Trichogramma brassicae is a an egg parasitoid, the most popular beneficial insect in the world, which is used in horticulture to control over 200 moth species. The parasite is very small in size, fits comfortably in the needle eye, despite this, it perfectly kills the eggs of Lepidoptera and solves the problem at the beginning. No caterpillars – no crop damage!
Trichogramma brassicae is a very small parasitoid insect, usually less than 1 mm in length. The head is hypognathous and round with bright red eyes. Mandibles have wavy edges and inner spine. The lower jaws have single-segmented palps, provided with 3 strong setae. Palp on lower lip single-segmented, strongly reduced, barely visible, with 2 strong setae. Antennae are small. There is a sexual sharply expressed dimorphism in their structure. Females antennae are short, compact; flagellum consists of 2 segments, usually similar in size; maces are differentiated, massive, the length twice its maximum width. Males antennae flagellum is longer and pubescent with more or less long hairs. The body is dense, wide, yellow or yellow and black with a metallic shade. The life span is for an average of 8 days in natural conditions, but no more than 14 days. T. brassiccae is photophilous, but avoids direct sunlight.
Females lay one or more eggs in the egg of a host insect. Having found pest egg T. brassiccae pierces its shell and deposits the tiny egg. Fertility is up to for an average of 40 – 50 eggs (maximum – 80 eggs). The egg is oblong, transparent with no traces of yolk, its length is 0,07-0,19 mm, width 0,04-0,1 mm.
The larvae are baggy, inactive, with small sickle-shaped mandibles, and have 3 instars. During oviposition, the female also injects the sex glands enzymes, which stop the host’s embryo development and its tissue lysis. Thus, the hatched larva enters the environment prepared for its nutrition. The Trichogramma’s larva hatches at a temperature of 25-30 °C within a few hours and immediately begins feeding. From its hatching, it is completely supplied with food. The host’s egg contents are enough for her to develop fully. At a temperature of 25-30 °C, its development lasts from 8 to 12 days, in case of temperature decreasing, the duration of development increases.
At a temperature of 25 °C, the duration of the 1st larval instar is 5-6 hours in the moth eggs. The 3rd instar larva develops for 30 hours. The larvae are adapted to feeding on the liquid host’s egg contents. Food absorption proceeds by suction due to a constantly pulsating esophagus. Most of the egg contents are absorbed by the 2d instar larva, the remainder one is absorbed by the 3d instar larva. Trichogramma, like other polyphagous parasitic Hymenoptera, has no synchronization in development with the hosts. Аt the end of the season in a temperate climatic zone, the formation period of the overwintering generation is critical because of the limited host number. Prolonged terms of coming into diapause and coming out of it are connected with wintering. The development continuity generations during the season are ensured by host changes.
Having finished development, the larva turns into the adult insect, destroys the eggshell, and flies out. The sex ratio in the population is about 75-90% of females. After several days the fertilized female searches for the host’s eggs. This is repeated up to 9-10 times per season.
The life cycle doesn’t connect with the development cycle of a particular host. Trichogramma can deposit eggs throughout all the vegetation period. But under natural conditions in spring, at the beginning of summer, the level of egg infection by Trichogramma is not high and accumulates in significant numbers with a females’ predominance by the end of summer due to the extremely rapid development.
Influence of temperature on Trichogramma development rate
|Instar of development
||Time of pre-imaginal development (days) at a constant temperature ° С
On the 1st day of life, the female lays the largest egg number. The life span of the adult insect depends on the temperature and air humidity: it decreases with temperature increasing and air humidity reducing. The lower developmental point is + 10ºC. It develops up to 9 of the parasite generations.