Temperature-based prediction and validation of pink bollworm, Pectinophora gossypiella infestation on cotton crop

2.1 Study sites and sampling plan of PBW infestation

The farmers areas were visited in intensive cotton-growing areas of 17 Punjab districts; Bahawalnagar, Bahawalpur, Bhakar, Dera Ghazi Khan, Faisalabad, Jahang, Khanewal, Layyah, Lodhran, Mianwali, Multan, Muzaffargarh, Rajanpur, Rahim Yar Khan, Sahiwal, Toba Tek Singh, and Vehari, during September-December 2012–2019 (Fig. 1).

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Fig 1

Survey districts where a collection of the samples was conducted from 2012 to 2019. This map was created with ArcGIS v 10.4 (https://www.arcgis.com). Here, grey color represents the survey sites, and white color represents no survey was conducted. Data of administrative boundaries were obtained from (https://gadm.org/download_country_v3.html).

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Pink bollworm infestations were collected by ZigZag method of pest scouting from 89 locations of those 17-major cotton-growing districts in Punjab Province at the maximum level of P. gossypiella infestation at stage (100–120 days after sowing) according to green boll maturity boll bursting stages representing PBW larval population at pre–picking and picking stages, respectively. The number of total and infested chosen bolls per plant was counted randomly from a unit area of one acre at each site in every cotton-growing region in order to measure the damage to cotton bolls caused by PBW infestation in a cotton field. A count of the infected bolls was made, and the percent infestation was calculated.

2.2 Identification of PBW

The collected pink bollworm larvae were identified by observing under a stereo microscope for their morphological characters such as shape, size, color (Saunders, 1843). The collected larvae were reared until the adult stage was reached and then correctly identified (Saunders, 1843) and sent to the National Agriculture Research Council Islamabad for further confirmation.

2.3 Modelling procedure

The multiple regression model was used to predict the impact of future climatic conditions on the infestation rate of the pink bollworm attack in different districts of Punjab. For the future climatic conditions, 2040 year was selected, and prevailing future climatic conditions were assessed using the minimum temperature, maximum temperature and relative humidity environmental layers in ArcGIS software. The following modelling formula was used for the assessment of the future impact on our cotton crop. $$\text{Y}=\text{Constant}+\text{B}1\left(\text{X}1\right)+\text{B}2\left(\text{X}2\right)+\text{B}3\left(\text{X}3\right)$$

Here, Y is the future estimated value in 2040; constant is the intercept value, B1, B2, and B3 are the independent factors, whereas X is the different coefficient values.

2.4 Collection and rearing of P. gossypiella

P. gossypiella infested bolls were collected during the cotton season of 2018–2019 from cotton crops randomly from different varieties raised at Cotton Research Institute, Multan (30°8′54.24″ N and 71°26′18.24″ E). The larvae were continuously reared more than 50 generations by feeding on artificial diet (Ali et al., 2020). For the oviposition, the Adults of P. gossypiella were put in cages (50 × 50 × 50 cm) and given 10 % honey solution to increase egg laying. White gauze and filter paper were placed in every cage for egg laying. The eggs which are in clusters form were collected every day during the total oviposition period.

2.5 Experimental setup

P. gossypiella stagewise life steps were tested in cohorts of single life stages at three constant temperatures 21, 28, and 35 °C. The colonies were managed in the controlled incubation chambers with relative humidity and photoperiods 65 ± 5 % and 12:12 L:D, respectively in the laboratory. Prior to the start of experiment, four pairs of mated adult moths were released separately into four plastic containers with white gauze and filter paper for egg laying at the evening. The eggs were counted from the filter paper after the removal of moths at the next morning. To test the effect of temperatures approximately total of 200 eggs which were 12 h old were placed in plastic box inside the growth chambers. This study has four repeats and each repeat with 50 eggs placed on different filter paper. Hatchability was recorded daily for each test of temperature.

For larval development study, each test of temperature was studied with around 100 larvae in a group of 25 (<12 h old) released on artificial diets in a plastic box (45 × 20 cm size). Development time, mortality of larvae, and number of successful pupal development were calculated at each set of constant temperatures with three repeats. Similarly for pupal durations, 100 newly formed pupae (∼12 h old) were separated from the reared colony and placed in the groups of 25 pupae into four different plastic boxes (15 × 10 cm size) inside the chambers for each test of constant temperatures. Emergence of adults from pupae and pupal durations were calculated every day for each test of constant temperatures. To test the oviposition rate of new emerged adults at different set of test temperatures, 10 pairs (∼12 h old) of P. gossypiella from stock colony were placed in plastic jar (30 × 15 cm size). For egg laying adults moths were given feeding of 10 % honey. Moths were moved to new oviposition boxes each day, and the number of eggs laid in each box were counted to determine each female's overall fecundity. Each adult male and female's longevity was measured at its appropriate test temperature. To ensure optimal mating and fertility, new male adults were offered to the females in the appropriate cages in the event that older male adults passed away.

