Introduction

Quantification of crop losses due to insect pests is required to estimate the cost–benefit ratios that justify the cost of implementing any control strategy. In sugarcane, stalk borers are considered as the most damaging pests and cause significant yield losses. Recent studies on two species, Chilo sacchariphagus (Lepidoptera: Crambidae) and Eldana saccharina (Lepidoptera: Pyralidae), showed significant reductions in terms of biomass and sucrose due to their damage (Goebel and Way 2009; Goebel et al. 2014). Diatraea (21 species) are the main pests of sugarcane in the Americas, particularly Diatraea tabernella Dyar in Costa Rica, Panama and Colombia. Internal stalk damage is associated with red rot (Colletotrichum falcatum Went) that causes inversion of sucrose (Alonso et al. 1991; Bonzi 2008). Díaz et al. (2003) found that 87.4 kg of sugar is lost per ha for a level as low as 2% of internodes bored. A more recent study (Gómez et al. 2009) indicated that losses from Diatraea spp. amounted to 145 kg of sugar/ha for each 1% of internodes bored.

In Panama, a preliminary study conducted in 2012 by a Cuban team highlighted the damage and impact on sugar yield from D. tabernella. Based on these unpublished data, it was decided to conduct a more detailed study to specifically determine the damage levels caused by this borer on commercial varieties in Panama and then investigate the impact not only on sugar losses but on stalk parameters. The impact of this species has never been determined.

Materials and Methods

The study was conducted at Compañía Azucarera La Estrella S.A. (CALESA), Natá, Panama, in January and February 2016 during the harvesting season. We tested eight commercial varieties (B74-125, BT77-42, B80-689, CR74-250 and four promising new varieties, E07-09, Na56-42, SP81-3250 and SP01-2050). Crops were aged from 10 to 12 months.

To assess overall borer damage, we sampled 100 stalks per variety taken at random, consisting of four replicates each of 25 millable stalks. These were taken from the four end points of each 2-ha field. Each stalk was then split longitudinally, and the number of bored internodes counted [percentage of internodes bored (% IB)] and the length of tunnels within each internode that were bored were measured (Fig. 1). We also recorded individual stalk length, stalk mass and diameter (data not presented).

Fig. 1
figure 1

Tunnels caused by D. tabernella (left) and classes of damage levels (right)

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To determine the impact on sugar quality and biomass, we took stalk samples (three replicates of five stalks each) from each of five damage levels: 0 = no internodes bored (IB); 1 = 1 IB, 2 = 2 IB, 3 = 3 IB and 4 = 4 IB (Fig. 1, right). Stalk samples were taken to the Laboratory of Quality and Sugar of CALESA for sugar quality and yield measurements: Pol, sucrose, Brix, purity, fiber and sugar yield (t/ha). Data were subjected to Analysis of Variance (ANOVA) using Statgraphics Plus 50, and regression graphs were generated to determine the relationship between damage and loss.

Results and Discussion

Overall Damage

Damage over the whole population of stalks (undamaged and damaged) was generally low in the eight varieties surveyed, with percentage of internodes bored ranging from 0.22% (SP01-2050) to 4.1% (B74-125) and with significant differences leading to two distinct groups of varieties, the ‘low damage level’ group (E07-09, Na56-42, SP01-2050 and SP81-3250) with internodes bored <0.40% and a ‘high damage level’ group (B74-125, B80-689, BT77-42, CR74-250) with internodes bored >1% (Fig. 2).

Fig. 2
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Damage level in each variety. High damage level (black columns) and low damage level (white columns)

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In terms of sucrose levels, cane yield and sugar yield, there was also significant differences with the highest sugar yields recorded for the varieties E07-09 (10.92 t/ha), Na56-42 (9.79 t/ha), SP01-2050 (9.03 t/ha) and SP81-3250 (9.03 t/ha) (Table 1) which were also the less damaged. Despite a relatively low level of internodes bored, these results showed an effect of borer damage on yield components. These findings confirm data recorded by Díaz et al. (2003).

Table 1 Damage levels, quality parameters and yields for each variety (mean ± SE)
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Impact of Borer Infestation Within Different Classes of Damage

The different groups of damage level from 0 (0% internodes bored) to 4 (16.17% with 28.7 cm of tunnel length) allowed us to better investigate the effect of borer attack on stalk parameters. Even though there was no significant difference in individual stalk length between classes of damage, there was a trend to a difference of 14 cm between the highest damaged samples (level 4) compared to undamaged stalks (level 0). Stalk mass was significantly affected with a loss of 21.9% in the level 4 group (Table 2).

Table 2 Parameters of stalks at different damage levels (all varieties) (mean ± SE)
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There were significant differences between damage levels, particularly between level 4 and level 0 for quality and yield parameters (Table 3). Brix, sucrose content, fiber content, cane and sugar yield were affected by borer damage. The increase in fiber content from 12.81% at level 0 to 13.57 at level 4 combined with a reduction in sucrose content (from 17.5% at level 0 to 14.7% at level 4). To reduce sugar yield (t/ha), with a maximum loss of 2.56 t/ha (yield level 0–yield level 4) representing a yield reduction of 26.5% when 16.2% of internodes are damaged (Tables 2, 3).

Table 3 Sugar quality parameters and yield at different damage levels (mean ± SE)
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Using the data generated by the eight commercial varieties (considering the real scenario of harvest) we generated an exact linear relationship negative (decreasing) between sugar yield (t/ha) and damage levels (y = −0.579x + 9.324, R 2 = 0.9069) (Fig. 3).

Fig. 3
figure 3

Relationship between internodes bored and sugar yield (varieties within the harvest)

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B74-125 is the variety with the highest % internodes bored (4.10) showed an exact linear relationship negative (decreasing) between sugar yield and internodes bored (y = −0.5636x + 8.7783; R 2 = 0.9908) (Fig. 4). For this variety, there was a yield reduction from 0.60 t/ha for level 1 damage to 2.23 t/ha for level 4 damage; a fourfold increase. These results are similar to those from other studies (Gupta and Avasthy 1959; Ferrer and Guedez 1990; Shrivastava and Solomon 2009). These studies confirmed that juice quality is reduced when the intensity of infestation is increased, confirming a negative impact of borer damage on this parameter.

Fig. 4
figure 4

Relationship between internodes bored and sugar yield for variety B74-125

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Conclusion

Our studies show that D. tabernella reduces the quantity and quality of sugar and of biomass. Fiber content increased with higher levels of internodes bored. This is a consequence of borer tunnels and reduction in water uptake that would be reflected by lower juice levels. These results are in accordance with other studies on stalk borers such as Eldana saccharina in Africa and Chilo sacchariphagus in Réunion (Goebel and Way 2009) and Indonesia (Goebel et al. 2014), even though the overall damage during our study was generally lower. Moreover, biomass loss appeared to be more difficult to estimate and require more research particularly to determine the relationship between internodes bored and cane yield components. The data from damage assessments can also be used to establish an approximate economic threshold for D. tabernella and examine the feasibility of developing a reliable model for predicting yield losses based on damage levels.