PROGRESS REPORT~~SFRI SOUTHERN FOREST RESEARCH INSTITUTE PUBLISHED Bl-MONTHLY JANUARY - FEBRUARY 1972 UNDER INVESTIGATION- Thanasimus dubius-an Important Predator of the Southern Pine Beetle Among the insect enemies of the southern pine beetle, Thanasimus dubius may not always be the most abundant; howeve,:-, this clerid beetle is the most obvious predator and very likely a most effective one. Good reasons ex.ist for making this assumption: both adults and larvae are predacious and decimate all stages of their prey from the egg to the full_y grown beetle. Furthermore, the pred­ ators maintain an amazing degree of spacial and temporal coincidence with their prey, and particularly so with southern pine be~tle populations. These observations are not new. In fact, early investi­ gators gave considerable attention to the clerid predators as potential control agents of the southern pine beetle. The species also has an T. dubius feeding on a important dictinction. In southern pine beetle 1892, A. D. Hopkins imported about 2,000 specimen of the European species Thanasimus formicarius from Germany for re­ lease in West Virginia. The insects failed to become established, but this effort now marks a historical event: the first importation of a predator into the United States for the biological control of a forest insect. In another early report published in 1908, W. F. Fiske reported in the proceedings of the Entomological Society of Washington, D. C. (p. 23-27) that it was not uncommon to find southern pine beetle infested trees in which the brood had been largely destroyed by T. dubius. He assumed that clerid predators were capable of reducing epidemic levels of their prey species but undertook no specific investigations. R. C. Thatcher and L. S. Pickard (J. Econ. Entomol. 59: 955-957 .1966) first reported the much needed information on the biology and habits of T. dubius. We now know that the clerid may have 2½ to 4 generations annually in the Gulf Coast region versus 6 to 8 for the southern pine beetle. Studies reveal that the adult predator, during a life span of 5-10 weeks, destroys at least 75 southern pine beetles under experimental conditions. Similarly, a single predator larva may destroy more than 100 immature stages of the bark beetle. However, both are probably less effective under field conditions since they must search for the prey before feeding begins. The adult predators are extremely active and intercept many attacking beetles before they bore into the bark. "In the field, the life cycles of T. dubius and the south­ ern pine beetle were found to be closely synchronized at the be­ ginning of an attack. As the southern pine beetles landed on the bark, the adults of T. dubius moved about rapidly catching and feeding upon them and mating and laying eggs." Numerically speaking, the laboratory experiments show that at least 15 clerids were necessary per 100 southern pine beetles to prevent attack on caged bolts. Clerid larva (15x) -2- In 1968, the discovery of frontalin as the attractant of the southern pine beetle by SFRI scientists revealed the amazing phenomenon which allows the predators to arrive simultaneously with their prey on newly infested pine trees. The secret was simple. The predators, in their search for food and mates, use the same odorous clues as their prey. Employing synthetic frontalin, SFRI re­ searchers experimentally demonstrated that T. du­ bius pursues olfactorily the aggregation of the southern pine beetles and intercepts the prey dur­ ing the short period of ambulant life (J. Insect Physiol. 16:233-239. 1970). This demonstration of the spontaneous re­ Mid-stage larva of T. dubius coin­ sponses of both sexes of T. dubius to the attrac­ cides with last stage of southern tant of the southern pine beetle exerted an impor­ pine beetle (2x). tant bearing on contemporary control strategies. It disclosed that control procedures using insecticides on felled pine trees infested with the southern pine beetle inadvertently became more effective against the insect enemies of the pest than against the pest itself because the felled trees con­ tinued to attract predators and parasites following application of the insecticide. (J. Econ. En­ tomol. 64:1440-1444. 1971). On the other hand, the manipulation of an important predator with attractants became a distinct possibility. This, in turn, prompted SFRI scientists to en­ deavor to clarify a number of questions relating to the potential manipulation of T. dubius populations. Two questions in particular arise: What is the actual impact of T. dubius populations on the population dynamics of the southern pine beetle? And, what is the impact of associated in­ sects on the predator population itself? There is no doubt that T. dubius can and does contribute to a decline in southern pine beetle populations; the question remains as to the degree of effec­ tiveness exerted by the predator. Are the predators actually capable of achieving, by themselves, a major reduction