2.6 Parameters/data collection

Data regarding hatching period, hatching percentage, duration of larval instars, rates of adult eclosion, consumption rate, male: female, fecundity, intrinsic rate of increase, survival rate, etc. were collected. Detail of the dependent variable/ data collected is given as following:

The age-specific survival rate (l_x) and age-specific fecundity (m_x) were calculated as: $$l_{x=}\sum _{j=1}^k{S}_{\mathit{xj}}$$ $$m_{x=}\frac{{\sum }_{j=1}^k{S}_{\mathit{xj}}{f}_{\mathit{xj}}}{{\sum }_{j=1}^k{S}_{\mathit{xj}}}$$

Where, k exhibits the number of stages.

The net reproductive rate (R₀) was computed as: $$R_{o=}\sum _{x=0}^{\infty }{l}_x{m}_x$$

The intrinsic rate of increase (r) with age indexed from 0 was corrected by the Euler-Lotka equation: $$\sum _{x=0}^{\infty }{e}^{-r\left(x+1\right)l_x{m}_x=1}$$

The following equation was used for the finite rate of increase (λ): $$\lambda =e^r$$

The mean generation time was demonstrated as $$\mathrm{T}=\frac{l_n{R}_o}{r}$$

The life expectancy (e_xj) was determined as: $$e_{\mathit{xj}}=\sum _{i=x}^{\infty }\sum _{y=j}^{\beta }{S}_{\mathit{iy}}$$

Where, s_iy shows the probability of survival of each individual of age × and stage j to age i and stage y by assuming s_xj = 1.

The reproductive rate (v_xj) was assessed according to (Li et al., 2012). $${\nu }_{\mathit{xj}}=e\frac{r(x+1)}{S_{\mathit{xj}}}\sum _{i=x}^{\infty }{e}^{-r(x+1)}\sum _{y=j}^{\beta }{S}_{\mathit{iy}}{S}_{\mathit{iy}}$$

2.7 Statistical analysis

The collected data was analyzed by using Age-Stage, Two-Sex Life Table Analysis (Copyright 1997–2021 Hsin Chi Version: 2017.09.01) for further analysis. The impacts of host plants on larval feeding rate and life history characters of P. gossypiella were evaluated by using the one-way analysis of variance, (ANOVA). In addition to life table parameters, age-stage specific survival rate sxj (where x = age and j = stage), age specific fecundity mx, age-stage specific fecundity fxj, age-specific maternity lxmx, age-stage specific life expectancy exj, age-stage specific reproductive value vxj, adult pre-oviposition period APOP, and total pre-ovipositional period TPOP for all temperatures were analyzed on a computer operated TWO SEX-MS Chart software for ecological studies related with life table.

3.1 Incidence of pink bollworm in cotton

The pink bollworms were collected from infested bolls from 17 districts in Punjab Province. The results of collecting population infestation during the studied period in cotton-growing areas of Punjab Province indicated a variable rate of infestation by PBW. Our observations of field visits performed during September – December 2012–2019 in cotton cultivated districts disclosed a continuous increase in cotton bolls attacked by P. gossypiella from less to medium to serious (>50 %) above the economic threshold level between cotton boll opening to further at cotton picking (Fig. 2).

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Fig 2

Pink bollworm infestation rate in different years on cotton crop in various districts of Punjab.

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From 36 districts of Punjab province, 17 were surveyed, and all were found highly susceptible for the invasion of pink bollworm infestation on the cotton crop (Fig. 2). The PBW infestation rate was found higher in district Bahawalnagar, Rajanpur, Rahimyar Khan, Sahiwal, Vehari, Bahawalpur, Faisalabad, and Jahang during surveys of 90–100 days of crop in 2015 (Fig. 2). The P. gossypiella infestation was observed at flowers and bolls of Bt cotton crop. In 2019, Dera Ghazi Khan cotton fields were highly infested (>50 %) by PBW than the other districts in Punjab Province (Fig. 2).

