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Effect of soil conditions and body size on digging by prediapause Colorado potato beetles (Coleoptera: Chrysomelidae) Christine Noronha and Conrad Cloutier
Abstract: The effect of soil temperature, density, and moisture on digging of prediapause Colorado potato beetles (Leptinotarsa decemlineata (Say)) was studied in 1.9 cm diameter soil columns. Beetles dug for between several days and a few weeks before finally resting. Speed of digging peaked during the initial hours and decreased slowly thereafter. A soil temperature near 16°C was the optimum in terms of incidence of digging (95%) and final diapausing depth. The intensity of these two responses was reduced at higher (≤ 24°C) and lower (≥ 8°C) temperatures. When given a choice of four soil densities, 50% of the beetles preferred the lightest soil (0.64 g/cm 3), but 11% chose the heaviest soil (1.14 g/cm3). When a range of nine soil densities was provided in a no-choice situation, digging speed and depth decreased exponentially with increasing soil density, suggesting that energy invested in digging is limited. Peak digging speed during the first 2 h averaged 4.33 cm/h in light soil and only 0.83 mm/h in heavily compacted soil. Dry soil (40–45% water by mass) inhibited digging, while nearly saturated soil (65%) was fully acceptable. Digging speed and depth were maximal in the 50–55% moisture range. Beetle size had a positive effect on digging. For each additional 1 mg of fresh mass, beetles dug for 1 day longer, reached 1 cm deeper in soil columns, and dug 0.5 mm/h faster during the initial burst of digging. Résumé : Les effets de la température, de la densité et de l’humidité du sol sur l’enfouissement de Doryphores de la pomme de terre (Leptinotarsa decemlineata (Say)) en prédiapause ont été étudiés dans des colonnes de sol de 1,9 cm de diamètre. Les doryphores ont creusé de quelques jours à quelques semaines avant de s’immobiliser. La vitesse a été maximale au cours des premières heures, se stabilisant par la suite. La fréquence et la profondeur maximales d’enfouissement ont été observées dans du sol à 16°C, température optimale dans l’intervalle de 8 à 24°C. Lors d’un test à quatre choix à des densités de sol de 0,64 à 1,14 g/cm3, 50% des individus ont choisi le sol le plus léger et 11% ont choisi le sol le plus dense. Lors d’un test sans choix impliquant neuf densités de sol, la vitesse et la profondeur ont diminué exponentiellement en function de la densité, suggérant que l’énergie investie dans cette activité est limitée. La vitesse de pointe au cours des 2 premières heures d’enfouissement était de 4,33 cm/h dans du sol léger, alors qu’elle n’était que de 0,83 mm/h dans du sol très compact. Les sols secs (40–45% d’eau) inhibaient l’enfouissement, alors que les sols presque saturés (65% d’eau) étaient tout à fait favorables à l’enfouissement. La vitesse et la profondeur maximales d’enfouissement ont été observées dans des sols contenant 50–55% d’eau. La taille des doryphores était en corrélation positive avec l’intensité de l’activité. Pour chaque 1 mg de masse fraîche additionnelle, les doryphores ont creusé 1 jour de plus, se sont immobilisés à 1 cm plus profondément et ont progressé à raison de 0,5 mm/h de plus durant les heures initiales d’activité intense . Noronha and Cloutier 1713
The Colorado potato beetle (Leptinotarsa decemlineata (Say)) is the primary insect pest of cultivated potatoes (Solanum tuberosum L.) in North America. Unchecked defoliation by larvae and adults over one or two generations can cause severe yield reduction (Hare 1990). Insecticides have traditionally been used to control the Colorado potato beetle, but the threat of widespread multiple resistance is forcing the development of new options (Roush et al. 1990). Alternative controls, including biological and physical control as well as plant resistance, are being intensively investigated (e.g., Boiteau et al. 1995). However, most of these methods Received September 16, 1997. Accepted April 16, 1998. C. Noronha and C. Cloutier.1 Département de biologie, Université Laval, Cité Universitaire, QC G1K 7P4, Canada. 1
Author to whom all correspondence should be addressed (e-mail:
[email protected]).