of the pest? Some entomologists argue that T. dubius cannot be a highly efficient biological control agent for the southern pine beetle because its life cycle appears poorly synchronized with the prey, and because the clerid diverts its activities to other bark beetle species and is preyed upon by insects such as Temnochila uirescens. As a matter of argument, the life cycle and behavior of T. dubius could also be considered extremely well synchronized with that of the southern pine beetle because their longer life span makes their predation more efficient. The adults of pest and predators arrive on the same tree simultaneously and deposit their eggs. The predators remain to attack the adult pests as long as the pests continue to arrive on the trees and follow the pests to new host trees. The young predator larvae feed on the eggs and young larvae of the pest species while the mid-stage and older predator larvae attack fully grown larvae, pupae and cal­ low adults of the pest. Termination of the feeding period of the predator larvae coincides with the emergence of the south­ ern pine beetle brood. In fact, the predator goes through the non-feeding stages of its development only in the absence of the southern pine beetle. Therefore, of necessity the larval de­ velopment of the clerid surpasses that of the prey by several weeks, and one predator generation roughly covers two pest generations. Furthermore, a model of the predator/prey rela­ tionship may show that the T. dubius populations compensate the developmental delay extremely well through their larger biomass, their prolonged span of adult life, and their reproduc­ tive capacity which is approximately tenfold that of the south­ ern pine beetle. Similarly, it could be argued that the existence of alternative prey species such as fps spp. is rather advan­ tageous for sustaining endemic predator populations and will not hinder the effectiveness of the predator on southern pine Temnochila virescens (4x) beetles unless they become overly distractive. -3- 229 North Bowie Street JASPER, TEXAS 75951 SFRI SOUTHERN FOREST RESEARCH · INSTITUTE " This appears unlikely, as present SFRI sponsored studies indicate that none of the various pheromones recently isolated and identified from lps grandicollis and lps calligraphus provide the efficiency in attracting T. dubius as does frontalin. The clerid shows some preference, how­ ever, for trans-verbenol, a pheromone released by a large variety df bark beetle species in contact with fresh, resinous host material. This likely provides a clue as to secondary sources of food for the clerids in the absence of the favored prey species. SFRI investigators began using frontalin baited sticky traps in 1969 to study population trends in the southern pine beetle and the effectiveness of such traps for population surveys. They were immediately impressed with the large proportion of T. dubius captured by the traps when compared with the number of southern pine beetles (Figure). As a possible measure of their impact on the pest population, the following trend becomes evident. Clerid populations remained high during 1969 at a time when the southern pine beetle epidemic appeared to col­ lapse. Predator numbers continued at high levels through the spring of 1970 while the pest - population declined rapidly. Subsequently, the predator population-declined- drastically, and the pest species began a steady increase. Figure - Relative numbers of southern pine beetles (SPB) and its clerid predator, T. dubius, caught on frontalin % baited survey traps during spring (Sp), summer (Su), fall (F), and winter (W) 1969-1971 [n=111,349]. 100 0 ------- o SPB . J) ... "" * PREDATOR* I "" * I '°O, I I "'"'O }I ,,, ,,, 75 I * o~--- ~~~if 60 45 >( X\, :\ ~ I • / 1c 30 0 • , -----* ... ,, , '\ 15 •., , * 0 Sp Su F w Sp Su F w Sp Su w [ 1969 ][ 1970 ][ 1971 ] PROGRESS REPORT, 229 N. Bowie, Jasper, Texas 75951 Jan/Feb 1972 -4- PUBLICATIONS ON SFRI SPONSORED PROJECTS AND RELATED RESEARCH - 1971 (II) J. P. Vite and J. A. A. Renwick. Inhibition of Dendroctonus frontalis response to frontalin by isomers of brevicomin. Naturwiss. 58:418. 1971. The olfactory response of flying male and female southern pine beetles, Dendrocto­ nus frontalis Zimm., to the female-produced attractant, frontalin, was inhibited under field conditions by an analogue, endo-brevicomin. Tests in the forest demonstrated that the at­ traction of D. frontalis to traps baited with frontalin and alpha-pinene was halted or dras­ tically reduced, but the response of its predator, Thanasimus dubius F., was unaffected by the addition of this material. (See: SFRI PROGRESS REPORT 1971:9-10.) J. P. Vite and J. A. A. Renwick. Population aggregating pheromone in the bark beetle, lps grandicollis. J. Insect. Physiol. 17:1699-1704. 1971. Males and females of the bark beetle, Ips grandicollis, aggregate on host material in response to 2-methyl-6-methylene-7-octen-4-ol, a pheromone characteristic for five-spined fps species.