42 



THE AGRICULTURAL NEWS. 



February 6, 190&, 



INSECT NOTES. 



Some Beneficial Insects. 



Beneficial insects may be classoil under three 

 headings : (a) those which tbrm products usefid to man, 

 of which the honey bee (Apis mcUifira) and the silk 

 worm (Bomht/.r mori) are the best-known examples : 

 (J>) insects which play such an important {lart in the cross 

 pollination of plants, as the bees and the hawk moths; 

 and ((■) those which, wp.ile thnmselves harmless, destroy 

 other insects that are harmful to cultivated crops. It 

 is this last class of insecis to which these notes refer. 

 They live in a variet}' of different ways : some are 

 pred.itory, either, like the l;idy-birds (Coccinellidae), 

 feeding in all stages on other insects, or, like the 

 predatory wasps, sturiig their nests with larvae and 

 spiders, to furnish a supply of food for their own larvae : 

 otiiers are parasitic, iHving their eggs in or on some 

 other insect, -.xnd the ri'sultiiig larvae from these eggs 

 pass through their whole met.-imorphosis at the e.xpense 

 of this insect host. 



The most important of these parasites are found in the 

 two orders Hymenoptera or the wasp-like insects, and the 

 Diptera or true Hies. There are afso some among the Coleop- 

 tera or beetles. 



In the Hymenoptera the following parasitic families are 

 found : the Pmetoti-ypidaf, Chalcidae, Ichneumonidae, and 

 Braconidae. These are the four mo.st important families, 

 though there are other smaller ones with parasitic habits. 



Among the Diptera, the chief oarasites are the Tachinid 

 flies. 



The Proctotrypidae are completely parasitic in the eggs 

 of other insects or si)iders. They are very small and include 

 what is thought to be the smallest e.xisting insect, viz., 

 Alaptns e.a-isiis, Westwood, the length of which is given as 

 ^ mm., or about ^l^, inch. Sometimes half a dozen of these 

 minute insecfs will lind snlHcient fnnd for their development 

 in a single egg. 



'fhe Chalcidae are also very small ; nearly .'),000 

 species are known, and there are doubtless still a great 

 nundjer of unknown species. They attack scale insects, 

 plant lice, bees, and Lcpidopterous in.sec'ts. 



The Ichneumonidae contal'ii larger forms of insect life ; 

 there are upwards of 6,000 s|)ecies known, most nf wliiih 

 Vwv ill Lei)idopterous larvae. 



'file Hraconidae are another large family, and are 

 parasitic on IjCpidopterous larvae. 



The I)i[itera include many families which eimtain 

 among thcni a few parasitic forms but, as stated ali.ive. the 

 Tachinidao are the most important. 



The Tachinidae are medium to large flics, but tiiey vary 

 a great ileal in size and details of stnu'ture. They are much 

 like the common house fly in general aiipearance. They are 

 parasitic on insects belonging to the Orthoptera, Hymenop- 

 tera, Coleoptera, and Lepidoptera, particularly on the last 

 mentioned. 



In the order Coleoptera, the Stylopidae are im]ioitanI 

 parasites on Hymenoptera, and on Hcmiptera or bugs. 



An attai-ked catcrpillai- continues to live and feed even 

 though it is being gradually devoured by the invading 



* Tlie Insect Nntes on tins i>age have lieen ccintribiUed l>y 

 Mr. ('. \V. Jenniiett, lately appninteil (idveiimient I'Jntnninlii- 

 .uist ti) S.aithern Nigeria, ami wIki has for tlieiiast few iimntlis 

 J een ennnected with the luiiicrial Dei>artnient of .Vgricidtiire. 



parasite. This is because the latter leaves the vital organs 

 of its so-called ' host ' untouched until it is ready to pupate. 

 If the larva dies toQ,soon, the parasites must neces.sarily die also. 

 The important part which these several parasitic families- 

 play in keeping in check the many pests of our crops caa 

 scarcely be overe.stimated. .-^ny unusually severe attack of 

 a pest is invariably followed by a corresponding increase in, 

 its particular parasite, owing to the abundance of the food 

 supi)ly of the latter ; and the planter or whoever the person, 

 interested may be, is surprised to find that the pest disap- 

 pears as quickly and as suddenly as it had arrived, often not 

 realizing to what he owes its disappearance. An e.xaniple in 

 point is instanced in Dr. Howard's ' Insect Book ' where the- 

 author mentions that in a certain year, in the cotton fields of 

 Northern Florida, 9.5 per cent, of the eggs tVotn which w^ould 

 have hatched the voracious cotton caterpillar were killed by 

 the minute Chalcid parasite Trivho<jraiiniKt jiretiosd. It is- 

 only in cases where a pest has long been established in 

 a country that the influence of these parasites is noticed. 

 Each siiecies of parasite has its own particular host species, 

 or at most only a few. In consequence, it freipiently happens- 

 that where a pest has been recently introrluced into a country 

 a parasite which preyed upon it in the country from whieh "it 

 came is not introduced at the same time, and so the jiest 

 multiplies for a while, unchecked by its natural enemy, until 

 either the parasite is introduced, or .some indigenous species- 

 alters its habits and becomes parasitic upon it. It is owing. 

 to the recognition of these facts that, in the United States and 

 elsewhere when .some imported pest has become established, .so- 

 much trouble is taken to discover, if po.ssible, its original home 

 in order that its natural parasites may be looked for, and if 

 feasible, may be introduced. Unfortunately, the effects of 

 this .satisfactory state of things are modified by the existence- 

 of other members of the fa:nilie.s, which themselves prey, 

 not on the pests, but on the parasites of the pests. Thus we- 

 have what are known as primary, secondary, and even, 

 tertiary parasites. For instance, we may have a bad attack 

 of a particular pest on .some crop. As it becomes more- 

 abundant, so does its primary parasite increa.se, and at last 

 gaining the upper hand, practically demolishes the pest. 

 Next season there are numbers of the parasite but there are- 

 few of the pest. Then the secondary parasite has its turn, 

 and finding its prey (the primary parasite) i)resent in lar<re- 

 number.s, increa.ses and destroys it, thus giving the orii'inal 

 pest another chance of a.sserting itself, and as stated above, 

 in some cases there may be a tertiary parasite w-hich destroys- 

 the secondary parasite, thus giving the primary parasite ai'aiu 

 the upper hand. 



In the West Indies, where the .scale in-^ects — Coccidae 



are .so abundant, one may frctpiently notice many of the- 

 scales with a large hole in them. It is through this lu,]^ 

 that the jjarasite, having killed the scale insect and com- 

 pleted its own development, lias escaped. 



Nearly all Lcpidopterous larvae also are preyed upon by 

 parasites. In the cotton field one can often find dead cotton 

 wnrm pupae with a small round liarrel-shaped cocoon Ivinw 

 in or on it. This is either a Tachinid or Sarcophagid cocoon 

 as the case may be, and on emerging, the fly hastens away to- 

 lay its eggs on other larvae, and thus ipiitc a number are 

 destroyed. 



Ml l'.all..u mentions in the WeM Indian Bulletin, 

 \<il \ 11, No. I, that Kvnnin lacviijnia, Orphion bltinmtttm, 

 Vliiik-is (ininil(tfifs, and Triclioiirnmma prefiosa — the last of 

 which was referred to earlier in these notes — are all of 

 frequent occurrence in the West Indies Theie are doubtless- 

 many others present, the life-histories of which will .scciner 

 or later be worked out. 



