15 Formulation and Structure of Residual
Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 31, 2018 | https://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0086.ch015
Insecticides for Bark Beetle Control R O B E R T L. L Y O N Pacific Southwest Forest and Range Experiment Station, Forest Service, U. S. Department of Agriculture, Berkeley, Calif.
The potential of "surface" and "tissue" (subsurface) deposits of synthetic organic insecticides to control bark beetles was investigated. The contact toxicity and deposit structure of surface deposits on fiberboard panels of six insecticides were studied in the laboratory: DDT, dieldrin, dinitrocresol, endrin, heptachlor, and lindane. Each insecticide was characterized by spontaneous crystallization after application in solution form. In field studies of lindane and dieldrin, tissue deposits appear to offer the greater promise. They were slightly but consistently more effective than surface deposits in killing beetles in infested bark and in protecting green logs. Fundamental research is needed on tissue deposits to clarify their properties and behavior in the bark and their effect on the bark beetle. Only then can formulation be lifted from its present empirical state.
•hemical control of the primary forest insect pest—the bark beetle— ^ is achieved mainly by spraying insecticide on the bark of the host tree. This strategy is dictated by the behavior of these insects, which spend all their life in or under the protective cover of bark except for a brief flight period as adults. Control is defined as (1) suppression or the destruction of beetle populations in trees or logs (postattack sprays), and (2) the protection of green trees or logs from beetle invasion (preattack sprays). Broadcast sprays as used on defoliators have not proved feasible. Systemic insecticides, though theoretically appealing, have not been successful in combating these pests (23, 35,38). Bark sprays should be used only after a thorough evaluation of the need (24). They are expensive and must be skillfully applied to be 192 Van Valkenburg; Pesticidal Formulations Research Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 31, 2018 | https://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0086.ch015
15.
LYON
Insecticides for Bark Beetle Control
193
effective (46). They are best suited for use on valuable trees which occur on cabin sites, in naval stores stands, of in seed production areas. Historically, the first bark sprays to be used widely were fumigants (18, 41). Called penetrating sprays, they are applied in high volumes (2 to 6 gal./100 sq. ft. of bark surface) to penetrate the bark and kill the bark beetles in their galleries. Through the years the fumigant ethylene dibromide in oil or water emulsion was developed and adopted for suppression of several bark beetle species as a penetrating spray ( 15, J9, 29, 30,42,45,47). The development of synthetic organic insecticides in the 1940's and thereafter stimulated research on their use as an alternative to the more expensive penetrating sprays. The promise of these modern insecticides lay in their potential for use at low volumes as postattack sprays for suppression but also, because of their residual properties, as preattack sprays for protection of green trees or logs (32). A recent review (27) indicates that the single insecticide lindane, either pure or as the active ingredient of benzene hexachloride (BHC), is the preferred chemical for bark beetle control. EDB (ethylene dibromide) is still used in some places, but such penetrating sprays are being replaced by the residual organic insecticides. This trend probably will continue. Indeed, lindane (or BHC) is gradually becoming a "universal" chemical for bark beetle control. Comparison of Formulations Our present understanding of formulation of the synthetic organic insecticides for bark beetle control is sketchy. The literature reveals only a few examples of formulation research. Field studies have emphasized the chlorinated hydrocarbon insecticides. Compared with other compounds, lindane has usually performed well in the field (J, 14, 20, 40, 43) and has ranked at or near the top in laboratory tests of contact toxicity (13, 26, 27, 31, 32, 33, 39). It has received more attention than any other insecticide, and most of the discussion that follows, by default, pertains mainly to this chemical. The very success of lindane has probably played a leading role in discouraging research on formulation of bark sprays. In a few studies, the major types of formulations have actually been compared in the same test at the same time. When this was done, solutions in diesel oil or No. 2 fuel oil, without exception, have been superior to emulsions (JO, 22, 40,43). Wettable powder suspensions were reported to be least effective in one study (40) and intermediate in another (22). A single report of the use of a dust (BHC) showed it least effective of all (22).
