ILLINOIS NATURAL HISTORY SURVEY

Reports ~ November/

December 1994

No. 330 1 Gateway to Invasion IN S ID E

Larval Fish Feeding

and Growth

3

New Exotic Insect

Pest on Illinois Pines

4

Effects of Hydrology

and Water

Chemistry on Wetland

Vegetation Zones

5

Species Spotlight: Praying Mantis

6

The Naturalist

Apprentice

7

Fingerprinting Bacteria

8

The zebra mussel (Dreissena polymorpha) may be the first foreign aquatic pest to invade the Mississippi drainage through the Great Lakes at Chicago, but it will not be the last. At least five other potential pest species are known to occur near this Chicago gateway. The northern Mississippi Basin and the rivers of Europe were once separated by a wide expanse of sea water, freshwater barriers such as Niagara Falls, and the drainage divide at Chicago. One by one, these barriers have fallen.

In 1900, the flow of the Chicago River was reversed, so water could flow by gravity from Lake Michigan, "up" the south branch of the Chicago River, then down the newly­completed Chicago Sanitary and Ship Canal and eventually into the Illinois. As its name indicates, one purpose of the

The zebra mussel (Dreissena polymorpha) may be the first foreign aquatic pest to invade the Mississippi drainage through the Great Lakes at Chicago, but it will not be the last.

canal was to carry waste away from Chicago, and thus maintain a clean drinking water supply in Lake Michigan. For a time, this waste created a pollution barrier isolating the Mississippi Basin from Great Lakes invaders. It is ironic that improved control of

industrial and urban pollution over the last two decades has lowered this final barrier. The increase in commercial and recreational boat traffic over this same period in the inland waterways created a particularly ripe opportunity for an organism like the European zebra mussel which can attach to boats and

hitchhike throughout the waterway system.

The zebra mussel was probably introduced to the Great Lakes in 1985 or 1986. By 1991, huge colonies were blocking water intake pipes in portions of the lakes, and the first individuals were showing up in the Illinois River and upper Mississippi River.

The population exploded in the lower Illinois River in 1993, where small (5/16 inch) mussels carpeted the bottom at average densities of 61,000 animals per square meter (tn2 = about l square yard). Oxygen levels in this portion of the river fell to 3.0 parts per million (ppm) on

31 August 1993 and to 1.5 ppm on 29 June 1994, well below the state water quality standard of 5 ppm. Although substandard oxygen levels of 3.6--4.9 ppm had been a problem in this part of the river for at least nine years prior to 1993, the ex­tremely low 1993-94 levels may be attributable to addi-

/NHS divers samplini mussel populations in the Illinois river

tional oxygen demand exerted by the zebra mussels. The low oxygen levels in combination with warm summer tempera­tures of 31 °c (88°F) apparently stressed the mussels because populations in the lower river declined to only 4,000/m2 by August 1994, and divers found up to 3,000 empty zebra mussel shells/m2

• Unfortunately, the low oxygen levels probably stressed native aquatic animals as well. As zebra mussels and other aquatic organisms die, the decomposition of their remains uses up additional oxygen, making a bad situation worse.

Zebra Mussels

t ll1'UIIUt!U I ,cl f

Zebra mussel populations elsewhere m the Illinois River and upper Mississippi River arc building up gradually, rather than explosively, and oxygen levels are not yet low. In the Illinois River at Peoria, the populations increased from about 2.000/m2 in 1993 to about 6,000/m2 10 July I 994. At Rock Island on the Mississippi, divers found only 1-2 zebra mussels/m' in July, 1994, but the numbers averaged 48/m2 in August. At another site 240 miles down­stream, divers found no zebra mussels. These gradual, patchy increases are probably attribut­able to sporadic low dens11y settlement of larvae

Wild swings in zebra mussel populations and environmental conditions will be very hard on native aquatic life. These swings could threaten sanctuar­ies ar.d set back years of recovery attnbutable to improvements in water quality. In the upper Illinoi\ River zebra mussels could threaten five native mussel species that recently recolonized an area where no mussels could exist in 1969. Over thirty nat1ve species, including the federally endangered Higgin's-eye (Lampsilis higginsii) and federally threatened spectacle case (Cumberlandia monodonta) occur in mussel sanctuaries the Illinois Depart-

,-------------------------. ment of Conservation estab

Ir--~~"' "~ s'!l:.:/J ... _~ \ ~~~~\• Lake lishcd on the upper Mississippi ,. · ( .._ I Michigan

