46 SIXTEENTH REPORT. 



PARTHENOGENESIS IN ANTHOTHRIPS VERBASCI. 



BY A. FRANKLIN SHULL. 



The mode of reproduction in Thysanoptera has been accurately deter- 

 mined in only a few species, especially in those of economic importance. 

 Parthenogenesis has been sho^\^l to occur in certain species by rearing them 

 in guarded cultures; but proof of sexual reproduction has not, so far as I can 

 find, been advanced for any species. The mode of reproduction has been, 

 as a rule, inferred from the relative frequenc}' of males, and from the oc- 

 currence of copulation in nature. Owing to the peculiar sex ratios found 

 (Shull, 1914, p. 239), this rule is difficult to apply. In some species, males 

 are unkno"uai (Anthoth-ips inyer). In others, they are uncommon {Thrips 

 tabaci). In Anaphothrips striatus, males are unknown in some regions 

 (Massachusetts), rare in others (Huron County, Mich., Shull, 1909, p. 224), 

 and fairly common in others (25 per cent of males at Douglas Lake, INIich.) 

 In Euthrips tritici, males are always abundant, though less numerous than 

 the females. In Anthothrips verbasci, there are about 24 per cent of males. 



In this series of species exhibiting increasing abundance of males, where 

 does parthenogenesis end and sexual reproduction begin? For the purpose 

 of answering this question, an extensive series of experiments was started 

 in the summer of 1914. The method of conducting these experiments was 

 to rear the food plant under cover from seed on sterilized soil, and at the 

 proper time transfer to it a single larva or pupa of some species. After 

 allowing sufficient time for the larva to emerge as an adult, and produce 

 offspring of its own, the food plant was examined carefulh' for thrips. 



In this way I have been able to demonstrate parthenogenesis in a few 

 species, mostly those in which it has been inferred before. In one species 

 (Anthothrips verbasci), however, parthenogenesis occurred wholly unexpect- 

 edly. Onl}' the experiments AA"ith this last-named species will be described 

 here. 



In the cultures of Anthothrips, the host plants (common mullein, Ver- 

 basciim thapsus) were not reared from seed, but were transplanted from a 

 spot in a field where a colony of very 3'oung plants had started near an ash 

 heap. No other mullein plants could be discovered within about 20 rods. 

 Since A. verbasci lives on no other plant, and does not migrate readily, this 

 distance made infection of such 3'oung plants quite unlikely. Moreover, 

 no thrips were on the plants at the time, this species of thrips l^eing quite 

 large, hence easily seen, and not at all inclined to hide in crevices on j'oung 

 plants. I conclude that the 3'oung mulleins grew from seeds brought in 

 with rubbish, and there had been as yet no opportunity for infection. 



To determine whether any eggs might be on the plants, the latter were 

 reared in the greenhouse for a month (about July 23 to August 28), before 

 being used in experiments. In that time no larvae appeared on them. All 

 of the original leaves had by that time l^een removed, so that an}' eggs that 

 might have remained unhatched longer than usual would have been removed 

 with them. Ten of the plants were kept nearly three months without the ap- 

 pearance of any larvae. Three of these ten are still being used as controls 



