670 



SCIENCE 



[N. S. Vol. LIV. No. 1409. 



beetles, when nematized, deposit from a few 

 to upwards of fifty of the nema larvae with 

 each egg. See Fig. 2. These soon mature 

 on the eggs or in the soil (where they can 

 live several weeks), moult, develop a more 

 perfect spear, and by its aid begin to make 

 their way into the body-cavity of the beetle 

 gi'ubs soon after the latter hatch out. That it 

 is rather improbable the nomas enter the host 

 by way of the mouth and alimentary canal is 

 illustrated in Fig. 4. The active young beetle 

 larvsB are armed with sharp-toothed, well de- 

 veloped mandibles. That the tender young 

 nomas could pass so relatively small a throat 

 and mouth, armed as the latter is, one hesi- 

 tates to believe. 



In plant-infesting triplonchs I have shown 

 the development of the so-called salivary 

 glands to be greatest in species noted for their 

 efficiency in destroying the tissues of the 

 host, and suggested that these glands aid in 

 dissolving the host tissues and thus supple- 

 ment the mechanical action of the spear or 

 onchium, which therefore should then act also 

 as a spewing channel. In light of this, it 

 may not be without significance that the sali- 

 vary glands of Howardula henigna appear 

 better developed than in some of its nearest 

 known relatives. Conceivably this secretion 

 is also antiseptic. Nomas of very many kinds 

 make their way through the tissues of their 

 hosts without causing fatal infections. The 

 existence of an antiseptic nema secretion or 

 excretion might explain this. In the case of 

 Diabrotica, there is no known trace left of 

 the relatively large breach made by the para- 

 site, a benignant result perhaps facilitated 

 by the parasite itself in the way indicated. 



The present investigations suggest how far 

 we are from appreciating the abundance and 

 importance of insect parasites and how back- 

 ward in attempting their control. How- 

 ardula is, beyond any reasonable question, 

 ages old, for on no other supposition can the 

 remarkable relationship of host and parasite 

 be explained. It is only one of a consider- 

 able number of parasites of the same de- 

 structive insect that have much to do with 

 the welfare of the host. Intelligently increas- 



ing the incidence of the parasites decreases 

 the ravages of the host. When we come to 

 understand these relationships, these " bal- 

 ances " between host and parasite, doubtless 

 we can do much toward inclining the " bal- 

 ance " in our favor. We hear more or less 

 of organisms introduced to new areas without 

 their enemies and parasites, and in conse- 

 quence becoming frightful pests, and we have, 

 very painfully and slowly it seems to some of 

 us, learned that searching for and introducing 

 these same enemies and parasites affords re- 

 lief. Marked successes of this kind at least 

 place it beyond doubt that this portion of 

 the field of economic parasitology will be care- 

 fully explored. But there is another very 

 important part of the field of which we hear 

 little if anything, and that is the comprehen- 

 sion and watchful control of what may be 

 termed indigenous or long-established " bal- 

 ances." 



The cucumber-beetle affords good enough 

 example of these latter to justify an appeal, 

 on the basis of it, to economic biologists to 

 scrutinize more carefully the ever changing 

 " balances " between pests and their parasites 

 and other enemies, including p»sts of long 

 standing, with a view to keeping the "bal- 

 ance " always inclined in our favor. I be- 

 lieve any well trained, experienced and 

 thoughtful biologist will agree that such a 

 course is bound finally to result in notable 

 economies. A case in point is the existence 

 of localities, among those here tested, in which 

 the total zoo-parasitic infestation of the beetles 

 reached only about two per cent. At the same 

 time not very far away there was a nema 

 infestation exceeding fifty per cent, and a dip- 

 terous infestation exceeding forty per cent. 

 The investigation showed that the transfer- 

 ence by post of these two parasites from the 

 highly infested areas to the low or non-in- 

 fested areas was easily feasible at small cost. 

 Posted in a ventilated box with a few cucurbit 

 leaves the infested beetles undergo a two to 

 four days' journey; set loose at night they 

 survive without apparent injury. 



N. A. Cobb 



United States Department op Agriculture 



