and T. soaresi, and 13 days for T. galloi (Fig. 3). The highest number of T. acacioi, T. annulata, T. atopovilia, T. bennetti, T. demoraesi, T. galloi, T. pretiosum, and T. soaresi adults emerged from A. kuehniella eggs stored up to 23 days; The highest number of T. brasiliensis and T. bruni adults emerged from A. kuehniella eggs stored up to 17 days (Fig. 2).

Trichogramma galloi showed the highest parasitism and adult emergence rates per card with eggs stored for 5 days, followed by T. acacioi. The lowest parasitism and adult emergence rates were observed in T. soaresi and T. benneti on eggs stored for < 1 day, T. pretiosum on eggs stored for 5 days, and T. brasiliensis and T. bruni on eggs stored for 10 days (Table 1). The number of emerged parasitoids per host egg was similar between treatments with an average of 1.0006 ± 0.0002.

Discussion

The success of biological control programs with egg parasitoids depends on the ability to produce these natural enemies in high numbers when pest problems occur. Storing host eggs for different time periods can help synchronize parasitoid production with demand.

The higher parasitism by T. acacioi, T. annulata, T. brasiliensis, T. bruni, T. pretiosum, and T. soaresi on eggs of A. kuehniella stored for longer periods may be related to factors such as these species having: 1) stronger ovipositors than the other parasitoids tested, 2) the ability to recognize chemical signals more efficiently, and/or 3) higher tolerance to changes in physical stimuli such as color, size and shape of chilled eggs (Stoepler et al. 2011). Except T. soaresi, the above species also showed high adult emergence from host stored for longer periods. The “exposure dose to cold” is a parameter use to describe a combination of exposure time and temperature. A decrease in temperature and/or an increase in exposure time can result in cumulative and irreversible chilling injuries (Colinet & Hance 2010). Several authors agree that survival of parasitoids decreases when host storage periods increase (Flanders 1938; Ayvaz et al. 2008; Chen et al. 2008; Colinet & Hance 2010; Abd El-Gawad et al. 2010).

The highest parasitism and adult emergence rates were observed in T. galloi parasitizing eggs stored for 5 days. Longer storage times may result in decreased nutritional quality for this parasitoid's embryos by deterioration of the yolk (Pratissoli et al. 2003). Parasitoid females can reject eggs with altered chemical and physical stimuli (Soares et al. 2009; Goubault et al. 2011; Penaflor et al. 2011; Stoepler et al. 2011) such as eggs that contain cryoprotectants (glycerol, alanine) to resist cooling (Rivers et al. 2000). Storage can also modify the egg shape and affect the host recognition by the parasitoids (Conti et al. 1996). Parasitism of Gonatocerus ashmeadi Girault (Hymenoptera: Mymaridae) on Coagulata homalodisca (Say) (Hemiptera: Cicadellidae) eggs was reduced when cold storage period increased (Chen & Leopold 2007). Emergence of G. ashmeadi from C. homalodisca eggs stored for 70 days was reduced by 48%, fecundity by 53%, female production by 19%, the development time was extended, and female longevity shortened, compared to parasitoids reared on non-stored hosts (Chen & Leopold 2007). Thus, storage of host eggs for long periods could lead to unfavorable conditions for parasitoids and only those most adapted can reproduce.

Emergence of Trichogrammatidae parasitoids per parasitized egg in our experiments (1.0006 ± 0.0002) was low compared with reports by Andrade et al. (2011) [1.5 T. pretiosum individuals per egg of Heliothis virescens (F.) (Lepidoptera: Noctuidae)], but similar to reports of Trichogramma maxacalii Vogelé & Pointel reared also on H. virescens (Oliveira et al. 2000; Soares et al. 2007). Larger eggs are the most important factor involved in allowing the emergence of more individuals per host (Andrade et al. 2011).

Trichogrammatidae species differ in their adaptability to use chilled hosts, and this fact should be considered when using this technology to mass rear parasitoids for biological control programs.

Fig. 1.

Percentages of Anagasta kuehniella (Lepidoptera: Pyralidae) eggs parasitized by each of 10 Trichogrammatidae species as function of the number of days that these eggs had been stored at 5 °C before they were parasitized. Each point on the graph represents 20 replications.

Fig. 2.

Percentages of adults of each of 10 Trichogrammatidae species that emerged from parasitized Anagasta kuehniella eggs as function of the number of days these eggs had been stored at 5 °C before they were parasitized. Each point on the graph represents 20 replications.

Fig. 3.

Number of adults of each of 10 Trichogrammatidae species that emerged per card of 40.70 ± 0.03 Anagasta kuehniella eggs as function of the number of days that these eggs had been stored at 5 °C before they were parasitized. Each point on the graph represents 20 replications.

Table 1.

Highest percentages of parasitized eggs and number of emerged adults of Trichogrammatoidea annulata and nine species of trichogramma (Hymenoptera: Trichogrammatidae) per card of Anagasta kuehniella (Lepidoptera: Pyralidae) eggs, as a function of storage period at Montes Claros, Minas Gerais State, Brazil.