·,,." t., -'--(" River, where zebra mussels are '" ' · iJ N=• ' ' d 11 . . ' 0t-ooA 'i Ct.cago .... gra ua y mcreasmg. ~ ( "- The experience with

'---. I introduced aquatic pests such M-Pl)I~...._,

"" - • as the common carp, the sea

>

---Map of the Chicai:11

area warn-wan cm111ecti11K Lake

Afichii:an to the ll/inoi.1 Rna. Inver. map of

North America. with tlu Mis.1imppi 1111d Great

l.11kcs/St. L,.1wre11n• drai11age1. (Map.

courtesy of D011gla.1 BIOlfl/clt. JNIIS Cell/er for Aquatic EcoloKY·)

drifting downstream from relatively small populations of adults that were carried into upstream areas by boats.

The massive settlement in the lower Illinois River must have originated from much denser populations of adults somewhere in the upper lllinois River or perhaps southern Lake Michi­gan. If events elsewhere follow the pattern m the lower Illinois River, these explosions will be followed by crashes attributable to competition and deteriorating environmental conditions.

lampre}, and no\\ the zebra mussel should convince anyone that 11 is far more effective to close the barn door before all the horses get out. Legislation now requires ocean-going ships to discharge freshwater ballast while they are still in salt water, before they reach the Great Lakes. It remains to be seen how effective this legislation will be; it certainly depends on strict compliance, monitoring, and enforcement.

Introducing nonnative mussel-eating fish to consume zebra mussels is highly questionable because these fish may prey on native snails, clams, and juvenile mussels before they get to the zebra mussels. This is unnecessary because there are several species of native fish and ducks that consume zebra mussels.

2

Whether introduced or native, mollusk-eaters are unlikely to eliminate zebra musseb because predator populations are limited by more than just the food supply; spawning or wintering habitat may be more critical, for example.

We also do not know whether mtroduced species carry pests and diseases of their own, or serve as supplemental hosts for native parasites. In either case, the incidence of parasitism and disease in native fish and mollusks could increase. When the Asian grass carp (Ctenoplzaryngodon idel/a)

was intentionally introduced by the Arkansas Game and Fish Commission to control aquatic plants, it carried a paras1t1c Asian tapeworm that infected the nalive red shiner (Notropis

lwrens,s). Although no one wants to

block pest invasions b} restor­ing the pollution barrier at the Chicago gateway. serious attention should be given to reducing their access to the M1ssiss1ppi system. Other options include ultrasound, thermal, or chemical barners in the lock chambers between Lake Michigan and the Chicago waterways, or in locks and constricted points further downstream. A side benefit of the dispersal barrier might be to reduce the number of zebra mussel larvae that are now seeding the lower Illinois River, 1f m fact the larvae are coming from the canal system or Lake Michigan.

Richard Sparks, wirlr Scoff \V/11tney,

Jwnes Stace kel, Eric· Ratcliff. Steren

Stm~el. R111/r Spark 1. Dou~la1

Hlodxe11. f.1/en Mar.11/en, and Daniel

Sc1111eida. Center for Aquatic Eco/og1•, 1111d with injom,atum from \1.S. itillur

a11d Ramlr l. Esh,•nroder, Gr,•ur lake.\

F11hery Cm11mr,sio11.

Larval Fish Feeding and Growth Virtually all species of fish consume zooplank.ton during part or all of their early develop­ment. Their chances of survival may in part be determined by the availability of suitable zooplank.ton prey during the first few weeks of life. This may be the case for the walleye (Stizostedion vitreum) which grows to be a large fish-eating predator, but when young relies on several taxa of zooplank.ton prey items. Most of the common species of freshwater zooplank.­ton in North America fall into three taxonomic groups; cladocerans, copepods, and rotifers. Cladocerans are round­bodied and relatively slow moving, frequently with spines, while copepods are more slender and move with a quick darting motion. Rotifer shapes vary greatly, but they are always small in relation to the other two taxa (see drawing). The differ­ences in shape, size, and overall abundance of prey may interact with the size of fish to determine which sorts of prey the walleye choose to eat. The prey which young walleye consume may in turn affect the fish's ability to grow and survive.

