20 S 



HORTICULTURE 



February 13, 1909 



Mrs. T. W. Lawson. One bench was near the wall with 

 a narrow walk separating them and in this walk at 

 about the height of the bench a return steam pipe from 

 the rose houses. The other was a center bench with 

 only the usual heating pipe below. On the first described 

 bench 9 per cent, of the buds were injured by thrips 

 while on the center bench there were 5 per cent, injured 

 buds during the season from November to June. The 

 difference decreased as the season advanced for in April 

 there was a difference of one per cent, in favor of the 

 center bench but in May tlie results were the same. 

 The conditions were unlike because of the radiation 

 from this return pipe but as the sun heat increased and 

 the artificial heat was less used, the conditions on the 

 two benches were more nearly equal and the thrips' 

 injury became proportionately equal as we would ex- 

 pect. These results are based on an examination of 27,- 

 038 buds and flowers during each week from Nov. 1, 

 1907, to June 1, 1908. 



The foregoing indicates what takes place under small 

 variations of temperature wheil other conditions are sim- 

 ilar. These warmer parts should be remedied when 

 possible but in all cases the plants growing therein 

 should be carefully watched for thrips and red spider. 



In cucumber houses, where the temperature is kept 

 above 60 F. at night, Thrips tabaci often increases when 

 unchecked to such an extent as to destroy the crops. On 

 the other hand so far as we have had opportunity to 

 study thrips in lettuce houses they are only abundant in 

 October and November and decrease as the season ad- 

 vances. In spring the crop in these houses is changed 

 so that there was no opportunity to learn if there was a 

 spring invasion. 



During April and May there were opportunities to 

 get more information on the effect of temperature on 

 thrips, since the difference between the day and night 

 temperatures was greater than in winter. The results 

 of the earlier observations were confirmed. Thrips tri- 

 tici on roses retire within the unfolding petals of the 

 flower as night approaches, where they feed along the 

 edges of the petals and especially in the point of the bud. 

 This night feeding in the point of the bud results in the 

 disfiguring of all the petals and renders the flower 

 worthless. In the morning as the temperature rises the 

 thrips of this species move out to more exposed positions 

 on the petals that have unrolled. Here they feed further 

 from the edges of the petals. It is a noteworthy fact 

 that if the sun is shining they place themselves so that 

 the petal is between themselves and the sun. In this 

 position they have all the advantage of the temperature 

 witli the least light. 



The thrips on carnations were less often seen out- 

 side the buds as the days become longer. This species 

 is more responsive to heat and light for on cucumbers 

 the adults could be seen late in the afternoon crawling 

 over the edge of the leaves to the upper surface where 

 they fed during the night. In the morning if the day 

 was cloudy they remained where they were but when 

 the sun came or;t they retired to the lower surface. On 

 a sunny morning it was interesting to observe the rela- 

 tive numbers of thrips on the upper surface of leaves 

 in sunny and sliaded parts of the house or even on the 

 same plant in sunshine and shadow. 



Thrips femoralis on the calla feeds on the lower side 

 of the leaves whether the day is cloudy or otherwise, but 

 it does feed to a certain extent on the upper side at night 

 as shown by the fact that the leaves show injury there. 

 The preference is for tlie lower surface. On tlic amarvl- 

 lis from observatimis mi a limited niniiliiT of plants 



this species prefers the upper surface where the result 

 of its feeding is seen in bright red spots which later 

 become brown and dead. 



In every case in which I have observed the develop- 

 ment of thrips in greenhouses on callas, carnations, cu- 

 cumbers and roses they made their appearance first and 

 caused greatest injury in the warmest part of the house. 

 Further than this the leaf infesting thrips in tiie imma- 

 ture stages are found on the under sides of the leaves 

 where they have hatched and are least exposed. On 

 roses the immature thrips were never observed feeding 

 exposed. Even the adult insect when feeding in carna- 

 tion buds and shoots remains almost wholly concealed. 



Mr. J. J. Davis, my assistant in certain phases of tlie 

 life history of this insect, made the following laboratory 

 experiments to determine the reactions of the thrips 

 towards heat and light. Adult Helithrips femoralis 

 were placed on carnation buds, twenty-five on each bud. 

 The buds were then placed in lamp chimneys, covered 

 with gauze to prevent the escape of the insects, and the 

 cages placed under the following conditions. Two cages 

 were placed side by side so the temperature was the 

 same, one of them being exposed to light and the other 

 covered so as to exclude all light. The other two were 

 placed one in the Insectary room and the last in the 

 office, the light intensity being apparently equal but the 

 temperature of the rooms unequal. In general the ex- 

 periments to determine the effect of light showed that 

 more thrips were out on the bud where light was totally 

 excluded but if the temperature became unequal the re- 

 sults were changed very noticeably. When the thrips 

 were out on the bud in the lighted cage they were on the 

 shaded side as far as possible, this being less marked 

 early in the morning than at mid-day. Temperatures 

 60 and 70 F. showed the greatest difference in activities 

 of thrips compared with any other equal difference of 

 temperature. In these cages also thrips avoided intense 

 light when possible. Mr. Davis concludes from the re- 

 sults of these tests that thrips activities are largely or 

 almost entirely occasioned by the temperature and that 

 light is only a secondary consideration. 



It appears that thrips do not thrive where the night 

 temperature is low with only a moderate rise in the day 

 tcmpei'ature but where this rise is great or when the low- 

 est temperature is above 60 they find conditions suited 

 to their welfare. In the first case thrips are less active 

 feeding but they respond quickly to- the rise in tempera- 

 ture in the morning due to the sun heat. In cucumber 

 houses the thrips are not hindered by any unfavorable 

 temperature. 



To secure the greatest results from the use of an in- 

 secticide for the destruction of thrips we must know its 

 habits for it is apparent to any one that they are diffi- 

 cult to kill when they have pushed their way down into 

 a carnation bud. Since it has been shown by observa- 

 tion and experiment that thrips are more active in the 

 morning and if they are outside the buds it is at this 

 time a comparison of the results of using insecticides in 

 the evening and in the morning is of interest. It was 

 found tliat when fumigating was done at five o'clock in 

 the morning during May tlie efficiency of Nicoti- 

 cide, Tobacine. Nikoteen was respectively increased 18, 

 21 and 20 per cent, over those obtained from using the 

 same insecticides in the evening. Tlie same reasoning 

 applies to spraying but as it is the general practice to 

 sprav in tlie morning no tests were made on this par- 

 licular ])oint. 



('Iiiiiii pai(jn. 111. 



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