Can. J. Zool. 76: 1705–1713 (1998)
more critically depend on a specific knowledge of the beetle’s ecology than does chemical control. The Colorado potato beetle in Quebec and neighbouring regions is close to the northern limit of its geographical distribution in eastern North America. The cycle is mostly univoltine, and diapause preparation in Quebec beetle populations occurs in late July to early August (C. Noronha and C. Cloutier, submitted for publication). These beetles feed intensely for 1 or 2 weeks and, when ready, will either walk to suitable overwintering sites within or along the borders of potato fields or fly to more remote sites (Voss and Ferro 1990; C. Noronha and C. Cloutier, submitted for publication). Having accepted a site where the soil is suitable, beetles will start digging until they reach a final resting position for diapause. Overwintering depth is variable, 10–30 cm being typical, with extremes from 0 to ≤100 cm reported (Gibson et al. 1925; Trouvelot 1935; Breny 1939; Minder and Petrova 1966; Weber and Ferro 1993). Mortality during the winter can be high (e.g., Ushatinskaya 1966b; Harcourt 1971; Lashomb et al. 1984; Cloutier et al. 1992). Several © 1998 NRC Canada
1706 Fig. 1. Soil-containing tubes for studying digging by prediapause Colorado potato beetles. (A) Clear acrylic tube holder with netting to confine the beetle on top of the soil column. (B) Tube holder with one 10 cm long section of black tubing packed with soil and observation ports 1 cm apart. (C) Same as in B, but with two soil-containing sections. (D) View of a potato beetle descending though the soil column and collapsing observation holes as it passes them in sequence.
Can. J. Zool. Vol. 76, 1998
ditions is limited, beetles would be forced to dig in suboptimal sites or to rest at less suitable depths. Despite the importance of Colorado potato beetles’ responses to external factors when they are preparing for overwintering in cold climates, a clear understanding is impossible to obtain from available reports, mainly because factors studied were often not precisely defined or were considered among a complex of varying conditions. We report here a study of the effects of soil temperature, soil density, and soil moisture on the digging behaviour of the Colorado potato beetle. Beetles may be using these soil variables as cues for maximizing their chances of winter survival. Alternatively, beetles may be constrained by such variables while digging, so their choice of a final resting position is limited and eventually their survival is affected. If such effects are significant and predictable, they should be known, as this may facilitate an understanding of the relationship between overwintering mortality and microecological conditions, and in turn allow better pest-management strategies against the Colorado potato beetle to be devised.
Beetles A Colorado potato beetle colony was established in 1993 using adults collected in potato plots at Laval University’s experimental farm in Sainte-Croix-de-Lotbinière, Quebec. They were reared in a glasshouse at 22°C (±2°C) and under a 16 h light (L) : 8 h dark (D) photoperiod. For digging tests, newly emerged beetles were put in a cage with a potato plant (cv Norland) and placed in a growth chamber under a 13.5 h L : 10.5 h D photoperiod and a daily oscillating temperature with a nighttime minimum of 6°C and daytime maximum of 18°C, these conditions being highly conducive to diapause in this beetle population. After 2 weeks, individuals were isolated in small plastic cages with access to foliage in order to verify cessation of prediapause feeding. Beetles that did not feed for 3–4 days were considered ready to dig.
Soil
factors can affect overwintering success, such as food quality during diapause preparation (Ushatinskaya 1966b, 1978; Hurst 1975) and physical conditions in overwintering sites (Minder 1966; Hurst 1975 and references therein). For example, mortality is higher in water-retaining soil than in sandy soils (Ushatinskaya and Kozarzhevskaya 1962; Minder 1966; Hurst 1975; Ushatinskaya 1978). Also a factor is the minimum soil temperature to which a beetle is exposed, which can vary widely depending on local climate, ground cover, and overwintering depth (Lashomb et al. 1984; Kung et al. 1992; Weber and Ferro 1993). Prediapause movement and variation in diapausing depth suggest that beetles may be able to adaptively use microenvironmental variations in soil or soil surface conditions at the time of digging as cues to site suitability. According to Breitenbacher (1918), beetles will not start digging if the air is moisture-saturated, which shows their sensitivity to their environment. Hurst (1975) suggested that a beetle might be able to adjust its overwintering depth in anticipation of winter severity. However, when the range of available soil con-
For all experiments, superior-grade topsoil obtained from a specialized supplier was used. The soil was first oven-dried and then sifted to remove coarse debris. Tap water was then added to obtain the desired moisture level in the soil. The required amount of soil was then compacted into digging tubes by gentle tapping (ensuring that density varied by ≤ 5% through the soil column) (see below). We controlled soil density by varying the volume of soil through compaction while maintaining water and solid matter at 55 and 45% by mass, respectively.
