February 13, 1909 



HORTICULTUKE 



205 



Thrips and Their Response to Light and Heat 



THE GENERAL DESCRirTION OF THRIPS 



Thrips are very small insects. The females of the 

 species considered in this article are from one twenty- 

 fifth to one-twentieth of an inch in length while the 

 males are smaller, those of E. tritici being about one 

 thirty-second of an inch long. Some species of thrips 

 infesting other plants are larger and some are smaller 

 than those described here. All the species known to 

 infest greenhouse plants are winged species, though at 

 certain seasons many or even all the individuals may be 

 wingless or else have only short wing pads. The wings 

 are four in number, long and membranous. They are 

 never folded. The hind margin is always and the fore 

 margin generally, fringed with long slender hairs, in 

 length greatly exceeding the breadth of the membran- 

 ous part of the wing. The name of the order, Thysan- 

 optera to which these insects belong, is derived from 

 two Greek words meaning tassel-wing and one of the 

 common names. Fringe Wings, is based on this peculiar 

 wing structure. 



The antenufp are very slender, six to eight Jointed 

 stiiictures. Besides the faceted eyes, two or three ocelli 

 are present on the winged forms, even though the indi- 

 vidual may not at the time possess wings. These ocelli 

 are not present in the larval stage. 



The feet are not terminated by a claw, but the tip is 

 fitted with a protrusile bladder-like organ. This gives 

 rise to the name Blasenfusse or Bladder-feet and the sci- 

 entific name Physopoda. 



The mouth parts are intermediate in form between 

 those of chewing and sucking insects but are believed to 

 be used for sucking. Some of the parts which form a 

 peculiar cone far back beneath the head are modified to 

 a piercing organ with which the insect pierces the ten- 

 der parts of plants and withdraws the sap. It is there- 

 fore impossible to kill thrips by spraying or dusting 

 poisons on the plant, notwithstanding the opinion of 

 some gi-owers. Any benefit, if such there be, is due to 

 other action of the material applied. 



REPRODUCTION 



Reproduction is oviparous, that is, multiplication by 

 means of eggs. Reproduction is either bisexual or par- 

 thenogenetic. The latter method, i. e., reproduction 

 without fertilization by the male, is thought by Dr. 

 Hinds to be the usual means of reproduction in at least 

 ten species and that it occurs in many others. He states 

 further that the females almost always outnumber the 

 males and that in some species males are found at cer- 

 tain-seasons, in others the males are rare and in others 

 unknown. The eggs of thrips, so far as they have been 

 observed, are bean- shaped and when fresh, clear white in 

 color. In size they are very minute, those of wheat 

 thrips being .009 x .003 of an inch. 



The female thrip lays her eggs in the green parts of 

 plants, first cutting a slit with her saw-like ovipositor, 



after which the egg is pushed down into the tissue be- 

 low the epidermis. The operation has rarely been ob- 

 served but so far as known requires only a minute and 

 a half. The short time required for this operation, 

 together with the fact that the insect, when possible, re- 

 mains concealed, makes it difficult to discover where the 

 insect lays its eggs. Again, it is impossible to know 

 when the egg laying is about to begin. Sometimes the 

 tissue is too hard and the insect is unable to withdraw 

 the ovipositor, and the insect is kept a prisoner until 

 relieved by death. 



In the pupal stage the insect is quite inactive, changes 

 color and usually retires to a quiet spot where it does 

 not feed. During this stage the wings, the ovipositor 

 and the genital organs develop. The insect emerges a 

 fully developed adult. It is a noteworthy fact that the 

 young thrips resemble the adult insects in general 

 form, in stracture of tlie mouth parts, and in food 

 habits. They differ in lacking wings, in color, which is 

 lighter than the mature insect, and in some of their 

 activities. 



Prof. Quaintance says that the life cycle of Thrips 

 tritici is twelve days under summer conditions in Flor- 

 ida. The egg state lasts approximately tliree days, the 

 larval stage five days, the nymph state four days. In 

 the north, especially in the high spring and autumn tem- 

 peratures in greenhouses, no doubt the life cycle ap- 

 proaches or equals this. The same writer finds the life 

 cycle of Onion thrips is approximately sixteen days, 

 divided as follows : egg state three and a half days, larval 

 stage seven to nine days, and pupal stage three days. 



HIBEHNATION 



The studies thus far made upon thrips indicate that 

 they pass the winter in the same place they have fed. 

 Those feeding on leaves and flowers hibernate under the 

 dead foliage and in the dried flowers. The grass-inliab- 

 iting species, among which are those doing the most 

 damage to cut flowers, pass the winter in the turf. In 

 "such situations they can endure very low temperatures 

 without suffering injury as shown by the fact that they 

 revive quickly when brought into warm quarters. They 

 emerge very early in the spring and begin their activ- 

 ities as soon as the growth of their food plants starts. 



Hibernating thrips can be destroyed by burning all 

 dead grass, weeds, flowers, remains of vegetables, leaves, 

 etc., in the vicinity of the greenliouses but to be effec- 

 tive this should be done before April 1st. 



THE EFFECT OF TEMPERATURE AND LIGHT ON THRIPS 



From the results of many observations it appears that 

 thrips are not very active at temperatures below about 

 65 degrees F. On cloudy and rainy days when the tem- 

 perature in carnation houses is more nearly that main- 

 tained at night (50-52 degrees) it has been found that 

 thrips are rarely seen outside the buds. Wlien the tem- 

 perature rises above 60 F. thrips become more active 

 and if the sunlight is not very intense they may be seen 

 outside the buds in the majority of cases in the morn- 

 ing. 



The effect of temperature on thrips activity is shown 

 by the well known fact that, like red spider, they cause 

 most damage in the warmer parts of the house. A case 

 in point was the result obtained from two benches of 