We conducted experiments in the laboratory and in field enclosures to examine the effects of prey taxa, size, and abundance on the foraging and success of young walleye. We divided the fish into three size groups representing large changes in morphology and behavior: small larvae (8-10 mm); large larvae (11-18 mm); and early juveniles(> 19 mm). In order to determine whether young walleye selectively consume certain types of prey,

we let them feed on mixed assemblages of zooplankton which included a variety of taxa and sizes across a range of densities. All sizes of fish avoided eating rotifers, even when they were abundant. Both taxa and size of prey appear to be important in determining prey selection, though their effect differs with size of the fish. In both laboratory and field enclosure experiments, small larvae selectively consumed medium sized (0.4-0.9 mm) cladocerans and avoided large (> 0.9 mm) prey of both cladocerans and copepods. Large larvae also preferred medium sized cladocerans, but selected against small ( < 0.4 mm) prey of both taxa. Early juvenile fish selected prey based primarily on size, selecting against all small prey. We found that prey taxa and size, but not density, interact with the size of the fish to influence selection of prey.

In order to test the advan­tages of different prey taxa for different sizes of walleye, we reared fish in the lab with either cladoceran, copepod, or rotifer prey. Though we did not detect differences in growth rates when fish were raised with different taxa of prey, we did see differences in survival rates. Both small larvae and large larvae survived better when they consumed cladoce­ran rather than copepod prey. Early juvenile fish fared equally well with all prey taxa. However, both size classes of smaller fish selected for medium cladocerans and had increased survival with cladoceran prey, indicating that

3

prey selection may lead to better success.

The differences in body shape of cladocerans and copepods may help explain patterns of selectivity and, hence, the advantage conferred by cladoceran prey on smaller fish. The round shape of cladocerans may be easier for smaller fish to detect. However, the greater width of cladocer

0.6

Cladoccran

1.0

0.3

Copepod

ans, especially large individuals, may impede consumption by smaller fish. Laboratory experiments in which we recorded walleyes attempting to feed on two sizes of copcpod or cladoceran prey supported this idea. When large larvae fed on large cladoceran prey, they frequently and repeatedly turned towards prey though they were very inefficient at capturing these prey items. Apparently, the large cladocerans provide a strong visual stimuli to small fish, inducing them to attack more frequently.

We found that taxa and size of prey interact with the size of the fish to influence prey selection. Also, we saw that prey type may affect survival in young fish, and may therefore influence the success of entire year classes of fish.

Christine M. Mayer and David H. Wahl, Ce111er for Aquatic Ecology

l.O o/

Rotifer

Shapes and relative si;:.es of three ta.rn of

:ooplankton used in the walleye feeding studies. ( Drawing, courtesy of Christine M. Mayer,

!NHS Center for Aquatic Ecology.)

Cross-section of a Scotch pine branch

infested with the larger pine shoot beetle.

lllinois counties quarantined for pine

shoot beetles. (Map. courtesy of

Charles G. Helm. /NHS Center for Economic

Entomology)

New Exotic Insect Pest of Illinois Pines

In July 1992 nursery inspectors discovered a peculiar beetle damaging the fresh shoots of pines in a Christmas tree

plantation near Cleveland, Ohio. Specimens of these beetles were submitted to specialists from the Animal and Plant Health Inspection Service (APHIS) of the U.S. Department of Agricul­ture who eventually identified them as the larger pine shoot beetle (Tomicus piniperda), a serious pest of pines in Europe. Alerted by APHIS officials, state and federal agencies began conducting surveys of pines throughout the Great Lakes region and were both surprised and alarmed to discover infesta­tions of this exotic pest in pines not only in Ohio, where it was first discovered, but also in Pennsylvania, New York, Michigan, Indiana, and several counties in northeastern Illinois. A federal quarantine restricting the movement of Christmas trees, nursery stock, and any pine lumber with intact bark was imposed by November 1992 to

attempt to prevent the spread of this new pest into major pine production areas.

