S. Ya. Reznik

The influence of previous host age on current host acceptance in Trichogramma

Trichogramma principium Sug. & Sor. females were sequentially offered two portions of the grain moth (Sitotroga cerealella Oliv.) eggs, either young (1‐day old) or old (eggs that had developed 6 days at a temperature of 20 °C). The probability of host acceptance depended not only on current host age, but also on the age of the previously offered host. Particularly, Trichogramma females more often oviposited in old host eggs when previously offered young eggs (35–45% of Trichogramma females laid eggs) compared to females which were sequentially offered two portions of old eggs (15–20% of Trichogramma females laid eggs). In other words, parasitization by Trichogramma was stable even when transferred from young (preferred) to old (usually rejected) eggs. Dissections showed that refusing females had significantly more mature eggs than ovipositing females, independent of host age. Among ovipositing females, wasps provided with young hosts had fewer mature ovarial eggs than wasps provided with old hosts. Supposedly, Trichogramma females offered old hosts require a higher motivation to oviposit and have a correspondingly higher egg load than females offered young (preferred) hosts.

Diapause induction in Trichogramma embryophagum Htg. (Hym., Trichogrammatidae): the dynamics of thermosensitivity

It is known that the low temperature is the most important factor inducing the pre‐pupal diapause in Trichogramma species. The position of the thermosensitive period over the life cycle and temporal variation of the degree of responsiveness were investigated in T. embryophagum Htg. by transferring pre‐imaginal stages between ‘neutral’ temperature of 15°C and ‘diapause‐inducing’ temperature of 10°C. Our experiments showed that 6 days long exposure at 10°C significantly increased the percentage of diapausing pre‐pupae when started during rather large part of development: from embryo up to early pre‐pupa. The highest thermosensitivity was recorded during the embryo and the larval stages, with some decrease during the hatching period. Treatments with shorter cold exposures (2–3 days) gave similar results. Even 24 h long exposure at 10°C increased the percentage of diapausing pre‐pupae when applied during egg or early larval stage. Being started at the same stage of development, longer cold exposures caused stronger increase in the percentage of diapausing individuals. The experiments did not reveal any significant daily changes in thermosensitivity: at 12 : 12 h light : dark, larvae subjected to the low temperature during six photophases showed practically the same percentage of diapausing individuals as those subjected to the low temperature during six scotophases, and as those subjected to the 3 days long uninterrupted cold exposure. Hence, in natural conditions even occasional short‐term cold periods could be accumulated.

Host population density influence on host acceptance in Trichogramma

In 7 Trichogramma species (Hymenoptera, Trichogrammatidae) the percentage of females, parasitizing at least one host, directly depends on the number of host eggs. Trichogramma females that refused Sitotroga eggs will still oviposit in natural hosts. These refusing females were yet active, moved in test tubes and sometimes contacted the host. It seems that the sequence of behavioural reactions resulting in parasitization is interrupted at the stage of arrestment and host recognition. Host density within a habitat proved to be an important factor in host acceptance. Possible mechanisms that formed the basis of the observed effect (behavioural response to host kairomones, the learning ability, endocrine processes) are discussed.

Weak photoperiodic response facilitates the biological invasion of the harlequin ladybird Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae)

