48 MISC. PUBLICATION 568, U. S. DEP'T. OF AGEICULTUEE 



may weaken a plant to such an extent that it falls prey to some fungus, 

 or the opposite might be true. Therefore the collector may need to do 

 considerable detective work before arriving at the true conclusion. 



Figure 33. — A wasp parasitized by a fungus (enlarged). Insects are commonly so 

 attacked in the Tropics. 



INSECT-PEST SPECIMENS 



The general remarks on the desirability of collecting specimens of 

 parasitic fungi apply equally well to insect pests. Any person working 

 with plants can contribute valuable information and prepare good 

 specimens even without a technical knowledge of entomology. One needs 

 to have a keen vision, a degree of patience, and to know a few funda- 

 mental facts about insects. 



Of the insects attacking living plants, the most frequently encountered 

 will be beetles, weevils, butterflies, moths, grasshoppers, leafhoppers. 

 ants, aphids (plant lice), true bugs (squash bugs), scale insects (coccids), 

 flies, thrips, and white flies. Mites, eelworms, and slugs also cause 

 damage, but these are not true insects. 



The beetles, weevils, butterflies, moths, ants, and flies form one 

 group (fig. 34, A, B, C) in that the adults differ in appearance from the 

 young stages. The eggs hatch into wormlike objects (larvae), variously 

 known as grubs, caterpillars, or maggots. After feeding for a time, the 

 larvae go into a resting period (pupa stage) and pass through a com- 

 plete internal transformation. Finally, the covering of the pupa splits 

 open and the adult insect emerges. 



In the other group (fig. 34, D, E, F, G, H), grasshoppers, leafhoppers, 

 aphids, true bugs, scale insects, thrips, and white flies, the young stages 

 (nymphs) are more or less similar in appearance to the adult form. The 

 nymphs progress to the adult stage by periodically shedding their skins 

 to emerge in a somewhat larger size. At each shedding they take on 

 more and more of the adult appearance. 



The nature of the damage to plants depends upon the type of mouth 

 part possessed by the insect, whether chewing or piercing-sucking. 

 Various types of injury are inflicted — externally, by chewing the leaves, 

 buds, flowers, fruits, seed, stems, or roots; internally, by boring or tun- 

 neling in these same parts; and by piercing the plant epidermis and 

 sucking out the sap. Sometimes injury occurs when an insect lays its 

 eggs in some part of the plant. 



In butterflies, moths, and flies, plant damage is confined entirely to 

 the larval stages, because only the larvae have chewing or rasping 

 mouth parts. In beetles and weevils both the adult and the larvae have 