Observations Specially designed digging tubes were used to monitor the beetle’s vertical descent through a uniform soil column with a cross section three to four times that of an average potato beetle (Fig. 1). A tube comprised three or more 10 cm long sections aligned end to end in a clear acrylic holder tube to accommodate various digging depths, as necessary. A section was made of 1.9 cm i.d. clear acrylic tubing with the external surface painted black to exclude light during observations. At 1-cm intervals, pinholes were bored diametrically through the tube and soil column, forming a series of miniature observation ports. A beetle’s descent through the column could be followed at any time by checking for obstructed observations ports. A small conical depression 5 mm in diameter and 5 mm deep was made at the top of the soil column, as preliminary tests showed that surface irregularities affect initial digging, uni© 1998 NRC Canada
Noronha and Cloutier formly flat soil being especially inhibitory when the soil was highly compacted. All individuals were sexed, measured (body length and width), and weighed (±0.5 mg) before and after testing. A test was started by releasing a beetle on top of the soil column. Hourly observations were made for the first 10 h and then once a day until no further downward movement was observed for ≥ 5 consecutive days. At this time it was assumed that a beetle had reached its final resting position for diapause. Beetles were then unearthed gently to verify that they had turned around to adopt the head-up resting position and formed a diapausing cell, and were weighed again. The factors tested were soil temperature, soil density, and soil water content. Each of these factors was tested separately, and beetles had no choice but to dig into the soil column onto which they were released. However, in a separate experiment involving soil density, beetles were given a choice of digging into one of four differently compacted soil columns. Beetle responses studied were the incidence of digging or the proportion of beetles entering soil columns (in both choice and no-choice tests) and digging time and final depth reached (no-choice tests only). All tests were performed in the growth chamber under a 13.5 h L : 10.5 h D photoperiod until the beetles had entered the soil. The lights were then turned off and the experiment proceeded in the dark. More experimental details are given below separately for each factor studied.
Soil temperature Tests were carried out at five soil temperatures, 8, 12, 16, 20, and 24°C, using a soil density of 0.79 g/cm3 and a moisture content of 55%. The experiment was repeated with 30 beetles of both sexes.
Soil density (i) The soil density preferred for digging was examined by providing beetles with a choice of four densities, 0.64, 0.79, 0.93, and 1.14 g/cm3, using soil with 55% water content at 20°C. These densities ranged from loose to highly packed in terms of the beetle’s ability to dig, based on preliminary tests. Beetles were tested in 10 cm diameter, 5 cm high clear plastic arenas with four holes in the bottom into which four digging tubes containing soil at different densities were inserted. The tops of the tube were flush with the bottom of the arena, forming a four leaf clover pattern in the centre of the arena. A test was started by releasing a beetle in the centre of the arena, its choice being recorded 24 h later by digging into each tube until the beetle was located. A total of 40 males and 40 females were tested. (ii) Final depth and digging time required to reach this depth were examined, in a no-choice situation, at nine soil densities, 0.57, 0.64, 0.71, 0.79, 0.86, 0.93, 1.00, 1.07, and 1.14 g/cm3, at a temperature of 20°C and a soil moisture content of 55%. The procedure was as described above.
Soil moisture Six soil water levels were tested, 40, 45, 50, 55, 60, and 65% (note that the saturation point of the soil used for all these experiments was 72% by mass, above which water flowed from the soil). Soil density and temperature were kept constant at 0.79 g/cm3 and 20°C, respectively.
Data analysis Beetles’ digging responses were regressed on soil density, temperature, and moisture using generalized linear models (McCullagh and Nelder 1989). For incidence of digging we used logistic regression, which is applicable to binary data. It allowed us to model y = log(p/1 – p), where p is the probability (incidence) of digging. For individuals that entered the soil, digging time, depth reached, and speed of digging were related to factor levels using regression models. Because nonlinear responses were expected over the tem-
1707 perature ranges used, the significance of quadratic effects was generally tested and included whenever significant. Predicted optima for quadratic regressions were calculated using xopt = –b1/(1 – b2), where b1 and b2 are parameter estimates for first- and second-order effects, respectively. Before regressing, distributions of digging speed and depth were checked for normality using Bartlett’s test, and data were log-transformed when appropriate. However, for easy visual interpretation of trends in responses to experimental variables, untransformed data scales are shown in the graphs. The significance of the covariates sex, fresh mass before digging, and biometrics (body length, width, and shape as defined by the length to width ratio) was tested in two steps. First, stepwise regression was applied to residuals of data from all three experiments after the effects of the 20 treatments (5 temperatures plus 9 densities plus 6 moisture levels) were removed, to identify significant covariates. The second step was to determine the proportion of residual variance explained by the covariates when included in the regression of responses to controlled factors. However, because the covariates were highly correlated (e.g., females are significantly larger and heavier than males), only the most significant covariate for any response was retained. All statistical analyses were performed using SAS statistical software (SAS Institute Inc. 1993) using the GENMOD and GLM procedures. When evidence of deviance from a Poisson distribution was detected in a logistic regression, data were scaled using the square root of deviance/degree of freedom, and goodness of fit was based on F ratios.
