306 REPORT OF THE COMMISSION"ER OF AGRICULTURE. 



compared either to the birds or to the parasitic insects in regard to the 

 number of cotton-worms which they destroy. True, the capacity of some 

 of them is great, but they either labor under disadvantages (such as being 

 comparatively confined to the ground as the carabid beetles) or are not 

 sufficiently numerous to do a very great amount of good. Still it is well 

 to know them and not destroy them, as thousands of worms are destroyed 

 by them, and it is only in a comparative ^^'ay that we speak at all de- 

 precatingly of them. The capacity of the rear-horses {Mantis Carolina) 

 has been shown by the statement that one individual has in one night 

 killed and devouretl eleven Colorado potato-beetles, and we have men- 

 tioned the fact that a young specimen of the wheel-bug {Frionotus cris- 

 taius [Rediivius norenariiis]) has been known to destroy ten caterpillars 

 in five hours, thus showing the amount of good which may be done by 

 the hemipterous enemies of the cotton-worm. The destructive powers 

 of the asilus tiies have been shown from Mr. Thompson's statement that 

 be has known one indi\'idual to destroy 141 bees in a day. The work of 

 ants in this direction has been discussed at length, and they are shown 

 to be the most valuable of the i^redaceous insect enemies of the colton- 

 worm. 



The destruction of the cotton -worms by their true parasites is a sub- 

 ject upon which interesting experiments may be made. The extent of 

 parasitism will undoubtedly vary much with the season of the year, the 

 last brood always seeming to be much more extensively parasitized than 

 any of the preceding broods. The probabilities are that they increase 

 with the increasing numbers of the worms, and that they also are affected 

 to a certain extent by the character of the season, although not com- 

 parably with the ants. From August 12 to August 28, 1,721 pupae prob- 

 ably belonging to the fourth brood were received at the department from 

 Alabama, for the purpose of ascertaining the extent of the parisitism. 

 The result hardly justified the anticipation. From this lot of 1,721 

 chrysalides there issued in all 1,455 moths, and from the remaining 266 

 chrysalides were bred the following parasites : Of Chalcis ovata, Say, 32 

 specimens ; of Taeldna aletiae, Riley, 3 specimens ; of Sarcophaga sp., 7 

 specimens; of Pimpla anuUpes, Br., 1 specimen; of Tacliina fraterna^ 2 

 specimens ; of JDidicktm zigzag sp., 32 specimens ; of the small Dipteron 

 FJiora aletiae, a very great number of specimens; making altogether of 

 the large parasites, 44, each singly from a chrysalis, and 120 chrysalides 

 destroyed by the small parasites, making a total of 164 out of 1,721, or 

 between 9 and 10 per cent. The i-emainiug 102 died from some unknown 

 cause. This percentage is small, but in the last brood it would undoubt- 

 edly be gTeatcr. 



The extravagant ideas of Dr. Gorham on the subject of the extent of 

 parasitism are easily accounted for. He collected his specimen chry- 

 salides tor observation late in the fall, after the hibernating moths had 

 issued. Naturally, no apparently sound chrysalides were left excepting 

 those containing parasites. These he collected, and parasites issued 

 from all ; hence his conclusions. A note from one of Professor Willet's 

 letters seems to indicate the greater abundance of pai-asites in the last 

 brood than in the earlier ones. He collected a number of newly-formed 

 chrysalides in November. Of these he says : 



About two dozen were placed in a box in my sitting-room, expccliiig to liatcli ont 

 some moths for exposure. Tlie following is tlio result : In some two weeks two moths 

 came out; 13i"y seemed delicate, and one lived only two days, the other four or five. 

 No other moths have appeared (I)ecemher 11). November 24, 1 found foiu- ichneumon 

 flies {Fimpla conqidsitor) out in one boll ; December 2, one more; and December 7, an- 

 other ; the sixth, the last, with no ovipositor (a male). In breaking open the diicd 

 chrysalides I destroyed two pupae of parasites. These make eight parasites in some 