Since its discovery in Ohio, Illinois Natural History Survey researchers have been cooper­ating with Illinois Department of Agriculture nursery inspec­tors and federal Plant Protec­tion and Quarantine officials to determine the extent of pine shoot beetle distribution throughout the state and to assess the potential impact beetles might have on Illinois pines. Like all bark beetles, the pine shoot beetle breeds and lays its eggs in a system of tunnels, or galleries, beneath the bark of recently cut logs and stumps, or in dying pine trees. Larvae hatch and feed under the bark from April to June, then tunnel out of their feeding galleries as new adults from July through October. As the name implies, newly emerged adults are attracted to fresh shoots of healthy pines. The beetles bore into the center of this new growth and tunnel toward the shoot tip. Affected shoots droop, tum yellow, and may fall from the tree. Al­though adult beetles are only 3 to 5 mm long (about the size of a match head), they have the potential to remove large quantities of foliage when their populations are high, thereby affecting both the height and diameter of trees. Fortunately, shoot damage is presently causing only minor injury to trees in Illinois. After a summer of feeding within pine shoots, adult beetles leave the shoots and move inside the thick bark at the base of pines to spend the winter. Overwin­tered beetles leave these

4

protected sites in early spring, lay their eggs, and begin the cycle again. These same adults may again move into pine shoots before they expire in the spring. Therefore, there is the possibility of a single genera­tion damaging pine shoots twice.

The current infestations of pine shoot beetles are thought to have arrived from Europe in wood dunnage used to stabilize cargo on trans-oceanic ships since this pest has previously been intercepted in such materials at U.S. ports of entry. Because the pine shoot beetle was able to become established in the northeastern U.S. with such apparent ease, researchers began to question whether other foreign bark beetles might also become established without their knowledge.

The pine shoot beetle was the only exotic species detected during the 1994 survey, resulting in three new pine shoot beetle infested counties. Illinois now has ten infested and quarantined counties. Most of the beetle finds have been at Christmas tree farms, pine plantations, or nurseries. Although "quarantine" may have ominous connotations for some, those purchasing Christ­mas trees need not be alarmed or concerned that this beetle is harmful to humans or house­holds in any way. On the other hand, commercial growers unable to ship trees because of quarantine restrictions, have certainly faced hardship as a result of this new pest that behaves like the Christmas grinch. Charles G. Helm and John E. Lloyd, Center for Economic £11tomology

Richard Cahill

sampling water at

Bluff Spring Fen

Effects of Hydrology and Water Chemistry On Wetland Vegetation Zones Survey staff working with Ri­chard Cahill of the Illinois State Geological Survey and Douglas Shaw of the University of Illi­nois Department of Civil Engi­neering are monitoring rela­tively undisturbed representa­tives of several emergent wet­land types (marshes, sedge meadows, and wet prairies) in­cluding Grass Lake Marsh, Wadsworth Prairie, West Chi­cago Prairie, and Collison Marsh.

The researchers are studying the influence of hydrology and water chemistry on vegetation zonation in these wetland com­munities. The studies will pro­vide useful baseline informa­tion for evaluating changes within the communities sampled over time, for creating models for wetland restorations, and for evaluating natural fluc­tuations within the wetland sys­tem when making management decisions.

Emergent wetland communi­ties include marshes, sedge meadows, wet prairies, seeps,

and fens. Plant species in most undisturbed, emergent wetland communities assemble into zones in response to subtle dif­ferences in the water levels and water chemistry at a site. For example, plants such as hard­stem bulrush, bur-reed, and ar­rowhead are usually found in deeper water than marsh milk­weed, tussock sedge, and prai­rie cordgrass. Other plants such as angelica and grass-of­Parnassus are found in more

mineral-rich water than fowl manna grass and Joe-Pye-weed. Because rainfall in Illinois is extremely variable, water levels (hydrology) and water chemis­try vary over seasons and years in natural, undisturbed wet­lands. Consequently, these veg­etation zones may move from year to year in response to changes in hydrology and water chemistry.

Natural areas managers, at­tempting to protect these com­munities and the species that inhabit them, must be able to identify threats to community

5

structure and diversity, and manage the community appro­priately. Ecologists attempting to restore wet prairie and sedge meadow communities must have info1mation about the presence and abundance of characteristic plant species, wa­ter levels over time (hydroperiods), and water chemistry pro­files and variation in order to set goats and success criteria. The wetland manager or restorationist is often making decisions without adequate information about

~ the hydrology, water ] chemistry, and veg­~ :;; elation for natural ~ " emergent wetland ,2

" ~ = ~

systems. Researchers found

"' ~ dramatic differences in the veg-f elation components of various in zones within emergent commu­} nities, based on water level £ changes of only a few inches. E I Water chemistry at many of the

sites, including the permanently groundwater-fed seeps, varies seasonally and geographically. For example, sodium levels were higher for sites in the northern portion of the state, whereas sulfates were higher for sites in the central portion of the state. Levels of some wa­ter chemistry parameters (sul­fate, phosphorus, and nitrate) were higher than current gen­eral-use water quality guide­lines. These guidelines are sometimes used as standards for wetland mitigation projects.