Soil temperature Increasing temperature had an increasing and then a decreasing influence on the incidence of digging (Fig. 2A). Logistic regression confirms both direct (χ12 df = 14.72, p ≤ 0.0001) and second-order effects of temperature (χ12 df = 14.89, p ≤ 0.0001). The relationship can be modelled by the logistic equation y = –5.112 + 0.893x – 0.028x2, which confirms the trend but poorly predicted the incidence of digging (R2 = 0.09, and nearly significant lack of fit, i.e., χ 22 df = 5.69, p = 0.058). The logistic model estimates 16°C as the optimum temperature, corresponding to 88% digging (96% observed). At the 8°C end of the range, the model predicts 56% entering (63% observed) compared with 57% at 24°C (74% observed). Temperatures of 12–20°C constitute the optimum for incidence of digging. For beetles that did enter soil, the overall period of digging activity before they finally came to rest was quite variable among individuals, lasting from a few days to several weeks (Fig. 2B). Digging time here represents the total duration (±1 day) of the period during which downward progress was detectable, but in fact, digging was discontinuous, with bursts of movement interrupted by variable periods of apparent resting. Average digging times ranged from a few days to a week or so at extremes of the temperature range and up to 3–4 weeks near 16°C (Fig. 2B). For modelling as a function of temperature, log-transformed digging time was regressed using the second-order model y = –1.177 + 0.478x – 0.014x2 (R2 = 0.12, F[2,113] = 7.70, p = 0.0007). Final depth reached increased markedly as the temperature rose from 8 to 16°C, but an additional increase to 20– 24°C produced little if any further change (Fig. 2C). The trend is best described by a second-order regression of log depth: y = –0.397 + 0.320x – 0.0081x2 (R2 = 0.26, F[2,113] = © 1998 NRC Canada
1708 Fig. 2. Effect of temperatures between 8 and 24°C on potato beetle digging. (A) Percent incidence of soil entry (N = 159) and box plots of digging time (B) and depth reached (C). The median (horizontal midline), mean (square), standard deviation (lower and upper limits of box), and range (vertical line) are shown in B and C (N = 116).
19.60, p ≤ 0.0001). The test temperature 16°C gave the highest observed mean final depth reached (18.65 cm), while the model predicts 19.7°C as the optimum temperature for a maximum predicted depth of 15.8 cm. Combining the average depth and digging time allowed us to calculate an average speed of digging at around 1 cm/day in the temperature range 8–20°C, increasing to about 2 cm/day at 24°C (see below for more on digging speed). Soil density Nearly 50% of beetles given a choice among four soil densities chose to dig into the lightest soil (0.64 g/cm3) (Fig. 3). This greatly departs from random choice (G23 df = 22.35, p ≤ 0.0001), indicating that beetles discriminate heavily packed soil surfaces as being less suitable to dig. However, despite a range of choices being available, soil at all densities was dug into, including the most compacted soil (twice as hard to penetrate as the lightest soil), which was chosen by 11% of the beetles tested. In the no-choice test involving nine soil densities, 70– 100% of the beetles tested penetrated the soil until they completely disappeared (Fig. 4A). Incidence of digging was also confirmed to be a negative function of soil density that can be modelled by the logistic equation y = 8.14 – 6.54x (R2 = 0.14, χ12 df = 22.16, p ≤ 0.0001). Most individuals dug only for 5–6 days before resting for good (Fig. 4B), a shorter average duration than in the other two experiments. Individual digging times were again vari-
Can. J. Zool. Vol. 76, 1998 Fig. 3. Incidence of digging by prediapause Colorado potato beetles within 24 h when a choice of four soil densities was given. The random choice hypothesis can be safely rejected (G23 df = 22.35, p ≤ 0.0001).
able, but, on average, seemed to follow a shallow domeshaped pattern as soil density increased. This was confirmed by regression of log-transformed digging time on both direct and quadratic effects of soil density, giving y = –3.019 + 11.504x – 7.205x2 (R2 = 0.09, F[2,226] = 11.25, p ≤ 0.0001). Final depth decreased sharply with soil density, from nearly 80 cm below the surface for the lightest soil to only 1–3 cm for the heaviest soil (Fig. 4C). This relationship is well described by a linear regression of log depth on density: y = 7.97 – 6.41x (R2 = 0.80, F[1,227] = 899.05, p ≤ 0.0001), indicating an exponentially rising limiting effect of soil density on final depth reached. Average digging speed also decreased markedly with soil density, from 22 cm/day in the loosest soil to