A subsequent survey of 17 districts showed that the mean green boll infestation above 5 % ETL in the majority of places except in districts Bhakar (in 2018, 2019), Jahang (in 2014, 2016, 2017, 2018, and 2019), Khanewal (in 2013–2016), Layyah (in 2012, 2013, 2014, 2016, 2017, and 2018), Lodhran (in 2012, 2013, 2015–2018), Mianwali (in 2012–2019), Multan (in 2012–2014, 2016–2018), Muzafargarh (in 2012–2014, 2016–2017), Rajanpur (in 2012–2019), Rahimyar Khan (in 2014, 2016, and 2017), Sahiwal (in 2016, 2017, and 2019), T. T. Singh (in 2015, and 2017), and Vehari (in 2012, 2018, and 2019) (Fig. 2).

Pink bollworm was not found in some years of Punjab Province districts such as 2019 in Bhakar, Jhang, Mianwali, and 2014 in Layyah, Mianwali, Muzaffargarh, Sahiwal, and 2015 in Mianwali during the surveys of study periods. The lowest infestation rate of PBW was found in Lodhran, Jhang, Layyah, Mianwali, and Rajanpur.

3.2 Prediction of PBW population

By applying weather parameters in ArcGIS software, we have found that PBW infestation level was varied in all districts of Punjab during the study. In 2012, PBW infestation risk levels were found in Bahawalpur, Rajanpur, and Bhakar Districts (Fig. 3).

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Fig 3

Pink bollworm infestation to different districts of Punjab at different levels. These maps were created with ArcGIS v 10.4 (https://www.arcgis.com). Data of administrative boundaries were obtained from (https://gadm.org/download_country_v3.html).

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In 2013, the PBW risk level was slightly decreased in Bahawalpur and Rajanpur, but the high-risk level in Bhakar, Multan, Khanewal, and Jahang. However, the PBW infestation risk level was increased in Punjab Province. More districts such as Bahawalpur, Dera Ghazi Khan, Rahimyar Khan, Bahawalnagar, and Faisalabad were affected in 2014. Subsequently, the medium to high-risk level was observed in some districts of Punjab in 2015, but Rahimyar Khan, and Rajanpur were highly infested by PBW, see Fig. 3. Similarly, in 2016 and 2017, the risk of PBW infestation was increased in surveyed districts of Punjab. However, low medium to high PBW infestation risk levels was found in all main cotton growing areas except the upper Punjab districts of Pakistan. In the 2014 predicted model, PBW infestation was found higher in all districts of Punjab except Bahawalnagar, T. T. Singh, Vehari, and Sahiwal.

3.3 Developmental duration, longevity, and fecundity

The mean hatching time of P. gossypiella has been presented in Table 1. Egg hatching duration was more rapid (2.95 days) followed by rearing at 35 °C and (3.70 days) at 28 °C but was prolonged (7.83 days) at 21 °C. The average egg-to-adult growth time at 21, 28 and 35 °C temperatures was 38.83, 20.90 and 18.13 days respectively. Development periods for the first instar were (5.74, 3.00, 2.59 days), (5.36, 3.15, 3.00 days) for the second, (5.11, 3.03, 2.84 days) for the third, (6.57, 3.31 and 3.16 days) for the fourth larval instars of P. gossypiella were recorded at 21, 28 and 35° C. Pupa completed its growth very rapidly, i.e., within 3.65, 4.90 at 28 and21 °C compared to 8.34 days at 35 °C. Pre-adult period from egg to adult at various temperatures was 38.83, 20.90 and 18.13 days for 21, 28 and 35 °C, respectively. Adult longevity of females was prolonged to 57.05 followed by 39.00 and 31.86 where for male was 49.71, followed by 26.56 and 23.22 days at temperatures of 21, 28 and 35 °C, respectively.

3.4 Comparison of reproductive and life table parameters

The mean adult oviposition period (APOP), Total pre-oviposition period of P. gossypiella have been presented in Table 2. Adult pre-oviposition period (APOP) at 21 °C was (2.94 days) followed by 28 °C (2.64) but was shortened (1.73 days) at 35 °C. Total pre-oviposition period (TPOP) was enhanced (41.47) as compared with 23.55 and 19.68 at 21, 28 and 35 °C, respectively. Oviposition period was 17.00 at 21 °C, 12.32 and 9.41 days at 28 and 35 °C, respectively (Table 2). The average oviposition days of female at 21, 28 and 35 °C temperatures were 17.00, 12.32 and 9.32 days, respectively. Fecundity 24.57, 123.05, 49.82 was observed at 21, 28 and 35 °C temperature, respectively. The intrinsic rate of increase (r), finite rate of increase (λ), net reproductive rate (Ro) and mean generation time (T) are given in Table 2. Rο of P. gossypiella under different temperature regimes was 10.31, 54.14 and 15.36 for 21, 28 and 35 °C, respectively. The longest generation time found was 48.845 d at 21 °C as compared with 29.53 and 24.31 d at 28 and 35 °C. λ has found higher than 1.0 at every test treatment.