Photoperiodic regulation of reproductive diapause in two invasive and two native populations of Harmonia axyridis and in one native population of Harmonia yedoensis was investigated in laboratory at 20°C, five photoperiods (day length of 10, 12, 14, 16 and 18 h) and two diets: (i) eggs of the Angoumois grain moth Sitotroga cerealella and (ii) the green peach aphid, Myzus persicae. Laboratory strains originated from native populations of H. axyridis from Irkutsk province of Siberia and H. yedoensis from South Korea showed a strong photoperiodic response: under short photoperiods (10–14 h and 10–12 h for H. axyridis and H. yedoensis, correspondingly), all females which fed on eggs and most of those fed on aphids did not start to lay eggs during 40 days after emergence, while under long photoperiods, all females fed on aphids and most of those fed on eggs oviposited. The photoperiodic response of H. axyridis from South Korea was less strong: on the both diets, the range of the photoperiodic response (the difference in the proportion of ovipositing females between the treatments with long and short days) was ca 40%. In the European (Czech Republic) and in the Caucasian (Sochi region, Russia) invasive populations of H. axyridis, the photoperiodic response was very weak: the proportion of females that started oviposition (when fed on aphids) or at least reproductive maturation (when fed on eggs) during 40 days after emergence was close to 100%, independently of the photoperiodic conditions. Obviously, instead of a rapid micro‐evolutionary adaptation of the critical day length to a new climate, the invasive populations of the harlequin ladybird decrease their dependence on photoperiod and thus the weak photoperiodic response of SE Asian population of H. axyridis can be considered as a pre‐adaptation further developed during the invasion.

Effects of photoperiod and diet on diapause tendency, maturation and fecundity in Harmonia axyridis (Coleoptera: Coccinellidae)

The effects of photoperiodic conditions of larval development and adult maturation (L : D = 12 : 12 vs. 18 : 6) and different diets (sugar solution, frozen eggs of Sitotroga cerealella, different numbers of aphids Myzus persicae, and their combinations) on survival, reproductive maturation and fecundity of Harmonia axyridis were studied in laboratory conditions. The fundamental aim of the work was to distinguish between cue effect of diet (neurohormonal triggering of reproduction) and direct effect of diet (nutritional maintenance of reproduction). When adults were kept under short‐day conditions, the proportion of ovipositing females decreased and the duration of the pre‐oviposition period increased. Moreover, a strong reaction to the direction of changes in the day length was demonstrated: when larvae and pupae developed at long day and adults were transferred to short day, the proportion of ovipositing females was much lower than in individuals that were permanently kept under short‐day conditions. The percentage of ovipositing females, the rate of their reproductive maturation and the average daily fecundity gradually increased in the following succession of diets: ‘sugar + 5 aphids per day < sugar + eggs < sugar + eggs + 5 aphids per day < sugar + 100 aphids per day’. However, dissection showed that most of the non‐laying females fed on these diets (particularly those kept under long‐day conditions) have started reproductive maturation, while even first stages of oogenesis were not found in females fed on sugar solution alone. We conclude that cue effect of diet (reproductive activation) can be achieved almost independently of the number of prey consumed, while nutritional effects (the rate of reproductive maturation and fecundity) are sensitive both to the quality and quantity of food.

Egg retention in the presence of a host in Trichogramma females

Abstract: Laboratory experiments with Trichogramma principium suggest that Trichogramma females may be in one of two alternative states: the parasitization state or the egg retention state. Dissections show that T. principium females at emergence have ≅ 20 mature ovarian eggs. Females in egg retention state have practically the same mean number of mature ovarian eggs, while the distribution is different, suggesting that both oosorption and oogenesis may occur during egg retention. In parasitizing females, additional maturation starts simultaneously with the oviposition. However, ovipositing females tend to lay eggs with maturation, which is why they mostly carry few (< 5) mature ovarian eggs. The mean number of eggs laid during the first 48 h of the oviposition does not depend on whether the parasitization is delayed or not, suggesting that egg retention is connected just with oviposition behaviour rather than with the intensity of oogenesis. We conclude that ‘voluntary’ egg retention in the presence of a host does not differ with the state of ovaria from ‘forced’ egg retention under host deprivation. Supposedly, egg retention with the availability of a suitable host may be considered as a sort of short‐term adult diapause‐like state when not oogenesis, but oviposition is subject to hormonal regulation.