Seo/I Simon and Marilyn Morris, Cemer for Wildlife Ecology, Dan Schneider, Cemer for Aq11aric Ecology

Marsh Phlox

During May the warm spring temperatures coax developing praying mantis eggs in their papier mache-like egg cases to hatch. The young, looking like miniature adults, wriggle out and hang head downward on a silken thread. Baby mantids are honey yellow in color, about the size of a mosquito, and vora­ciously hungry! There is no sibling bonding here, brother or sister is likely to become a first meal. For each 1,000 mantids

it is about 2 inches long and variable in color. The mantis most people encounter, however, is the introduced Chinese mantis, measuring at least 3 inches long and brown or green in color.

Praying mantids have slender e longated bodies, with strong hind legs. The protho­rax, the first thoracic segment, is long and tubelike. The front legs, found on the prothorax, are long, equipped with spines, and are habitually folded like hands in prayer. Mantids have a triangular face with two

· bulging compound eyes and a free swiveling head. These are the only insects that are able to look from side-to-side and over their shoulder.

forelegs clasped and held out like the arms of a praying monk, inspired the common name praying mantis. The name "preying" mantis would be more fitting as the insect is not waiting for inspiration from above, but insect prey to satisfy its hunger--cven when it is stuffed it waits for more. From the moment of hatching, mantids are carnivorous and ready to battle with most any insect foe. Mantid prey includes grasshoppers, crickets, beetles, bumblebees, wasps, and even other mantids. The mantis grasps prey in a deadly embrace with its heavily spined forelegs, uses its mandibles to cut into the flesh, and then eats the "victim" alive.

Don't be fooled by the Praving Mantis' spiritual pose, it fa probah/~ more read,· to attack

tha11pra) .

that hatch, entomologists estimate that only two will survive to become an adult.

Illinois has one species of native praying mantis, Stagomantis carolina, the Carolina mantis. Found in the sou them two-thirds of the state,

Mantids are masters of camouflage. The Carolina mantids are grey-black, brown, or green, allowing them to resemble tree bark, grass stems, or twigs. These colors and patterns enable them to blend with the natural surroundings and make themselves harder to see for both their insect prey and their enemies, such as birds.

The ability to hang motion­less for hours at a time with its

Mantids overwinter as eggs, hatch in the spring, and spend the summer eating and growing. In early autumn they mate and the female hangs head down­ward on a weed stem or twig and deposits two hundred or more eggs inside a frothy mass that soon hardens into a tough waterproof egg case. This egg case is called an ootheca and acts like insulation to protect the eggs throughout the winter.

Teacher's Guide to "The Naturalist's Apprentice" (facing page)

Objective:

Materials:

students learn some of the diverse places insects lay their eggs to repro­duce multiple copies of Where the Eggs Are

Vocabulary: reproduction, nurture, complete metamorphosis, incomplete metamor­phosis

Comments: The modes of insect reproduction are nearly as numerous as the kinds of insects that exist on earth. In this issue of The Naturalist's Apprentice we explore egg-laying and the way a few insects treat their young. Parental care in insects ranges from nonexistent (walking sticks merely drop their eggs to the ground while walking along) to a high level of nurturing (bees and wasps have a highly developed social structure and adults feed young until they arc grown). Along the way we find insects whose young are cared for by parents for awhile, but if they stay around too long, they may

become a meal for a parent (earwigs), or other species that produce live young that stay near the mother, soon reproduce themselves. and eventually form large colonies (the aphids). In general, if the mind can conceive of a method of reproduction, the insects have likely incorporated it into some bizarre and wonderful life style, somewhere on earth. Procedure:

1. Introduce the subject of insect reproduction/egg laying by using the examples given in the comments section and in Species Spotlight.

2. Distribute copies of Where the Eggs Are and have students match the description in column 1 (next page) with the correct egg picture in column two with the correct adult picture in column three. Answers: A-5-V, B-3-Y, C-4-X, D-2-Z, E-1-W

3. Have students list other places where they think insects might lay their eggs. What food would these newly hatched insects likely feed on? Which insects in the exercise have incomplete metamorphosis; complete metamorphosis?