3.5 Development rates

Development rates (1/developmental time) of P. gossypiella female were regressed against temperature (Table 3). Results revealed that temperature contributed 92.17, 84.51, 66.01, 70.69, 62.99, 99.43, and 84.46 % variation in egg hatching, development of first, second, third, fourth instar larvae, pupa and longevity of adult stage, respectively for P. gossypiella female. The lower developmental threshold of P. gossypiella female for egg, first, second, third, fourth instar larvae, pupa and egg to the adult stage was recorded as 11.72, 8.32, 1.33, 1.79, 6.13, 9.88 and 7.57 °C, respectively. Concerning thermal constant (K), 66.26-degree day units were required for egg hatching, 66.093, 95.41, 89.52, 85.25, 90.86 for first, second, third, fourth larval instar and pupa development. In this way concerning thermal constant (K) 476.19-degree day units were required for P. gossypiella female to complete egg to adult stage. Regression equation depicted that temperature has negative correlation with the developmental stages of P. gossypiella with very minute change in temperature there was 17 % adverse change in egg hatching, 12 % negative effect on of first instar larvae and 10 % on pupal development of pink bollworm.

3.6 Survival rate, life expectancy, and reproductive value

From the detailed age-stage survival rate (Sxj) of PBW for different set of constant temperatures (Fig. 4), the observations showed the prospect of new born P. gossypiella living to age x and stage j. Developmental stages showed significantly difference as seen in curves due to the rate of development varies between individuals at each set of test temperatures. It is observed that age is inversely proportional to the survival rate of individuals for different temperatures treatment. The survival peak was found higher for all PBW life steps at 21 °C with respect to other temperature treatments. The age-specific survival rate (lx), the age-specific fecundity of total population (mx), age-stage specific fecundity (fxj) and the age-specific maternity (lxmx) curves designed at different temperatures shown in Fig. 5.

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Fig 4

Effect of three different temperatures ((A) 21 °C, (B) 28 °C and (C) 35 °C) on the age stage- specific survival rate (sxj) of the P. gossypiella. L1 = First Instar, L2 = Second Instar, L3 = Third Instar, and L4 = Fourth Instar.

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Fig 5

Effect of three different temperatures ((A) 21 °C, (B) 28 °C and (C) 35 °C) on the survival rate (lx), female age-specific fecundity (fx), age-specific fecundity (mx) and age specific maternity.

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Basic form of the age-stage survival rate Sxj found to be inversely proportional at 21 °C whereas it was observed direct in proportion to 28 °C and 35 °C as shown in curves. Age-stage specific fecundity (fxj) were observed higher at 21 °C then the 28 °C and 35 °C. The curve of mx shows that reproduction start prior at 35 °C and 28 °C then 21 °C. In our results, the impact of the given set of temperatures on the projected regular life expectancy (exj) of the population at each phase of P. gossypiella (Fig. 6) were observed.

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Fig 6

Effect of three different temperatures ((A) 21 °C, (B) 28 °C and (C) 35 °C) on the life expectancy (exj) of the P. gossypiella. L1 = First Instar, L2 = Second Instar, L3 = Third Instar, and L4 = Fourth Instar.

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A progress movement of all phases were found at 21 °C with dissimilarity in other developing phases. The age-stage reproductive value (Vxj) defines the portion of an individual of age × and stage j in the direction of the forthcoming population (i.e., the scale of population forecasting). The curves of Vxj were found significantly lower at 28 °C and 35 °C with respect to the 21 °C. But, at 28 °C the female reproductive value was observed higher as compared to the other temperature sets as shown in Fig. 7.

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Fig 7

Effect of three different temperatures ((A) 21 °C, (B) 28 °C and (C) 35 °C) on the age-stage reproductive value (vxj) of the P. gossypiella. L1 = First Instar, L2 = Second Instar, L3 = Third Instar, and L4 = Fourth Instar.

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