Van Valkenburg; Pesticidal Formulations Research Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 31, 2018 | https://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0086.ch015
194
PESTICIDAL FORMULATIONS RESEARCH
At a given spray concentration and application rate, solutions are more reliable. Emulsions of lindane usually work well enough, however, if applied in larger volume or at higher concentration. Elimination of the oil carrier reduces the cost and also the hazard of phytotoxic reactions where living trees are sprayed for protection. Several early studies report the excellent control afforded by lindane emulsions (4, 5, 12, 14, 20, 23), and reports on the excellent performance of lindane emulsions on a wide assortment of bark beetle species have recently grown (6, 7, 8, 9, 10, 17, 37, 40, 44). Massey (28) has reported on the successful use of an emulsion of DDT to protect living trees. Formulations of lindane now used for various bark beetle species are quite similar. They are usually 0.5-1.5% (w/w) and are applied either to bark wetness or to runoff, amounting to 1/2 to 2 gallons/100 sq. ft. of bark surface. The higher volumes are more often used ( application to the point of runoff) and give only a slight cost advantage over penetrating sprays. This procedure does not fully exploit the low volume potential of lindane and other residual insecticides. Doane ( 14) reported that lindane killed 98% of emerging elm bark beetles [Scolytus multistriatus (Marsham)] when a 6% emulsion was applied at the low volume of less than 1 pint per 100 sq. ft. of bark surface. Moore (32) found in preliminary tests with fogging equipment that a light coating of 6 or 12% DDT or 10% lindane gave very high kills of bark beetles. We know of no examples in the literature of a concerted study of the potential of such low volumes. Deposit Structure
A crucial omission in the research on residual insecticides for bark beetle control has been the absence of corollary studies relating control effectiveness to the physical structure of insecticide deposits on and in bark. The importance of deposit structure has been well documented (3, 21, 36). The structure of the deposit affects its availability to the insect contacting it and governs its toxicity. On an absorbent surface like bark, two major types of residues can be created: deposits on the surface and deposits in the bark tissue. Henceforth, these will be referred to as "surface" and "tissue" deposits, respectively. Dusts and wettable powders form surface deposits. Solutions and emulsions penetrate and form mainly tissue deposits, though they may not remain in the tissue. The insecticide may crystallize out of solution, forming a deposit of fine crystals on the bark surface. A study of the structure of surface deposits of residual insecticides aimed specifically at bark beetle control was undertaken between 1956 and 1962 at the U.S. Forest Services Pacific Southwest Forest and Range
Van Valkenburg; Pesticidal Formulations Research Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 31, 2018 | https://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0086.ch015
15.
LYON
Insecticides for Bark Beetle Control
195
Experiment Station (27). Portions of that work relevant to the present discussion are cited here. Crystal Blooms. A preliminary examination of crystal blooms of selected insecticides on various species of conifer bark (32) soon demonstrated a wide variation and high unpredictability in crystallization patterns. Because of this variation and the difficulty of bioassay with irregular bark surfaces,fiberboardpanels were chosen as the test surface. Fiberboard is not a substitute for bark and does not simulate bark except in a general way and so was used mainly for convenience in studying the potential toxicity of crystal blooms and the broad aspects of crystallization patterns. Acetone was the primary choice of solvent because of the rapid and spontaneous crystallization characteristics of low boiling solvents (3) from which insecticides may crystallize within a few hours after application. The Californiafive-spinedips [Ips confusus (Le Conte)] was used as the experimental bark beetle for most of the study. The probit method of Finney (16) was used to analyze time-mortality relationships. The best deposits of all six insecticides studied—dieldrin, lindane, endrin, dinitrocresol, heptachlor, and DDT—produced 50% mortality with exposure periods of less than 2 minutes ( L T ). All mortality estimates were based on a 72-hour post treatment holding period at about 70°F. and 60% relative humidity. Lindane and dieldrin were the most toxic. One percent solutions produced 50% mortality with less than 2-seconds'exposure (Table I). [1% solutions: weight/volume: 10mg./ml. = 1%; this applies to all further references to concentration.] This left little doubt that surface deposits could be made toxic enough by contact to kill bark beetles in nature even though they may be in contact with the bark surface of their host trees for as little as 1 minute or less. 50
Table I.
Contact Toxicity of Crystalline Deposits on Fiberboard to Ips confusus Exposure Time for 50% Kill (LT ), min. 0
50
Insecticide Dieldrin Lindane Endrin Dinitrocresol Heptachlor DDT
Median