6

Study the three columns below and choose one item from each column to fonn a complete description of various types of egg-laying insect behaviors.

Column 1

A. I lay my eggs in the soil in a group called a pod. My close relatives, the katydids, usually lay their eggs in a slit in a plant stem.

B. My eggs are protected from most predators that happen along with a hungry stomach. When I'm grown, I have lacy wings and long antennae.

C. When my babies hatch they eat the insides of other insects. I put my eggs where the babies can find an easy meal. My relatives are ants and bees.

D. My young must search for their food, so I lay my eggs close together on a plant where they can find their favorite food, the aphid. I belong to the largest group of insects, the beetles.

E. I drop my eggs wherever I may be. I do put them in a nice, neat package, though, like my relative the praying mantis.

Column2 Column 3

1

w

"The Naturalist's Apprentice" presents educational activities for middle school students. Teachers are invited to photocopy this page for classroom use.

ILLINOIS NATURAL HISTORY SURVEY

607 East Peabody Drive, Champaign, Illinois

61820USA

ll/i11oi., Na111ral History S11ri-ey Reports is published bimonthly by the Illinois Natural History Survey, 607 East Peabody Driw. Champ,1ign. IL 61820. Hcad4uar1cred on the campus of the University of lllinois at Urbana-Champaign, the Survey is a division of the Illinois Department of Energy and Natural Resources.

!NHS Repon, is edited by Charlie Warwick and printed on recycled and recyclable paper. Design by Otto-Walker Communications.

Fingerprinting Bacteria The bacterium Bacillus thuringiensis, subspecies israelensis, is the only microbial insecticide registered in Illinois for use against mosquitoes which utilize waste tires as larval habitat. This bacterium is commonly referred to as "Bti." Bti produces a protein crystal that is toxic to larval mosquitoes when ingested, but is harmless to nontarget insects, fish, birds, and mammals. The Medical Entomology Program is inter­ested in detennining if there are native strains of Bti in Illinois, and if so, detennining their distribution within the state. The answer to this question is complicated by the widespread use of Bti within Illinois by Mosquito Abatement Districts as well as individuals. If we recover Bti from mosquito habitat, how do we know that it wasn't sprayed there by man? The answer to this last question lies in the composition of the envelope surrounding the bacterial cell.

Using a technique know as "gas liquid chromatography" (GLC), in collaboration with Dr. A. Ray Smith of the Ul's Department of Veterinary Pathobiology, we concentrate on major building blocks of the cell envelope - fatty acids. GLC

enables us to identify both the fatty acids that comprise the cell envelope, as well as their percent composition. Each strain of Bacillus thuringiensis has a unique pattern, analogous to a fingerprint, which we refer to as a profile. Profiles can be compared by a variety of statistical techniques. Conse­quently, when we recover Bti in the field, we can compare it to our library of commercial products and determine if it is a native strain. To date, we have discovered seven strains of Bti native to Illinois that are toxic to the following vector species: Aedes aegypti, Aedes albopictus, Aedes triseriatus, Culex pipiens, and Culex quinquefasciatus. These discoveries have come from material collected from flood plains and waste tires. In fact, waste tires appear to be excellent collectors of Bacillus thuringiensis, perhaps because bacterial spores settle into tires from the air, or the spores are present in dirt deposited within tires.

In the next phase of our re­search, we will concentrate on whether the presence of native strains of Bti within a tire affect the ability of mosquitoes, which serve as vectors for human and

Non-Profit Org. U.S. Postage

PAID Champaign, lL 61820

Permit75

Bacillus thuringiemis ( Bti) at JOOOX (oil immersion lens),

usmg phaie contras/ illummatwn

animal diseases, to become estab­lished. As a secondary benefit, our profiles will help us deter­mine the usage pattern of com­mercially produced Bti within Illinois, as well as its persistence. Information of this nature is es­sential to answer the seemingly simple question of why mosqui­toes are present in tire A and not tire B, when both tires seem iden­tical to us. As a sidelight, we can identify the three subspecies of Bacillus thuringiensis used in agriculture and determine their persistence in the environment, as well.

Joel P. Siegel and Robert J. Novak,

Medical Entomology Program. Cemer

for Economic E11tomology, l/linois

Natural Hi5tory Survey,

A. Ray Smith. Departme fll of

Veterinary Pathobiology, College of

Veterinary Medicine, Univenity of

Illinois at Urbana-Champaign