by Piter Kehoma Boll
After beetles, which make up the order Coleoptera, the second most diverse group of insects is the order Lepidoptera, which includes butterflies and moths. However, the order Hymenoptera has the potential to eventually surpass Lepidoptera and get closer to the beetles because a lot of new species are being constantly described.
The most widely known hymenopterans are bees, ants and wasps, but a huge part of their diversity is made up by the so-called sawflies. One of these species is commonly known as the turnip sawfly and scientifically named Athalia rosae.
The turnip sawfly is found throughout the Paleartic Ecozone, from western Europe to Japan, and his common name comes from the fact that its larvae feed on plants of the family Brassicaceae, which includes the turnip, as well as the cabbage, among others. The larvae is considerably large and resembles a caterpillar, having a dark gray, almost black dorsal color, and is lighter close to the feet. When they are about to turn into a pupa, they dig into the ground, build a cocoon and remain there until they become adults.
The adults measure about 6 to 8 mm in length, the females being larger than the males. The body and the legs have a yellow to orange color, darker on the dorsal surface of the thorax, which also has two large black spots. The head and the antennae are black.
Hymenopterans in general are characterized by a unique sexual determination in which females are diploid, i.e., have two sets of chromosomes, and males are haploid, having only one set. Matings conducted in the laboratory with the turnip sawfly, however, were able to produce anomalous combinations, including diploid males and triploid females and males. Apparently this is possible due to sex being determined by one allele in one chromosome, so that males are always homozygous and females always heterozygous, but this must be explained in another post. The fact is that the study of this peculiar system in this species is helping to understand how sex determination evolved in hymenopterans.
One last interesting thing to mention about the turnip sawfly is that it is able to bypass the defense mechanisms of the plants on which its larva feeds. Plants in the family Brassicaceae produce a group of compounds called glucosinolates that give them their characteristic pungency and bitterness, such as in mustard and horseradish. These compounds are used by the plant as a defense against pests that feed on them. However, the turnip sawfly is resistant to this compounds and is able to sequestrate them and store them in their hemolymph, i.e., their “blood” in concentrations much higher than found in the plants. When attacked by a predators, such as ants, the larva releases drops of its hemolymph in a sort of defensive bleeding and can stop the attack.
The turnip sawfly may be a nuisance for humans and their crops, but it is certainly a fascinating animal.
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Müller C, Agerbirk N, Olsen CE, Boevé JL, Schaffner U, Brakefield PM (2001) Sequestration of host plant glucosinolates in the defensive hemolymph of the sawfly Athala rosae. Journal of Chemical Ecology 27(12): 2505–2516. doi: 10.1023/A:1013631616141
Müller C, Boevé JL, Brakefield PM (2002) Host plant derived feeding deterrence towards ants in the turnip sawfly Athalia rosae. In: Nielsen J.K., Kjær C., Schoonhoven L.M. (eds) Proceedings of the 11th International Symposium on Insect-Plant Relationships. Series Entomologica, vol 57. Springer, Dordrecht. doi: 10.1007/978-94-017-2776-1_18
Naito T, Suzuki H (1991) Sex determination in the sawfly, Athalia rosae ruficornis (Hymenoptera): occurrence of triploid males. Journal of Heredity 82(2): 101–104. 10.1093/oxfordjournals.jhered.a111042
Oishi K, Sawa M, Hatakeyama M, Kageyama Y (1993) Genetics and biology of the sawfly, Athalia rosae (Hymenoptera). Genetica 88(2–3): 119–127. doi: 10.1007/BF02424468
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