Maternal influence on diapause induction in Trichogramma (Hymenoptera: Trichogrammatidae): the dynamics of photosensitivity

It is known that the prepupal diapause in Trichogramma is dependent on temperature conditions of embryonic and larval development and on the photoperiodic conditions of preimaginal development of maternal females. However, the relative photosensitivity of different preimaginal stages has been never investigated. We studied the position of the photosensitive period over the preimaginal development of maternal females in T. embryophagum Htg. and T. principium Sug. et Sor. by transferring preimaginal stages between diapause‐averting ‘long’ day (L : D = 20 : 4) and diapause‐inducing ‘short’ day (L : D = 12 : 12). Results showed that the influence of maternal photoperiod on progeny diapause was determined during the late pupal stage (last days of preimaginal development at 20°C). During this time, the critical duration of the photoperiodic induction was extremely small: one short or long day caused almost the same effect as the permanent development under these conditions. As a whole, photoperiodic response in Trichogramma was found to be unusually rapid, labile and easily reversible which is probably explained by extremely small size and fast development of these egg parasitoids. The results of this study could be used for elaboration of optimal methods for Trichogramma mass rearing and storage and for prediction of its seasonal cycles under natural conditions.

Photoperiodic regulation of the diapause of the progeny in Trichogramma embryophagum Htg. (Hymenoptera, Trichogrammatidae): Dynamics of sensitivity to photoperiod at the immature stages of maternal females

Photoperiodic regulation of T. embryophagum progeny prepupal diapause was investigated under laboratory conditions. Maternal females developed at 20°C, the position of the photosensitive period was detected by transferring immature stages between the diapause-inducing “short” day (L:D = 12:12) and the diapause-averting “long” day (L:D = 20:4). Progeny generation developed at L:D = 12:12 under the near-threshold temperature of 13°, 14°, or 15°C. Experiments showed that, at least under the used photo-thermal regimens, only the pupal stage was sensitive to the photoperiod influence. The highest photosensitivity was recorded during the last two days before the adult emergence. In this critical period, even a single short day cycle induced diapause in most of individuals. However, the reaction to the long day was weaker: at least two long light days immediately preceding the maternal adult emergence were necessary to avert the diapause. If a short day was preceded by a long day by more than 2–3 days before the adult emergence, the stepwise photoperiodic response caused an increase in the tendency to diapause. The results of this study could be used in pest biocontrol practice for elaboration of optimal methods for Trichogramma mass rearing and storage and also for prediction of its seasonal cycles under natural conditions.

Ecological and evolutionary aspects of photothermal regulation of diapause in trichogrammatidae

Facultative winter diapause in the genus of Trichogramma Westw. (Hymenoptera, Trichogrammatidae), like in most other insect species, is regulated by photoperiod and temperature. Their prepupae are diapausing, but sensitivity to temperature and day length (environmental cue factors inducing diapause) is characteristic of all stages of development from embryo to the egglaying female. The environmental cues can affect not only the current, but also the next generations. Under the natural conditions, the photothermal regulation provides the timely diapause induction coordinated both with the astronomical season (the photoperiodic response) and with the peculiarity of the given year (the thermal response). The special experiments revealed “rudimentary responses” that had lost their adaptive role. The results of these studies have proved once more that the specificity of photoperiodic and thermal diapause-regulating responses, their relative importance, and association with sensitive stages of development are determined not only by the ecological peculiarities of different insect taxa, but also by their previous evolution.

Temperature Effects on Induction of Parasitization by Females of Trichogramma principium (Hymenoptera, Trichogrammatidae)

The effects of temperature on induction and stability of parasitization of less preferred factitious host (the Angoumois grain moth, Sitotroga cerealella) eggs by Trichogramma principium females were studied under laboratory conditions. The percentage of parasitizing females at 15°C was significantly lower than that at 20, 25, and 30°C. At constant temperatures of 15, 20, 25, and 30°C, mean durations of pre-oviposition periods were 6.5, 2.1, 0.9, and 0.6 days, respectively (differences were significant between all regimens). In most of females, parasitization induced by high temperature (25°C) continued after the temperature decrease to 15°C. These results suggest that although emerged females are ready to lay eggs, rather prolonged pre-oviposition period may precede parasitization of less preferred hosts, and the duration of this period may depend on temperature.